colorspace-faq - purpose of the faq ?
Timo Autiokari
Dear Mr. Poynton,
What is the purpose of your GammaFAQ?
In your forewords you say:
***"In video, computer graphics and image processing, gamma represents a
numerical parameter that describes the nonlinearity of intensity reproduction.
Having a good understanding of the theory and practice of gamma will enable you
to get good results when you create, process and display pictures." ***
Many people understand the above as if it would apply to
digital-photographic-imaging as well.
However you do not say that it would apply. Instead you speak about "image
processing" and about "create, process and display pictures". These differ from
digital-photographic-imaging in two aspect:
Digital photographic images (1) originates form the real world, and they are
then (2) enhanced where as "image processing" and "create, process and display
pictures" refers to something that originates from calculations or algorithms,
synthetically, and are then displayed as they are, they are not enhanced.
So, I find many fundamental issues in your GammaFAQ that are, to say the least,
misleading when they are applied to digital-photographic-imaging. I take only
one example from the GammaFAQ, the same text is in the ColorFAQ as well:
>>13. How many bits do I need to smoothly shade from black
>>to white?
>>...To shade smoothly over this range, so as to produce no
>>perceptible steps, at the black end of the scale it is necessary
>>to have coding that represents different intensity levels 1.00, 1.01,
>>1.02 and so on. If linear light coding is used, the "delta" of 0.01 must
>>be maintained all the way up the scale to white. This requires about
>>9,900 codes, or about fourteen bits per component. If you use nonlinear
>>coding, then the 1.01 "delta" required at the black end of the scale
>>applies as a ratio, not an absolute increment, and progresses like
>>compound interest up to white. This results in about 460 codes, or about
>>nine bits per component. Eight bits, nonlinearly coded according to
>>Rec. 709, is sufficient for broadcast-quality digital television at a contrast
>>ratio of about 50:1....
Now, the conditional expressions: "If linear light coding is used..." and "If
you use nonlinear coding...". are fundamentally faulty when applied to digital
photographic imaging. Also the meaning of what actually the "to code" means
gets to be a bit blurry.
In digital-photographic-imaging where a CCD imager is used (cameras and
scanners) there is absolutely no possibility to choose what coding is used. The
CCD sees the light linearly and the coding happens linearly by the AD converter
that is inside the CCD device. _This_ is where the coding happens. And it
happens linearly.
So, the data that comes out from the CCD imager device (the integrated circuit)
is already coded and that has been done linearly, always. The light-to-data
coding has been done at this point.
There after it is only possible to alter the _result_ of this coding, in other
words to make the image data suitable for an output device or an other. This is
the same as compensation.
In other words after the coding is done it is not possible to just calculate any
better shading (or better human perception) into the data. That would require
more data.
On the other hand if you _create_ computer generated graphics (like virtual
reality 'images') using algorithms then you have all the possibilities to affect
to the 'coding' freely because the 'coding' is done by algorithms or
calculations, so any intensity values can be freely chosen. There you can have
the better shading of the black and the better perception for the eye,
simultaneously.
So my question is: Does your GammaFAQ cover digital-photographic-imaging and if
your answer is "yes" then how do you explain the better perceptual coding above?
Timo Autiokari
http://www.clinet.fi/~timothy/calibration/index.htm
Stephen H. Westin
tim...@clinet.fi (Timo Autiokari) writes:
> Dear Mr. Poynton,
>
> What is the purpose of your GammaFAQ?
>
> In your forewords you say:
> ***"In video, computer graphics and image processing, gamma
> represents a numerical parameter that describes the nonlinearity of
> intensity reproduction. Having a good understanding of the theory
> and practice of gamma will enable you to get good results when you
> create, process and display pictures." ***
> Many people understand the above as if it would apply to
> digital-photographic-imaging as well.
Actually, he is dealing simply with the issues of display of a known
digital image on a CRT. As you point out, there are additional issues
involved with image acquisition.
<snip>
> So, I find many fundamental issues in your GammaFAQ that are, to say
> the least, misleading when they are applied to
> digital-photographic-imaging. I take only one example from the
> GammaFAQ, the same text is in the ColorFAQ as well:
> >>13. How many bits do I need to smoothly shade from black
> >>to white?
> >>...To shade smoothly over this range, so as to produce no
> >>perceptible steps, at the black end of the scale it is necessary
> >>to have coding that represents different intensity levels 1.00, 1.01,
> >>1.02 and so on. If linear light coding is used, the "delta" of 0.01 must
> >>be maintained all the way up the scale to white. This requires about
> >>9,900 codes, or about fourteen bits per component. If you use nonlinear
> >>coding, then the 1.01 "delta" required at the black end of the scale
> >>applies as a ratio, not an absolute increment, and progresses like
> >>compound interest up to white. This results in about 460 codes, or about
> >>nine bits per component. Eight bits, nonlinearly coded according to
> >>Rec. 709, is sufficient for broadcast-quality digital television at a contrast
> >>ratio of about 50:1....
I think this description is a bit pessimistic. I would put the minimum
for a digital image displayed on a CRT at about 12 bits minimum, if
linear encoding is used.
> Now, the conditional expressions: "If linear light coding is
> used..." and "If you use nonlinear coding...". are fundamentally
> faulty when applied to digital photographic imaging. Also the
> meaning of what actually the "to code" means gets to be a bit
> blurry. In digital-photographic-imaging where a CCD imager is used
> (cameras and scanners) there is absolutely no possibility to choose
> what coding is used. The CCD sees the light linearly and the coding
> happens linearly by the AD converter that is inside the CCD device.
> _This_ is where the coding happens. And it happens linearly.
Then how does our Kodak DCS420 camera deliver images with a gamma
correction of around 1.6? We know; we measured it. And I think you
will find a similar nonlinearity in most commercially-available
cameras.
<snip>
> In other words after the coding is done it is not possible to just
> calculate any better shading (or better human perception) into the
> data. That would require more data.
Yup. That's why you have to think about these issues. As do
manufacturers of digital cameras.
<snip>
Another subtle point is that it is, in general, impossible to
transform an acquired image into any standard tristimulus color space;
since the three filter responses are always different from the CIE
matching functions, there will always be cases where the
transformation will give the wrong answer. Fortunately, most spectra
in the real world don't incur egregious errors in such a process. Just
found this out a few weeks ago, myself.
--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.
Michael McGuire
0]>
....snippage
: In digital-photographic-imaging where a CCD imager is used (cameras and
: scanners) there is absolutely no possibility to choose what coding is used. The
: CCD sees the light linearly and the coding happens linearly by the AD converter
: that is inside the CCD device. _This_ is where the coding happens. And it
: happens linearly.
: So, the data that comes out from the CCD imager device (the integrated circuit)
: is already coded and that has been done linearly, always. The light-to-data
: coding has been done at this point.
...snippage
: Timo Autiokari
: http://www.clinet.fi/~timothy/calibration/index.htm
A/D convertors in cameras and scanners are in fact external to the CCD's--not
on the same chip, and thus accessible to adjustment by other than the maker of
the CCD. Further there is no requirement that an A/D converter have an output
linearly proportional to its input voltage. An implementation of an A/D
consists of N voltage comparators and a voltage divider comprised of a string
of N resistors fed by a stabilized voltage. Each node of the divider is
connected to the plus input of a voltage comparator. The minus input of each of
the comparators is connected to the input signal. The output is the number of
comparators that are turned on by the input signal, that is all those whose
voltage divider (plus) input is less than the input signal. If the resistor
values in the divider are all equal then you get linear output. But they could
just as well be a power law sequence for a power law relationship of output to
input or whatever function you like.
