Some Elementary Questions About RAW Images & PTgui

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Roger D. Williams

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Jan 27, 2009, 5:46:28 AM1/27/09
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I am rather confused about the way RAW images and the data
that specify how the RAW images should be displayed actually
work.

I know that changes are not made to the RAW image itself,
but to the accompanying data.

What is the relationship between this data and EXIF data?
Or are they two quite different things? I'd like to get
that straight first. I know that EXIF data includes things
like aperture, exposure, lens FL etc., but want to know
if it extends to the way the image is to be displayed.

DXO says that if the EXIF data of an image is changed, it
will refuse to process that image. But it doesn't refuse
to process images that I have tweaked in Silkypix. Is
that because Silkypix stores the changes I make somewhere
else? Or because modifying the way the image is displayed
has nothing to do with EXIF data? DXO refuses to process
the JPEGs or TIFFs I create... although Silkypix will
happily continue to work on them.

Silkypix creates a whole subdirectory of little
files which I assume comprise the instructions I have
given for the images I process. But that's just a
guess. What do I know?

And does PTgui read this data, whether created by DXO or
Silkypix?

Or does PTgui only work on the original unmodified RAW
image, disregarding any modifications that other programs
may have made?

Roger W.

If someone can point me to a book or a series of articles
that walk me through this stuff I would be grateful. I only
need to have it explained. It can't be all that difficult...

--
Work: www.adex-japan.com

Kevin Wilton

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Jan 27, 2009, 8:04:43 AM1/27/09
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Roger

You are quite right about what the Exif information
contains, it's basically the vital statistics of the capture
the moment you pressed the shutter. RAW is something quite
different.

A RAW file as you also say cannot be changed by any process,
it is read only information. When you appear to make changes
to your file you are actually viewing a PREVIEW of what it
WILL look like when you eventually do process it out to be a
JPG, TIFF or whatever. When you make adjustments to the
appearance of a RAW image you are actually only writing a
series of complex instructions.

To best demonstrate this, make some small adjustments to any
RAW file. Now go to the newly created .XML file with the
same name as your original RAW file and open that in Excel.
You can do this by changing the properties so all .XML files
associate with Excel or you can simply open Excel and then
go find the .XML file and open it that way, with File, Open,
Browse. Voila, a long list of instructions.

And to prove you have not altered the read only RAW file in
any way, delete the same test .XML file and open the
original RAW file again. When you do, it will be exactly as
it was before you made any of the test adjustments. Why?
Because you haven't made any, you only think you have
because you can see the preview.

Does this help?

Kevin

Erik Krause

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Jan 27, 2009, 2:19:01 PM1/27/09
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Roger D. Williams wrote:

> I know that changes are not made to the RAW image itself,
> but to the accompanying data.
>
> What is the relationship between this data and EXIF data?

All the EXIF data that can be found in a Jpeg or TIFF after RAW
conversion is already present inside the RAW file - but not in the EXIF
format. Some RAW files even encrypt those data. Thanks to dave coffin
(author of dcraw) most of this RAW formats where hacked and hence are
available outside the proprietary RAW converters.

The so called "sidecar" files are created by RAW converters in order to
store the specific conversion data - basically all settings made in the
RAW converter. This information is stored separately in order not to
change the RAW file at all. dcraw and hence PTGui does neither read nor
write those files (as far as I'm informed). Those files are often in
XML format and hence human readable.

EXIF data is found in a variety of image file formats like JPEG, TIFF or
PNG. It is either written directly by the camera software or by the RAW
converter. What EXIF information is written into a file is not
standardized, you can't rely on it, neither on the values nor on it's
existence. The sidecar data often is written to the RAW conversion
result file (JPEG, TIFF) as well. XMP f.e. is designed to be embedded as
well as written to a stand alone file.

More on EXIF: http://www.sno.phy.queensu.ca/~phil/exiftool/
More on dcraw: http://www.cybercom.net/~dcoffin/dcraw/
More on XMP: http://en.wikipedia.org/wiki/Extensible_Metadata_Platform

--
Erik Krause
http://www.erik-krause.de

Roger D. Williams

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Jan 27, 2009, 7:13:34 PM1/27/09
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Thanks, Kevin, that's very clear, and just about what I
thought. Erik's reply takes it a little further and
answers some important questions. I am most grateful for
the list and its helpful members!

