change resolution
No One
I scan images in at 300dpi, some of which I want to downsample to 72dpi
for my website. Whenever I use the scale image dialog, I change
resolution only at the cost of it automatically and irritatingly
changing the print size to something else.
All I want to do is to choose an exact size of the image and change the
resolution ONLY.
I've flipped through the manual and looked through the forums. Some have
suggested that after changing the resolution you can just change the
print size. Wrong. Wrong. Irritating. Bad. Why is the resolution
forcibly tied to the print size?
I can't be alone in this since this is such an super basic thing to do.
Peter-Josef Meisch
> I'm confused by a seemingly simple problem with GIMP 2.0.
>
> I scan images in at 300dpi, some of which I want to downsample to
> 72dpi
> for my website.
Why 72? I have a monitor with a resolution of 1280x1024 that has 100dpi.
Forget about the dpi and resize your images to fixed pixel sizes, i.e.
800x600 pix.
> Whenever I use the scale image dialog, I change
> resolution only at the cost of it automatically and irritatingly
> changing the print size to something else.
>
> All I want to do is to choose an exact size of the image and change
> the resolution ONLY.
As I mentioned, the resoultion in dpi is irrelevant for the web because
you don't know which resolution the monitor has with which people are
looking at your website. If you scale an image with 72dpi to be 5
inches wide, on my screen it will be 3.6 inches wide.
P.J.
--
Peter-Josef Meisch
Doctor J. Frink
>I'm confused by a seemingly simple problem with GIMP 2.0.
>
>I scan images in at 300dpi, some of which I want to downsample to 72dpi
>for my website. Whenever I use the scale image dialog, I change
>resolution only at the cost of it automatically and irritatingly
>changing the print size to something else.
That's because pixel count, image size and resolution are intimately
linked! Something that is 300pixels wide, at 300dpi will be one inch
wide, if you drop the resolution to 100dpi, still at 300pixels width, it
will be 3inches wide if printed at that resolution.
>All I want to do is to choose an exact size of the image and change the
>resolution ONLY.
Resolution has no meaning *at all* for raster images when displaying on
the web. Take an image, keep the pixel count the same and change the dpi
to 72, 300, 600 etc (never mind about the 'print size'), save and then
display in a web browser. Notice that they all look *exactly* the same
and have the *exact same* filesize.
The only place where image resolution comes into play on a monitor is if
you are adding text, where the size of the text will be dependent on the
resolution.
When it comes to monitors choose your size in pixels, that's all you
have to do and that's all graphics cards are aware of. The "use 72dpi
for web images" idea is total rubbish.
Frink
--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse
No One
user with false information: changing the resolution away from 300dpi to
anything smaller tells the user that the new dimensions are very large,
say moving 3.5" square to 14" inches square. Even if I take "print size"
to mean only printer output, it is the only information I have to go on
in the dialog box. Why would I look at that and think the dimensions
will remain the same on the website? You're right in that it does, but I
find that I need to move back and forth a lot more between the browser
and web software to make the images look the way I want them to,
especially in relation to images given to me that were already scanned
at 72dpi.
Maybe the idea of 72dpi for web stuff is total rubbish. It could very
well be. But all the web manuals strewn across my desk say this 72dpi
business is convention and I dislike having to ask a newsgroup how to do
something that no other graphic program ever gave me trouble with.
Roel Schroeven
> In other words, GIMP won't do this very simple task and misleads the
> user with false information: changing the resolution away from 300dpi
> to anything smaller tells the user that the new dimensions are very
> large, say moving 3.5" square to 14" inches square.
Yes, because that's exactly what happens. Say you have a picture with a
width of 1200 pixels. With a resolution of 300dpi, that will be printed
4 inches wide. With a resolution of say 100dpi, it will be printed 12
inches wide.
> Even if I take "print size" to mean only printer output, it is the
> only information I have to go on in the dialog box.
What about 'New Width' and 'Height' under 'Pixel Dimensions'? I would
think that would be much more useful.
> Why would I look at that and think the dimensions will remain the
> same on the website?
Because the pixel dimensions remain the same unless you change them.
Changing the resolution doesn't change them, and since web browsers
don't look at the resolution in order to scale images themselves, the
resolution doesn't have any effect.