From the point of view of optimizing signal-to-noise performance, a square root
scaled sequence would be superior for CCD's given the Poisson law fluctuation
of the incoming photon flux--yet another expression of Nature's preference for
the non-linear, and in the same direction from linear as CRT gamma, printer
correction, and perceptual linearity.
Mike
--
Michael McGuire Hewlett Packard Laboratories
email:xmcg...@xhpl.xhp.com P.0. Box 10490 (1501 Page Mill Rd.)
(remove x's from email if not Palo Alto, CA 94303-0971
a spammer)
Phone: (650)-857-5491
************BE SURE TO DOUBLE CLUTCH WHEN YOU PARADIGM SHIFT.**********
Charles Poynton
Concerning an ongoing public and private attempt to educate Timo Autiokari
concerning transfer functions, in article
<34f66f5b...@news.clinet.fi>, tim...@clinet.fi (Timo Autiokari)
wrote under the heading "What is the purpose of your GammaFAQ?":
> the conditional expressions: "If linear light coding is used..." and
> "If you use nonlinear coding...". are fundamentally faulty when
> applied to digital photographic imaging.
The statements are not faulty, and they apply quite well to digital
photographic imaging.
Timo continues,
> In digital-photographic-imaging where a CCD imager is used (cameras and
> scanners) there is absolutely no possibility to choose what coding is
used. The
> CCD sees the light linearly and the coding happens linearly by the AD
converter
> that is inside the CCD device. _This_ is where the coding happens. And it
> happens linearly.
>
> So, the data that comes out from the CCD imager device (the integrated
circuit)
> is already coded and that has been done linearly, always. The light-to-data
> coding has been done at this point.
In a note attached to this follow-up, I reiterate what I told Timo, in
private e-mail, yesterday. (He acknowledged receiving it from me.) The
note explains how scanners and cameras use CCD devices. I see that Michael
McGuire and Mitch Valburg have already posted follow-ups concerning CCDs
and ADCs, and I expect (and hope for) several more third-party follow-ups
in the next few days. I'm sorry that Timo paid little attention to my
message, because he could have cleared up some confusion, instead of
creating some more.
C.
p.s. Timo: I took care in my previous posting to direct follow-ups to just
<news:sci.image.processing> and <news:rec.photo.digital>. Perhaps you
could do the same, instead of continuing to cross-post to 6 groups.
--
Charles Poynton
<mailto:poy...@poynton.com> [Mac Eudora/MIME/BinHex/uu]
<http://www.inforamp.net/~poynton/>
--
A Rough note for Timo concerning the Gamma FAQ
as it applies to CCDs and ADCs in scanners and cameras
Copyright (c) 1998-02-26
Charles Poynton
Nearly all contemporary CCDs are intrinsically analog devices - or more
properly, their output is sampled but not quantized. (For definitions of
sampling and quantization, see Chapter 1 of my book; that chapter is on
the web.) Today, only a few CCD devices have integral A-to-D converters.
(Soon, many will, but solutions to the 8-bit linear light problem must
first be found.)
Contemporary desktop scanners generally take one of two approaches:
- Some have a 10-bit (or sometimes a 12-bit) A-to-D converter, followed
by a digital hardware lookup table where a nonlinear correction is
performed, producing 8 bits out.
- Some have an analog nonlinear correction circuit, followed by an
8-bit A-to-D converter.
Video cameras invariably take the second approach, except for the most
sophisticated studio cameras, costing $80,000 or more, which employ 12-bit
converters and digital gamma correction with 10 (nonlinear) bits out.
Industrial machine vision cameras, and astrophotography cameras, sometimes
directly produce linear-light intensity output.
A very low-end, cheap scanner might get away with no analog processing, a
CCD and an 8-bit ADC. (I'm not certain whether this is done even in in
cheap commercial units, I've never taken a really cheap unit apart. Maybe
a QuickTake 100 internally codes 8-bit linear-light, does anyone know?)
But such a scanner cannot reproduce smooth shades in dark regions of the
image. As I mention in my book, the quantization requirements near black
are relaxed when the contrast ratio of the display medium is low. Scanners
for print work generally have less demanding requirements than video
cameras, because offset printing generally has a lower contrast ratio than
television. Most demanding of all is motion picture film, or projected 35
mm transparencies [slides] in a dark room. (Most desktop computer
applications are not demanding of good shading near black, because most
desktop computer environments are brightly lit, consequently the contrast
ratio is poor.)
Timo suggested (in private e-mail) that the images in the following three
cases would appear exactly identical, assuming 8 bits per color component:
1. Raw, linear-light image data is processed through a display driver
that imposes a lookup table exactly compensating the nonlinearity
of the CRT.
2. Raw, linear-light image data is gamma-corrected by 8-bit per channel
image manipulation software, processed through no lookup table (or a
lookup table containing a ramp), and then displayed on a CRT.
3. Software in the camera or the scanner applies gamma
correction; the resulting 8-bit image displayed directly on the CRT.
Timo incorrectly concludes that these three cases appear exactly
identical. His cases 1 and 3 correspond to the second and first rows
respectively of Figure 6.8 in my book. (The same figure is in question 14
of the Gamma FAQ.)
I'm sure that Timo correctly concluded that, in each of his 3 cases, black
is reproduced correctly, and white is reproduced correctly, and mid grey
is reproduced (almost) correctly. That's not the problem.
The problem is that the boundary between adjacent code values in dark
areas of the picture is more or less visible - or perhaps objectionable -
depending which of these schemes is used! In decent viewing conditions,
with a video camera or a decent scanner, the images will _not_ appear
exactly the same. In case 1, the dark shades will exhibit banding, and
case 3 they will not.
In case 1 (Figure 6.8, second row, CG), the rounding of intensities in the
dark shades to the same code value - by conversion of the intensity value
to 8 bits - erases distinctions between dark shades (intensities) that I
can see - that my vision can easily distinguish.
If you have seen banding in a (supposedly) continuous-tone image, at 24
bits per pixel, then you can be fairly certain that the image data has
been subjected to 8-bit linear-light coding (or poorly-chosen nonlinear
coding) someplace along the path from creation, capture, processing,
recording, placing in a framebuffer, running a lookup table, converting to
analog, and displaying.
--
Timo Autiokari
On 25 Feb 1998 23:40:19 GMT, mi...@xhplmmcg.xhpl.xhp.com (Michael McGuire) wrote:
>A/D convertors in cameras and scanners are in fact external to the CCD's--not
>on the same chip, and thus accessible to adjustment by other than the maker of
>the CCD. Further there is no requirement that an A/D converter have an output
>linearly proportional to its input voltage. An implementation of an A/D
>consists of N voltage comparators and a voltage divider comprised of a string
>of N resistors fed by a stabilized voltage. Each node of the divider is
>connected to the plus input of a voltage comparator. The minus input of each of
>the comparators is connected to the input signal. The output is the number of
>comparators that are turned on by the input signal, that is all those whose
>voltage divider (plus) input is less than the input signal. If the resistor
>values in the divider are all equal then you get linear output. But they could
>just as well be a power law sequence for a power law relationship of output to
>input or whatever function you like.