Roger
--
Work: www.adex-japan.com

michael crane

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Jan 27, 2009, 7:18:40 PM1/27/09
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2009/1/28 Roger D. Williams <ro...@adex-japan.com>:

>
> Thanks, Kevin, that's very clear, and just about what I
> thought. Erik's reply takes it a little further and
> answers some important questions. I am most grateful for
> the list and its helpful members!

<dabs tearful eye > go get those photos Roger :-)
mick

Roger D. Williams

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Jan 27, 2009, 7:21:18 PM1/27/09
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On Wed, 28 Jan 2009 04:19:01 +0900, Erik Krause <erik....@gmx.de> wrote:

>
> Roger D. Williams wrote:
>
>> I know that changes are not made to the RAW image itself,
>> but to the accompanying data.
>>
>> What is the relationship between this data and EXIF data?
>
> All the EXIF data that can be found in a Jpeg or TIFF after RAW
> conversion is already present inside the RAW file - but not in the EXIF
> format. Some RAW files even encrypt those data. Thanks to dave coffin
> (author of dcraw) most of this RAW formats where hacked and hence are
> available outside the proprietary RAW converters.
>
> The so called "sidecar" files are created by RAW converters in order to
> store the specific conversion data - basically all settings made in the
> RAW converter. This information is stored separately in order not to
> change the RAW file at all. dcraw and hence PTGui does neither read nor
> write those files (as far as I'm informed). Those files are often in
> XML format and hence human readable.

As Kevin pointed out--mentioning a way to access the data, too.

So PTgui does not read the XML files that contain the display instructions
I added in Silkypix. I thought that was what was happening. The CA that
I had removed is still present in the stitched image. And there is no way
of handling CA within PTgui at the moment.

So the only way I can be sure PTgui will profit from CA correction is by
actually exporting a TIFF file... which is what I was doing before I
started using PTgui's RAW handling function. So it is of little benefit
to me, especially as so many of my shots are full of people moving
around. I will keep it in mind for more static shots with greater
dynamic range. A very useful fall-back but not quite all I was hoping
for...

> EXIF data is found in a variety of image file formats like JPEG, TIFF or
> PNG. It is either written directly by the camera software or by the RAW
> converter. What EXIF information is written into a file is not
> standardized, you can't rely on it, neither on the values nor on it's
> existence. The sidecar data often is written to the RAW conversion
> result file (JPEG, TIFF) as well. XMP f.e. is designed to be embedded as
> well as written to a stand alone file.
>
> More on EXIF: http://www.sno.phy.queensu.ca/~phil/exiftool/
> More on dcraw: http://www.cybercom.net/~dcoffin/dcraw/
> More on XMP: http://en.wikipedia.org/wiki/Extensible_Metadata_Platform

Thanks for these references. I will read them up over the next couple of
days...

Roger W.

--
Work: www.adex-japan.com

Keith Martin

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Jan 28, 2009, 5:37:26 AM1/28/09
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Sometime around 28/1/09 (at 09:21 +0900) Roger D. Williams said:

>PTgui does not read the XML files that contain the display instructions
>I added in Silkypix.

Yep. I know of no RAW processor that reads the settings files created
by other RAW processors. I can see some benefit for those jumping
between multiple tools, but that has to be a vanishingly small
percentage of users.


>the only way I can be sure PTgui will profit from CA correction is by
>actually exporting a TIFF file...

Yep again. As mentioned in a different thread, this feature is
undeniably useful for some, but it isn't intended for everyone.

k

Kevin Wilton

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Jan 29, 2009, 5:09:10 PM1/29/09
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Roger Wrote:

So the only way I can be sure PTgui will profit from CA
correction is by actually exporting a TIFF file... which is
what I was doing before I
started using PTgui's RAW handling function. So it is of
little benefit
to me, especially as so many of my shots are full of people
moving
around. I will keep it in mind for more static shots with
greater
dynamic range. A very useful fall-back but not quite all I
was hoping
for...

And he is right, why would you allow a program to
automatically adjust your RAW files? The adjustments you
make in ACR, Lightroom (my favourite) or whatever are quite
personal and location specific, all of which is information
the new RAW handling facility isn't privy to.

Why would you want to do that?

Kev

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