> You're right in that it does, but I find that I need to move back and
> forth a lot more between the browser and web software to make the
> images look the way I want them to, especially in relation to images
> given to me that were already scanned at 72dpi.
I think I don't really understand your way of working. When I create
images for the web, I'm always thinking in terms of pixel dimensions. I
want an image 600 pixels wide, or 300 pixels heigh, whatever. I just
scale it to that dimension.
> Maybe the idea of 72dpi for web stuff is total rubbish. It could very
> well be. But all the web manuals strewn across my desk say this
> 72dpi business is convention and I dislike having to ask a newsgroup
> how to do something that no other graphic program ever gave me
> trouble with.
I don't think it is *total* rubbish: if you scan an image and want it to
be more or less the same size on screen as the paper original, you
should scan with about 72dpi (though more and more with larger
resolutions: 100 or even 120 dpi are more accurate for modern high
resolution screens). But that is the scanner setting; changing the
number in Gimp doesn't do anything except changing the number itself.
I don't really have experience with other graphic programs; exactly what
do they do when you change the resolution? Do they actually change the
pixel dimensions and thus change the pixel data itself?
--
"Codito ergo sum"
Roel Schroeven
Zundark
> In other words, GIMP won't do this very simple task and misleads
> the user with false information: changing the resolution away from
> 300dpi to anything smaller tells the user that the new dimensions
> are very large, say moving 3.5" square to 14" inches square.
That's not false information. If you don't change the image itself,
then printing it with fewer pixels per inch will obviously take up
more space.
In any case, there shouldn't be any need for you to fiddle with the
dpi when all you're doing is creating images for a web page. Even
for non-web use, the dpi of a digital image is usually irrelevant,
and most image formats allow it to be omitted.
> Even if I take "print size" to mean only printer output,
What else could it mean?
> it is the only information I have to go on in the dialog box.
If you're talking about the Scale Image dialog, then you must be
ignoring the top half, which is the important part. You would be
better off ignoring the dpi and print size.
> Why would I look at that and think the dimensions
> will remain the same on the website?
Because it should be obvious that only the pixels matter. What
meaning do you expect "dpi" or "print size" to have for an image
on a web page?
> You're right in that it does, but I find that I need to move back
> and forth a lot more between the browser and web software to make
> the images look the way I want them to, especially in relation to
> images given to me that were already scanned at 72dpi.
How does other software do it? In the GIMP, you simply set the image
size to the size you want (in pixels). It was the same in Paint Shop Pro
last time I looked. It's not obvious how it could be simpler than this.
> Maybe the idea of 72dpi for web stuff is total rubbish.
Yes. Even if dpi had any meaning for web images (which it doesn't),
72 dpi is surely too low to take as a typical monitor resolution.
> But all the web manuals strewn across my desk say this 72dpi business
> is convention and I dislike having to ask a newsgroup how to do
> something that no other graphic program ever gave me trouble with.
As I said above, it's already about as simple as it could be.
You're just making it difficult by worrying about the dpi.
krONik
> I see what's confusing you :-)
>
> Other photoediting packages allow you to combine two seperate operations
> together and the GIMP doesn't.
>
> First..
>
> DPI (Or PPI) is nothing more than the number of pixels you spread
> across an inch of paper (or whatever you print on). You can determine
> the desired print size in inches by setting the DPI.
>
> So...
>
> If you have a 900 pixel wide file set to 300 DPI the print size will
> show as 3 inches wide.
>
> If you change the DPI in this 900 pixel wide image to 100, then
> the size in inches becomes 9 inches. (900 / 100 = 9)
>
> If you grab a calculator, and play with inches and PPI in any
> photo editing software, you'll see the PPI is ALWAYS the
> proposed *print* size in inches divided by the number of
> pixels in the file. That's all DPI (or PPI) is, and nothing
> more.
>
> But..
>
> Suppose you want to print this 900 pixel wide image at 100 DPI
> and have the resulting image only 3 inches wide.
>
> You CAN'T spread 900 pixels evenly across 3 inches and wind up
> with 100 pixels in each inch. It's mathematically impossible.
> you HAVE to have 900 / 3 = 300 Pixels in each inch, or 300 DPI.