In the article:
http://x4.dejanews.com/getdoc.xp?AN=296741174&CONTEXT=888477590.22675835&hitnum=1
you yourself say:
<< A well set up CCD--probably needs to be cooled--can put out 14 bit data
<< where all the noise intrinsic to the CCD is in the lowest order bit. CCD's
<< respond linearly to light intensity so 14 bits amounts to 14 doublings of
<< light intensity which is to say 14 stops. ...
<< Mike
<< --
<< Michael McGuire Hewlett Packard Laboratories
So please tell me what are you trying to say now ?
In your reply you use the wording "the could just be" in your illusion: "If the
resistor values in the divider are all equal then you get linear output. But
they could just as well be a power law sequence for a power law relationship of
output to input or whatever function you like."
There are two good reasons why the coding is linear in AD converter:
(1) It would be rather foolish for the manufacturer of the converter to create
an AD converter that is very accurate in design, so that it can detect a very
small change on the other end of the range and then make the rest of the range
to be much more loose.
(2) There are technical problems in the production of AD converters. It is
relatively easy to produce 8 resistors onto the chip that all have accurately
the same value. In an AD ladder it does not matter what the exact value of the
resistors is, only the value needs to be the same for each resistor in the
ladder. This is the very basic issue that the production of AD converters relies
on. It becomes increasingly difficult to produce more than 8 resistors that have
exactly the same value. This is the reason why higher bit-dept converters are
more expensive and that there are not many over 16 bit accuracy. On the other
hand producing 8 resistors that each have different value, but so that these
values accurately follows some non-linear function would be *very* difficult.
There is no technology in the horizon that could make this approach feasible, it
is much easier to produce larger amount of linearly coded bits than the
equivalent amount of non-linearly compressed bits.
Below are links to AD converter pages of Analog Devices Inc., National
Semiconductor Corp., and Fujitsu Microelectronic Inc., if anyone is interested,
please see the specifications. All the converters are linear. It is easiest to
just locate the *nonlinearity error* or the *linearity* spec.
http://products.analog.com/products/list_generics.asp?category=86
http://products.analog.com/products/list_generics.asp?category=85
http://www.national.com/catalog/AnalogDataAcquisition.html
http://fujitsumicro.com/products/analog/data.html
If anyone knows on-line specifications of CCD devices could you please post the
links.
And yes, there are both internal and external AD conversions with CCD imaging
systems. It does not change the fact that the conversion happens linearly,
inside an integrated circuit where the coding is in the from of physical
resistor elements on the chip.
It is also possible (unlikely but possible) that in some very specific camera
there is an non-liner analog signal conditioning between the CCD output and the
AD converter input. If Mr. Poynton's pages are targeted for such exotic systems
then in my opinion it would be nice if he could say indicate this on these
pages.
Timo Autiokari
Charles Poynton
In article <34f637f...@news.clinet.fi>, tim...@clinet.fi (Timo
Autiokari) wrote:
> It is also possible (unlikely but possible) that in some very specific camera
> there is an non-linear analog signal conditioning between the CCD output
> and the AD converter input. If Mr. Poynton's pages are targeted for such
> exotic systems then in my opinion it would be nice if he could say indicate
> this on these pages.
Could I ask for a few volunteers in s.e.t.a or s.e.t.b to explain to Mr.
Autiokari that this not only possible and likely, and not just in "exotic"
systems, but in ***ALL*** consumer and most professional video cameras
work? I have tried repeatedly in private e-mail and public posts to
explain, to no avail. A short note like this ought to suffice:
Dear Mr. Autiokari,
In every consumer video camera, and most professional video cameras, there
is nonlinear analog signal conditioning between the CCD output and the AD
converter input.
Certain high-end professional studio video cameras have no nonlinear
analog signal conditioning between the CCD output and the AD converter
input; in these cameras, the output of the CCD has more than 8 bits, and
nonlinear processing is performed digitally.
Alan Roberts
Charles Poynton (poy...@poynton.com) wrote:
: In article <34f637f...@news.clinet.fi>, tim...@clinet.fi (Timo
: Autiokari) wrote:
:
: > It is also possible (unlikely but possible) that in some very specific camera
: > there is an non-linear analog signal conditioning between the CCD output
: > and the AD converter input. If Mr. Poynton's pages are targeted for such
: > exotic systems then in my opinion it would be nice if he could say indicate
: > this on these pages.
:
: Could I ask for a few volunteers in s.e.t.a or s.e.t.b to explain to Mr.
: Autiokari that this not only possible and likely, and not just in "exotic"
: systems, but in ***ALL*** consumer and most professional video cameras
: work? I have tried repeatedly in private e-mail and public posts to
: explain, to no avail. A short note like this ought to suffice:
I did that yesterday, recommending him to look at manufacturers data sheets
on real cameras. He hasn't responded to me yet. He should do some reading if
he's going to try to catch up with my 30 years in the camera business, let
alone your experience :-)
--
******* Alan Roberts ******* BBC Research & Development Department *******
* My views, not necessarily Auntie's, but they might be, you never know. *
**************************************************************************
Timo Autiokari
On Thu, 26 Feb 1998 02:04:45 -0500, poy...@poynton.com (Charles Poynton) wrote:
>Today, only a few CCD devices have integral A-to-D converters.
>(Soon, many will, but solutions to the 8-bit linear light problem must
>first be found.)
No, the problem is the monitor gamma. It is not a problem for TV and video
because the images are not edited. But in the case the image is gamma
compensated then the image manipulation software sees the compensated (and
possibly compressed) image. Editing such an un-natural image gives poor quality.
E.g. Photoshop makes it easy to edit gamma compensated images (this is why it
has the two gamma settings). It shows the image properly on monitor but the data
is kept in the gamma space and the problem is that image editing is done to the
gamma compensated image data. So the Photoshop effectively hides the appearance
of the actual compensated data. To get a feeling what the compensated data
actually looks like just apply a gamma say 1/2.0 to an decent image.
>Video cameras invariably take the second approach,
My concern is not the video, it is digital photographic imaging.
>A very low-end, cheap scanner might get away with no analog processing, a
>CCD and an 8-bit ADC. (I'm not certain whether this is done even in in
>cheap commercial units, I've never taken a really cheap unit apart. ...)
So, you are not certain. Most of the 'really cheap' digital cameras have the 8
bit/color CCD. see e.g. http://plugin.com/dcg2.html . Are you similarly
un-certain about them too ? There really is no non-linear analog amplifiers in
them. Non-linear analog amplifiers are expensive and they eat lot of current
(such amplifiers need to have stable ambient temperature in order to be
accurate, so small miniature ovens are usually used to achieve this, similar
ovens are used for accurate frequency generation in counters and function
generators to keep the crystal stable).
>Timo suggested (in private e-mail) that the images in the following three
>cases would appear exactly identical, assuming 8 bits per color component:
No I did not say that. In my private e-mail I said:
"In the below three cases the image will appear *exactly* the same. There will
be no differences at all (still considering the 8bit/color CCD):"
What a master you are in the art of twisting words. You then go on and say:
>In decent viewing conditions, with a video camera or a decent
>scanner, the images will _not_ appear exactly the same. In
>case 1, the dark shades will exhibit banding, and case 3 they
>will not.
So here you change the "8bit/color CCD" into "a video camera or a decent
scanner". Again my concern is not the video. My concern are cameras and
scanners in the area of digital photographic imaging. Maybe to you a "decent
scanner" is only a 12 bit/color scanner, many people do have 'cheap' 8 bit/color
scanners.
May I please suggest to you that you place a warning on your FAQ pages like:
Not suitable for really cheap 8bit/color digital cameras nor for scanners that
are not decent. Or simply: Suitable only for video cameras and only displaying
images.
Timo Autiokari
Timo Autiokari
On Thu, 26 Feb 1998 09:22:56 -0500, poy...@poynton.com (Charles Poynton) wrote:
>In every consumer video camera, and most professional video cameras, there
>is nonlinear analog signal conditioning between the CCD output and the AD
>converter input.
Really, I'm not concerned about the video. I've actually said that for video the
gamma compensation is good. You seem to have trouble with handling the digital
photographic imaging and image editing aspect, as you avoid to mention about
them in your on-line documents also.
The problem with your FAQs are that they are being applied to digital
photographic imaging and there the tool is digital camera. It is different form
the video cameras.
Here is a simple way to see if there is a non-linear analog signal conditioning
in the camera:
-open any decently exposed image into Photoshop that is acquired using a
8bit/color CCD device using a gamma setting other than 1.0 in the acquire module
(in case there is such setting) .
-choose Image/Histogram.
-in Photoshop the Luminosity -channel is smoothed so select the red, green or
blue from the dropdow-box.
Now, if you do not see gaps in the histogram then there is the non-linear analog
signal conditioning in the camera or the scanner. But if you do see the gaps
then the data is just modified by the software of the camera or scanner. To
verify, open a couple of other images and do the same to see that the gaps
appear generally in the same places.
Timo Autiokari
Keith Jack
Timo Autiokari wrote in message <34f637f...@news.clinet.fi>...
>On 25 Feb 1998 23:40:19 GMT, mi...@xhplmmcg.xhpl.xhp.com (Michael McGuire)
wrote:
>
[edit]
>(2) There are technical problems in the production of AD converters. It is
>relatively easy to produce 8 resistors onto the chip that all have
accurately
>the same value. In an AD ladder it does not matter what the exact value of
the
>resistors is, only the value needs to be the same for each resistor in the
>ladder. This is the very basic issue that the production of AD converters
relies
>on. It becomes increasingly difficult to produce more than 8 resistors that
have
>exactly the same value. This is the reason why higher bit-dept converters
are
>more expensive and that there are not many over 16 bit accuracy. On the
other
>hand producing 8 resistors that each have different value, but so that
these
>values accurately follows some non-linear function would be *very*
difficult.
>There is no technology in the horizon that could make this approach
feasible, it
>is much easier to produce larger amount of linearly coded bits than the
>equivalent amount of non-linearly compressed bits.
Actually, at the last company I worked at, we developed an ADC that
was 8-bit flash (255 resistors), and designed to perform a nonlinear
function. Was just as simple to make as a standard ADC (the design
change took less than an hour), and was designed specifically for
interfacing
to CCDs. Amazing what you can do when you have a physicist available --
a simple experiment on his kitchen table proved the concept and the
first
silicon worked great.
Michael McGuire
0]>
: On 25 Feb 1998 23:40:19 GMT, mi...@xhplmmcg.xhpl.xhp.com (Michael McGuire) wrote:
: >A/D convertors in cameras and scanners are in fact external to the CCD's--not
: >on the same chip, and thus accessible to adjustment by other than the maker of
: >the CCD.
.....
: >values in the divider are all equal then you get linear output. But they could
: >just as well be a power law sequence for a power law relationship of output to
: >input or whatever function you like.
: In the article:
: http://x4.dejanews.com/getdoc.xp?AN=296741174&CONTEXT=888477590.22675835&hitnum=1
: you yourself say:
: << A well set up CCD--probably needs to be cooled--can put out 14 bit data
: << where all the noise intrinsic to the CCD is in the lowest order bit. CCD's
: << respond linearly to light intensity so 14 bits amounts to 14 doublings of
: << light intensity which is to say 14 stops. ...
: << Mike
: << --
: << Michael McGuire Hewlett Packard Laboratories
: So please tell me what are you trying to say now ?
------------------------------------------------------------------------------
Good juvenile lawyering, Timo, but irrelevant to the subject at hand. The
question there was about the possible dynamic range of CCD's, and not about the
details of the A/D conversion or output encoding. Expressed in linear bits
that's what can be achieved. I was not anticipating cross examination by you,
or I would have pointed out that the 14 linear bits could be encoded with no
perceptual loss with fewer bits with the appropriate non-linear function, as
all the rest of the world knowledgeable about this subject has been trying to
show you.
--------------------------------------------------------------------------------
: In your reply you use the wording "the could just be" in your illusion: "If the
: resistor values in the divider are all equal then you get linear output. But
: they could just as well be a power law sequence for a power law relationship of
: output to input or whatever function you like."
: There are two good reasons why the coding is linear in AD converter:
: (1) It would be rather foolish for the manufacturer of the converter to create
: an AD converter that is very accurate in design, so that it can detect a very
: small change on the other end of the range and then make the rest of the range
: to be much more loose.
--------------------------------------------------------------------------------
But, stripping away your pejorative verbiage, this is exactly the description
of a possible non-linear A/D, small steps at one end of the scale and
progressively larger to the other end. Obviously such an A/D would be designed
for a particular purpose and not offered for general purpose uses. An
alternative general purpose possibility would be a programmable A/D.
--------------------------------------------------------------------------------
: (2) There are technical problems in the production of AD converters. It is
: relatively easy to produce 8 resistors onto the chip that all have accurately
: the same value. In an AD ladder it does not matter what the exact value of the
: resistors is, only the value needs to be the same for each resistor in the
: ladder. This is the very basic issue that the production of AD converters relies
: on. It becomes increasingly difficult to produce more than 8 resistors that have
: exactly the same value. This is the reason why higher bit-dept converters are
: more expensive and that there are not many over 16 bit accuracy. On the other
: hand producing 8 resistors that each have different value, but so that these
: values accurately follows some non-linear function would be *very* difficult.
: There is no technology in the horizon that could make this approach feasible, it
: is much easier to produce larger amount of linearly coded bits than the
: equivalent amount of non-linearly compressed bits.
-----------------------------------------------------------------------------
But as other posters to this thread have remarked, non-linear A/D's have been
made for and used in high end video cameras. You have conceded below my first
point of my original post, that in cameras and scanners, A/D's are not usually
combined with CCD's--apparently there are a few counter examples. Here we see
my other point that they need not have a linear output. But in the overall
context of this thread, it really doesn't matter. If a linear A/D has
sufficient dynamic range and a step size less than the noise level of the CCD,
then the transformation to non-linear encoding can be done digitally with
no perceptual loss.
------------------------------------------------------------------------------
: Below are links to AD converter pages of Analog Devices Inc., National
: Semiconductor Corp., and Fujitsu Microelectronic Inc., if anyone is interested,
: please see the specifications. All the converters are linear. It is easiest to
: just locate the *nonlinearity error* or the *linearity* spec.
: http://products.analog.com/products/list_generics.asp?category=86
: http://products.analog.com/products/list_generics.asp?category=85
: http://www.national.com/catalog/AnalogDataAcquisition.html
: http://fujitsumicro.com/products/analog/data.html
------------------------------------------------------------------------------
It is completely unsurprising and irrelevant to this discussion that general
purpose A/D's are linear.
------------------------------------------------------------------------------
: If anyone knows on-line specifications of CCD devices could you please post the
: links.
-------------------------------------------------------------------------------
Try digging in the web pages for Sony, Toshiba, or Philips. They all make them.
-------------------------------------------------------------------------------
: And yes, there are both internal and external AD conversions with CCD imaging
: systems. It does not change the fact that the conversion happens linearly,
: inside an integrated circuit where the coding is in the from of physical
: resistor elements on the chip.
----------------------------------------------------------------------------
Irrelevant. I can write programs for a digital computer that computes values
of both linear and non-linear functions. Are the bits that represent these
values linear or non-linear? Non-linear of course, they are either on or off.
Depending on the resistor values, I can go A/D linearly or non-linearly, but
of course the resistors are linear--so what?
-----------------------------------------------------------------------------
: It is also possible (unlikely but possible) that in some very specific camera
: there is an non-liner analog signal conditioning between the CCD output and the
: AD converter input. If Mr. Poynton's pages are targeted for such exotic systems
: then in my opinion it would be nice if he could say indicate this on these
: pages.
------------------------------------------------------------------------------
No, the understanding that everyone but you seems to have, is that encoding of
images for the same perceptual quality--minimization of contouring etc--can be
achieved with noticeably fewer bits with the right non-linear encoding, than
with linear.
------------------------------------------------------------------------------
Mike
--
Michael McGuire Hewlett Packard Laboratories
Alan Roberts
Timo Autiokari (tim...@clinet.fi) wrote:
: It is also possible (unlikely but possible) that in some very specific camera
: there is an non-liner analog signal conditioning between the CCD output and the
: AD converter input. If Mr. Poynton's pages are targeted for such exotic systems
: then in my opinion it would be nice if he could say indicate this on these
: pages.
You should check your facts more carefully. Most TV cameras sold to
broadcasters have a non-linear circuit between the ccd and the ADC. It
isn't a standard gamma circuit, that's done in the digits, the "pre-gamma"
circuit is used to compress the video signal in a known way, including
knee function to handle high overloads. The precise nature of this curve
is used in the digital processing to notionally recover the linear signal
before applying the required non-linear processing. Cameras are really a
lot more complex than you seem to imagine. Have you actually looked at
one? I've been doing exactly that for 30 years now, and even some domestic
camcorders are processed in this way, hardly exotic.
Timo Autiokari
On 27 Feb 1998 02:58:02 GMT, mi...@xhplmmcg.xhpl.xhp.com (Michael McGuire) wrote:
>Good juvenile lawyering, Timo, but irrelevant to the subject at hand.
In the article:
<<>http://x4.dejanews.com/getdoc.xp?AN=296741174&CONTEXT=888477590.22675835&hitnum=1
<<>: you said:
"CCD's respond linearly to light intensity"
That is not irrelevant to this subject.
>I was not anticipating cross examination by you, or I would have pointed
>out that the 14 linear bits could be encoded with no perceptual loss with
>fewer bits with the appropriate non-linear function, as all the rest of the
>world knowledgeable about this subject has been trying to show you.
I have never said that it couldn't be done. I *have* said that it can be done.
And I have said it is good for TV and video.
Non-linear functions in general have the ability to compress data. This is
rather basic knowledge and has been widely used e.g. in audio.
What I have been saying is that even if it is good in TV and video to encode the
gamma correction and bith-depth compression into the video signal, it is not at
all good for digital photographic imaging.
It should not be difficult to see that when the acquire device does the gamma
compensation then the image editor will see the compensated and compressed
"signal" that we, in digital photographic imaging, call as the _image_.
In TV and video the signal (image) is suitable for the eye only after the
monitor does the gamma.
Now, Mr. Poynton's humbug about the perception mis-leads people in digital
photographic imaging, badly.
The gamma compensation in TV and video cameras is done because the effective
gamma from the scene to the eye needs to be linear (or very near to linear).
In TV this has been done from the early days of television in the TV camera. It
was so chosen in the beginning, so that a difficult and expensive pure analog
non-linear correction was not needed in the receivers.
There is no problem in this since no-one is looking the signal itself. They look
the picture on the TV and the CRT first apples the gamma. Only after that the
signal is proper, natural, for the eye again.
So, all the TV sets have the gamma therefore the gamma compensation must be done
today and in the future also, in the TV and video camera (not necessarily in the
camera but before sending the signal)
Now, technology has advanced so that the gamma of the television can be actually
useful. Best broadcast quality TV cameras can now acquire 10 bit or 12 bit data
and because of the non-linearly of the monitor we get an other benefit for free,
bit-depth compression.
The problem is that Mr. Poynton says that the "non-linear coding" is done
*because* it gives better image quality. This is not true, this way.
The better perception is there for TV and video, but it is the *consequence* of
the improved technology. And for TV and video it is a free benefit, due to the
unavoidable gamma correction that must be done anyway.
The benefit however is only there for TV and video, since there it is the
question of transmission only. The transmitted signal is gamma compensated and
it can be bit-depth compressed.
If the TV signal that is on the transmission path is converted into image
without applying the same gamma that the CRT applies, the image can not be
edited properly. Because it is heavily un-natural form. But this is what Mr
Poynton is suggesting to every one. And it mis-leads people in digital
photographic imaging badly.
Some image manipulation software like Photoshop makes it possible to edit such
gamma compensated image. This is why it has the two gamma settings. It shows the
image properly but the image data is still in gamma space and the image
manipulation operations are done to the data. To see how the data looks like,
just apply a gamma 1/2.0 to a image that show properly. Then think what the
various image editing operations might do with that.
>But, stripping away your pejorative verbiage
>But as other posters to this thread have remarked, non-linear A/D's have been
>made for and used in high end video cameras.
But, but, but, maybe he is kind enough to reveal the type code of this device. I
work as a component specialist for 12th year now and have not seen any info
about non-linear AD converters. I'm quite surprised that Mr. Michael McGuire
from Hewlett-Packard Laboratories believes that such thing exists.
>If a linear A/D has sufficient dynamic range and a step size less than
>the noise level of the CCD, then the transformation to non-linear encoding
>can be done digitally with no perceptual loss.
The above is only partially true. You need also to say *where* the signal (or
data) is perceived. If it is perceived on TV or on uncalibrated monitor then
this is so because the CRT makes the image proper again by applying the gamma.
And again, the image editor perceives the signal (data) before the monitor and
some rather heavy perceivable problems exists there.
Timo Autiokari
Timo Autiokari
On 27 Feb 1998 09:26:29 GMT, al...@rd.bbc.co.uk (Alan Roberts) wrote:
>You should check your facts more carefully. Most TV cameras sold to
>broadcasters have a non-linear circuit between the ccd and the ADC.
Firstly, TV and Video cameras necessarily need *not* an ADC (Analog Digital
Converter) at all. You should know considering your 30 year experience. There
was no ADCs 30 year ago, but there was the television.
TV and video cameras supply the video signal, this signal is analog, not a
digital one. However all modern TV and video cameras do have ADC even if it is
not obvious.
>It isn't a standard gamma circuit, that's done in the digits, the "pre-gamma"
>circuit is used to compress the video signal in a known way, including
>knee function to handle high overloads.
It was Mr Poynton that insists that there is an analog non-linear conversion
between CCD and ADC and the context was that it would do the gamma compensation.
Now such an *analog* amplifier that does the *gamma* compensation is very
difficult one. If there is such devices in cameras today, they are rare.
I do know that there is pre-conditioning circuits. They are piecewise linear
*not* non-linear. Then there are signal processors for this purpose and even if
they seem to be analog devices (so that they have analog input for the CCD and
analog video outputs) the actually have a flash ADC inside them. Because of the
speed that is needed with video they are often only 6 bit or 8 bit devices.
There are fast 10 bit and 12 bit flashs but they are very expensive. Such are
only used in broadcast quality systems. Then there are other rather genius
methods in achieving the "no missing codes" that is often seen on the specs.
Do you know what happens to the CCD signal (information) when it goes through
such analog-digital-analog device (or a chain of devices). Have you ever seen a
spec of those devices? Their overall error is around 1% for a high quality
device. For lower grade devices the error is usually expressed (for some
reasons) in decibels and a value of 1 dB seems to be quite common. That
translates to 10% overall error.
1% is equal to 2.5 levels in linearly coded light and 10% is equal to 25
levels. Now compare: Mr Poynton is worried about the "perception" issues below
1 level (0.4%) of linearly coded light. While this small portion of his signal
path generates in a broadcast quality system 1% and in a consumer grade system
some 10 % errors.
>The precise nature of this curve is used in the digital processing to notionally
>recover the linear signal before applying the required non-linear processing.
Yes. As I explained above. And it generates large errors.
>Cameras are really a lot more complex than you seem to imagine. Have you
>actually looked at one? I've been doing exactly that for 30 years now, and
>even some domestic camcorders are processed in this way, hardly exotic.
Again: an *analog* amplifier that does the *gamma* compensation is very
difficult one. If there is such devices they are rare. That was the context by
Mr. Poynton.
Congratulations, you have got one thing right: as you say "even some domestic
camcorders are processed in this way"
_Some_ domestic camcorders do. Some other do not.
But Mr Poynton says his pages are suitable for anyone.
And the issue is not about TV or video cameras. My question was if Mr. Poynton's
pages are applicable to digital photographic imaging, where we use digital
cameras. There are such things also, even if they are cameras, they are not TV
nor video cameras.
And the digital cameras do not have video circuitry inside them. If they provide
gamma compensated images then it is done by software. If the ADC in the camera
is 8bit then nothing is gained only the images are damaged.
In my other message in this thread there is a simple procedure to see if the
camera provides such gamma compensated images, if anyone is interested to
experiment a bit.
Timo Autiokari
Ed Ellers
Alan Roberts wrote:
"Cameras are really a lot more complex than you seem to imagine."
At least they are if they're any good. :-)
Stephen H. Westin
tim...@clinet.fi (Timo Autiokari) writes:
<snip>
> Again: an *analog* amplifier that does the *gamma* compensation is very
> difficult one. If there is such devices they are rare. That was the context by
> Mr. Poynton.
Well, thirty years ago, these amplifiers were ubiquitous; do you
really think that broadcast TV cameras have been digital since the
'40s? After all, they all have built-in gamma correction.
<snip>
> And the digital cameras do not have video circuitry inside them. If they provide
> gamma compensated images then it is done by software. If the ADC in the camera
> is 8bit then nothing is gained only the images are damaged.
You keep saying this; where dod you find this out?
<snip>
I still find it astonishing that you keep on arguing with people of
the stature of Mr. Roberts and Mr. Poynton, with no authoritative
references to back you up. Roberts and Poynton can serve as their
*own* authoritative references.
JG Smith
tim...@clinet.fi (Timo Autiokari) wrote:
>Dear Mr. Poynton,
>
>What is the purpose of your GammaFAQ? ... ... etc., etc.
>
This is a very amusing thread. Judging from the commentary, apparently
neither side understands what the other is talking about.
Rather typical of "appointed" authority (as compared to "recognized"
authority) in the academic field. Leaves one to wonder if either knows
what he's talking about ... hmmm!
Alan Roberts
Timo Autiokari (tim...@clinet.fi) wrote:
: On 27 Feb 1998 09:26:29 GMT, al...@rd.bbc.co.uk (Alan Roberts) wrote:
:
: >You should check your facts more carefully. Most TV cameras sold to
: >broadcasters have a non-linear circuit between the ccd and the ADC.
:
: Firstly, TV and Video cameras necessarily need *not* an ADC (Analog Digital
: Converter) at all. You should know considering your 30 year experience. There
: was no ADCs 30 year ago, but there was the television.
:
: TV and video cameras supply the video signal, this signal is analog, not a
: digital one. However all modern TV and video cameras do have ADC even if it is
: not obvious.
Oh, dear, I say to you again, check your facts. You are wrong. Quite wrong.
This is getting rather tedious, do you ever listen to anyone?
TV cameras come in all varieties, as I have been trying to tell you, you
cannot make blanket statements about them. They start with vhs camcorders
and end at studio cameras, each of which may be analogue or digital at any
stage of the processing. The latest breed of DOMESTIC cameras are digital,
but they still have analogue preprocessing so that the digital processing
is easier. If you can't understand that, I suggest you go away and read
the manuals on some of them. That's how I got to understand it, why can't you?
: It was Mr Poynton that insists that there is an analog non-linear conversion
: between CCD and ADC and the context was that it would do the gamma compensation.
:
: Now such an *analog* amplifier that does the *gamma* compensation is very
: difficult one. If there is such devices in cameras today, they are rare.
Nonsense, it's very easy. EVERY tv camera has one. Why not check your facts,
is that so hard to do.?
: I do know that there is pre-conditioning circuits. They are piecewise linear
: *not* non-linear.
Again, nonsense, they are not piecewise linear. Check the circuit diagrams.
: Then there are signal processors for this purpose and even if
: they seem to be analog devices (so that they have analog input for the CCD and
: analog video outputs) the actually have a flash ADC inside them. Because of the
: speed that is needed with video they are often only 6 bit or 8 bit devices.
: There are fast 10 bit and 12 bit flashs but they are very expensive. Such are
: only used in broadcast quality systems. Then there are other rather genius
: methods in achieving the "no missing codes" that is often seen on the specs.
Not so. Check your facts. There are truly analogue circuits that do this,
and they are in common usage in tv cameras. Your depth of lisunderstanding
is breathtaking.
: >Cameras are really a lot more complex than you seem to imagine. Have you
: >actually looked at one? I've been doing exactly that for 30 years now, and
: >even some domestic camcorders are processed in this way, hardly exotic.
:
: Again: an *analog* amplifier that does the *gamma* compensation is very
: difficult one. If there is such devices they are rare. That was the context by
: Mr. Poynton.
Nonsense, when will you actually try to find out the truth by actually
looking at real cameras instead of inventing difficulties?
Alan Roberts
Stephen H. Westin (westin*nos...@graphics.cornell.edu) wrote:
: I still find it astonishing that you keep on arguing with people of
: the stature of Mr. Roberts and Mr. Poynton, with no authoritative
: references to back you up. Roberts and Poynton can serve as their
: *own* authoritative references.
Thanks Stephen, it makes a change to be recognised.
I intend to retreat from this thread now, clearly Timo has made his mind up
how the universe works, and will not be dissuaded.
Walter Hafner
tim...@clinet.fi (Timo Autiokari) writes:
> If anyone knows on-line specifications of CCD devices could you please post the
> links.
Sure. Have a look at:
I think its a very good page! CCD technology explained to the max.
Under http://www.photomet.com/ref/refgain.html the "gain" is described:
Photometrics cameras are indeed linear (this is an exception) by
default but can be changed up to 4 in hardware (at least that's my
interpretation of the page)
The term "linearity" (http://www.photomet.com/ref/reflin.html) on the
photometrics pages refer to a different concept. I qoute:
: Hence, non-linearity is a measure of
: the deviation from the following relationship:
: Digital Signal = Constant x Amount of Incident Light
-Walter
--
Walter Hafner_____________________________ haf...@forwiss.tu-muenchen.de
<A href=http://www.forwiss.tu-muenchen.de/~hafner/>*CLICK*</A>
The best observation I can make is that the BSD Daemon logo is _much_
cooler than that Penguin :-) (Donald Whiteside)
Dave Martindale
tim...@clinet.fi (Timo Autiokari) writes:
>It is also possible (unlikely but possible) that in some very specific camera
>there is an non-liner analog signal conditioning between the CCD output and the
>AD converter input. If Mr. Poynton's pages are targeted for such exotic systems
>then in my opinion it would be nice if he could say indicate this on these
>pages.
Is a frame grabber digitizing the output of an analog video camera an
"exotic system"? The video camera is required to apply a non-linear
transform between the output of the CCD and the video voltage representing
light intensity, since this non-linear transformation is part of the
specification of the video signal for both NTSC and PAL. (The transform
is called "gamma correction"). The frame grabber simply does A/D conversion
on the video signal, so the non-linear transfer characteristic is built
into the digital sample values as well.
In addition, flatbed scanners and at least some digital cameras perform
their own sort of gamma correction on the voltages coming from the
CCD. They have to, or the images would look poor on most PCs.
>And yes, there are both internal and external AD conversions with CCD imaging
>systems. It does not change the fact that the conversion happens linearly,
>inside an integrated circuit where the coding is in the from of physical
>resistor elements on the chip.
But how the A/D conversion itself is performed is essentially irrelevant.
What you *care* about is how the digital sample values are related to
the original light intensity seen by the CCD. And it is a simple,
measurable fact of life that many image sensors have some sort of
nonlinear transform built in. In some cases, it is a nonlinear analog
circuit between the CCD and the A/D. In other cases, the A/D is directly
digitizing the CCD output to 12 or 14 bits, but this data is passed through
a lookup table to produce 8 or 10 bits of data to be stored in the image
file. The nonlinear transfer function is performed by the lookup table.
Timo, I'll happily believe that if you build a CCD camera the output
will be linearly proportional to intensity. But that just isn't the way
most real cameras and scanners are built, for a variety of good reasons.
Dave
Dave Martindale
tim...@clinet.fi (Timo Autiokari) writes:
>Here is a simple way to see if there is a non-linear analog signal conditioning
>in the camera:
>
>-open any decently exposed image into Photoshop that is acquired using a
>8bit/color CCD device using a gamma setting other than 1.0 in the acquire module
>(in case there is such setting) .
>-choose Image/Histogram.
>-in Photoshop the Luminosity -channel is smoothed so select the red, green or
>blue from the dropdow-box.
>
>Now, if you do not see gaps in the histogram then there is the non-linear analog
>signal conditioning in the camera or the scanner. But if you do see the gaps
>then the data is just modified by the software of the camera or scanner. To
>verify, open a couple of other images and do the same to see that the gaps
>appear generally in the same places.
This test is useless. It will tell you if an image has been modified
by a particular sort of lookup table that is altering the gamma by mapping
8-bit input values to 8-bit output values. But if the processing has
been done with samples that are wider than this, you won't see the
discontinuities in the histogram.
For example, I have worked with a particular image in the following
sequence:
- the image was photographed on film, with a gamma of 0.6 relative to
the original scene.
- the film was digitized with a CCD and 14 bit A/D convertor
- the output of the A/D convertor was run through a lookup table to
produce a 10-bit value that is proportional to film density, then
these values are stored in image file #1. (This is a nonlinear
transform)
- this file was read in by another program which converted from negative
density space back into an approximation of the light intensity in the
original scene. This new image was written with 16 bits per sample.
(Another nonlinear transform)
- File #2 was read into Photoshop in 16-bit mode. A gamma correction
factor was applied using the Levels control panel. In addition,
a small fraction of the 16-bit intensity space is expanded to fill
the whole range (equivalent to about a 3 f-stop exposure change).
- finally, the image was converted to 8 bits per sample and written out
again.
If you were to look at a histogram of the 10-bit data or the 16-bit
data with a tool that showed you all 1024 or 65536 "bins", you would
see discontinuities caused by using lookup tables for the nonlinear
transforms. But viewed in Photoshop, with its histograms that always
have 256 bins, you won't see anything. And the final resulting image,
despite having undergone *three* nonlinear transforms, has a nice
smooth histogram. This is because the nonlinear operations were done
with enough bits that the quantization errors remain well below the
size of the errors caused by the 8-bit output format itself.
In fact, I'd argue that *all* image processing should be done with
more than 8 bits, using 8 bits per sample only for storing final images
that will not undergo further processing.
Dave
Dave Martindale
tim...@clinet.fi (Timo Autiokari) writes:
>Non-linear functions in general have the ability to compress data. This is
>rather basic knowledge and has been widely used e.g. in audio.
>
>What I have been saying is that even if it is good in TV and video to encode the
>gamma correction and bith-depth compression into the video signal, it is not at
>all good for digital photographic imaging.
Gamma correction in video turns out to be good for two things in video.
The first one, as you note, is to compensate for the nonlinearity of the
CRT. This compensation has to be done somewhere, and doing it in the
camera (of which there are few) is cheaper than doing it in the TV set
(of which there are many).
But it *also* turns out that gamma correction is closely related to the
nonlinear way that the human visual system perceives intensity. Gamma
correction causes more of the video voltage range (or code space, in
digital video) to be used for the darker portions of the picture and
less for the brighter parts of the picture, which matches the eye's
response. The net effect is that we can *usually* represent an image
without banding artifacts (Mach bands) using only 8 bits per sample
using gamma correction, while if the samples were linearly proportional
to intensity we'd need about 12 bits for the same performance.
And this second advantage has nothing at all to do with video, or with
the ultimate display device. It simply says that nonlinear representations
need fewer bits for the same quality, or give better quality with the same
number of bits, than a linear representation.
In addition, there is nothing wrong with this. Nowhere is there a stone
tablet with a commandment saying that the digital sample values in an image
file should be linearly related to intensity. And, in fact, few real
images really use a linear encoding for storing pixels. I do it sometimes,
but I'm usually careful to use about 16 bits per sample to avoid creating
artifacts. Sometimes I use 32-bit floating point for sample values.
It works great! But it's not efficient for storage.
>Now, Mr. Poynton's humbug about the perception mis-leads people in digital
>photographic imaging, badly.
No he doesn't. The *only* place where it is common for the sample values
stored in image files to be linearly proportional to intensity is in
computer graphics. And that's mostly because the graphics people don't
know any better.
>The problem is that Mr. Poynton says that the "non-linear coding" is done
>*because* it gives better image quality. This is not true, this way.
No, it *is* true. See above.
>If the TV signal that is on the transmission path is converted into image
>without applying the same gamma that the CRT applies, the image can not be
>edited properly. Because it is heavily un-natural form.
What do you mean by "cannot be edited properly", and "un-natural"?
Do you mean that pixel values are no longer linearly related to light
intensity, and thus standard linear image processing operations like
blur, sharpen, and resize will not give the correct answer when applied
to these nonlinear images? If so, you're right.
There are two approaches for dealing with this. One is to realize that
the values in the file are just numbers, not light intensity, and you
can convert the numbers to (linear) intensity any time you want. So you
can take your 8-bit gamma-corrected image, convert it to a linear form
(remember to use at least 12 bits to avoid artifacts), do your image
processing operation, then re-convert the pixel values to gamma-corrected
8 bit samples. This adds only a little bit of roundoff error, and the
whole process has a lot less error than you would get by converting the
image to 8 bit linear and working with it in that space. (Though using
12 bit linear everywhere would be slightly better yet).
The other approach is to apply linear image processing operations to
the 8-bit pixels even though it is mathematically incorrect. This is
commonly done in video. When the process is done within a control
loop, with a human operator looking at the result and modifying the
parameters until they see what they like, this works quite well.
It doesn't matter much if the math is wrong, since nobody is making
measurements from the images - they just want something that looks good.
>Some image manipulation software like Photoshop makes it possible to edit such
>gamma compensated image. This is why it has the two gamma settings. It shows the
>image properly but the image data is still in gamma space and the image
>manipulation operations are done to the data.
Yes, this is approach #2 above.
>To see how the data looks like,
>just apply a gamma 1/2.0 to a image that show properly. Then think what the
>various image editing operations might do with that.
No, the data is intended to be viewed just as you see it. When you apply
the 0.5 gamma to the image, *you* are distorting the image to something
that it is not intended to be.
Timo, how old are you? Forgive me if this doesn't apply to you, but
you sound exactly like an undergrad who has always assumed that sample
values in images files should be linearly related to intensity because
that's the mathematically *right* way to do it. And you defend with
great vigour the purity of your conception of how images *ought* to
be stored in data files.
Sadly, it just isn't this simple. If you used floating-point for
storage and computation, a linear representation would be natural.
But real physical devices have to be built with no more bits than
necessary in order to keep the costs down, and nonlinear encoding
of pixels provides a way to maintain quality with fewer bits.
There are good reasons for this, mostly because our vision is
itself nonlinear. As you learn about perception and image processing,
you will see that there is no perfect way to do something. There are
just a bunch of tradeoffs, and all you can do is make intelligent ones.
In some cases, we are stuck with tradeoffs that made sense at the time
but no longer do. (e.g. using 20% of the video bandwidth to transmit
non-image information). In other cases, the original design was done
by people who really did understand the tradeoffs, and they are still
valid today.
Dave
Michael McGuire
...... snipping Timo's usual hymn to linearity
: >But, stripping away your pejorative verbiage
: >But as other posters to this thread have remarked, non-linear A/D's have been
: >made for and used in high end video cameras.
: But, but, but, maybe he is kind enough to reveal the type code of this device. I
: work as a component specialist for 12th year now and have not seen any info
: about non-linear AD converters. I'm quite surprised that Mr. Michael McGuire
: from Hewlett-Packard Laboratories believes that such thing exists.
------------------------------------------------------------------------------
12 years a component specialist and never heard of an ASIC. That's an
Application Specific Integrated Circuit to enlighten him. One of the
competive advantages we have here is the ability to make our chips. We
generally do not make them available or publish their specifications.
------------------------------------------------------------------------------
: >If a linear A/D has sufficient dynamic range and a step size less than
: >the noise level of the CCD, then the transformation to non-linear encoding
: >can be done digitally with no perceptual loss.
: The above is only partially true. You need also to say *where* the signal (or
: data) is perceived. If it is perceived on TV or on uncalibrated monitor then
: this is so because the CRT makes the image proper again by applying the gamma.
>>>: And again, the image editor perceives the signal (data) before the monitor and
>>>: some rather heavy perceivable problems exists there.
-------------------------------------------------------------------------------
This last sentence is not true if one considers the most likely destinations
of the image or the most likely operations in an image editor that might cause
problems. If the destination is a CRT, the Timo apparently already agrees that
the correction for gamma = 2.2 is correct especially if initially done at
higher bit depth. Now consider taking this to an Mac system where the
gamma = 1.8. The necessary correction from 2.2 -> 1.8 is gamma = 1.1. This is
much gentler and more accurate at 8 bits than banging it all the way from
gamma = 1.0. The other likely destination of the image is a printer. All the
printers I have tested and that's quite a few, at least at their native dot
resolution, have a response similar in shape to a CRT but usually somewhat more
radical. This is a consequence of round dots having to completely cover square
pixels to avoid gaps in solid fills. Put down dots covering 1/4 of the pixels,
but not overlapping, and you get pi/8 coverage of the paper, not 1/4. The
correction curve for a printer lies somewhere above that for a gamma = 2.2
monitor. Again going there from being already accurately corrected for
gamma = 2.2 is going to be gentler and more accurate that going there from 1.0.
This leaves the question what might be done in an image editor keeping in mind
these destinations for the image. Of course anything can be done, but likely
corrections to normal images are linear stretching or compression of the tonal
scale and gamma-curve-like corrections to enhance contrast in the shadows or
highlights. Any of these operations can produce contouring if applied strongly
enough, but they will do it sooner if applied to a gamma 1.0 image which is
then corrected for screen or printer destinations.
I don't expect Timo will agree with much of this any more than he has with
Alan Roberts or Charles Poynton, but then he doesn't agree with anybody.
Valburg
To all the extremely knowledgeable gentlemen weighing in on this thread,
I say thank you! This has been very stimulating and informative.
Could one of you answer a couple of questions which are, perhaps, of
practical interest to those of us just working on still images with
commonly available tools, and not involved in R&D in this field?
The first is a repeat of a question I posed previously: Could you tell
us whether there is a way to determine the native gamma
of a particular scanner model, in order to avoid reducing the 8 bits
worth of information by adjusting the gamma or midpoint to another
setting? Or is this, perhaps, a concern of more significance in theory
than in practice; that is, perhaps the extent of mid-point adjustments
commonly practiced are gentle enough so as to make little difference in
image quality (loss of "bins" through quantization error?)?
Is there commonly available image manipulation software which allows
working at bit-depths greater than 8 and then reducing bit-depth to 8
for output or storage (as mentioned most recently in posts by Dave
Martindale)?
Thanks!
Mitch Valburg
Walter Hafner
Valburg <lk...@psu.edu> writes:
[snip]
> The first is a repeat of a question I posed previously: Could you tell
> us whether there is a way to determine the native gamma
> of a particular scanner model, in order to avoid reducing the 8 bits
[snap]
Related question:
Is the method of finding the gamma-factor of particular displays as
described in
http://www.povray.org/binaries/ (last paragraph)
any good?