>
> The only way to do it is resample the image. If you resample
> the image from 900 down to 300 pixels wide and print the
> 300 pixel wide image at 100 DPI, you've got what you want.
>
> Resampling, (or changing the number of pixels) is a completely
> seperate operation. You aren't dealing with printing, you're
> altering the image file itself by adding or discarding pixels.
>
> Programs like Photoshop have a box in the 'image size' dialog that
> you can check that allows for image resampling if your printing
> parameters (PPI and Inches) are mathematically impossible.
>
> In the case of photoshop, if you take your 900 pixel wide
> image and set the DPI to 100 and the inches to 3 with the
> resample box checked, then the pixel width will change
> from 900 to 300. You're resampling the image so your
> print parameters will work.
>
> What causes the 72 DPI confusion is the fact you can check this
> box.
>
> Take a 3000 pixel wide image. In Photoshop, set it to print 5 inches
> wide at 72 DPI.. The image will be resampled to a width of 360 pixels
> so when you spread these pixels across your page, you'll have 72 of them
> in each inch. 360 pixels / 72 dpi = 5 inches
>
> A 3000 pixel wide image won't fit on a screen that only displays
> 1024 pixels across. Most of the 3000 pixels will scroll off the side
> BUT, the resampled image of 360 pixels WILL fit in a 1024 screen.
>
> Does it fit because you changed it to 72 DPI or does it fit because it
> was resampled ? Obviously it fits because it was resampled. It fits
> because the number of the pixels in the file was reduced.
>
> Many don't understand that resampling an image is a completely different
> operation than simply printing an image which is taking the existing pixels
> and spreading them on paper... Hence the confusion.
>
> So WHY mess with 72 DPI and inches for displaying images on monitors,
> when you could have completely ignored the DPI and inches and just
> resampled the image from 3000 pixels to 360 ?
>
> One last thing.. I doubt you could find a monitor/video card combo
> nowadays that actually displays 72 DPI.. Many modern systems are
> over 100 DPI.
>
> Setting 72 DPI at 6 inches wide will NOT display an image that is
> 6 inches wide on 99% of monitors out there. If you grab a ruler
> and measure your images on your monitor, and compare them with the
> inches @ 72 dpi you set, you'll see they aren't exact.
>
> Not only that, but the image will be different widths, depending on
> the screen resolution, and size of the monitor viewing it.
>
> If I want to make an image fill EXACTLY 1/2 of my screen that's 1280
> pixels wide, I ignore inches DPI and just change the pixel width
> which resamples the image to 640 pixels. A 640 pixel image will always
> fill 1/2 my screen regardless of the DPI.
>
> How would I do this using 72 DPI and width in inches :-) :-)
>
> PS.. If you want to know YOUR monitor DPI, first find what it's set
> at (eg 1024 x 768 pixels). Grab a ruler and measure the screen.
>
> If your screen is 12.25 inches wide and 1024 pixels make up the full
> screen width, then the DPI is 1024 pixels / 12.5 = 81.92 DPI (My laptop).
>
> When sizing images for the screen, care not about inches or DPI.. Just
> care about pixels.
This is a very informative post. Thanks for the info Jim!
--
cBe
krONik =8^)
===================================================================
"Somewhere, something incredible is waiting to be known."
--- Dr Carl Sagan (1934 - 1996) ---
===================================================================
Registered Linux User: #296186 @ http://counter.li.org/
Mandrake 10.0 Official ROCKS!
William Kendrick
> When it comes to monitors choose your size in pixels, that's all you
> have to do and that's all graphics cards are aware of. The "use 72dpi
> for web images" idea is total rubbish.
Yeah, I'm often asked to produce screenshots or mockups of games I'm
developing for a wireless phone carrier, so they can add it to a brochure
or something.
They want it in 300dpi. WTF does that even MEAN? I'm taking a SCREENSHOT.
It will be EXACTLY X pixels wide by Y pixels tall! When they ask for a
file at a particular dpi, they should ALSO be specifying the width and height,
in inches! Sheesh!
What I normally do is just estimate "about business-card sized" and then
use The Gimp to convert a 150x200 pixel screenshot into something that
fits in the business-card size at 300dpi. Since it's a screenshot, I do
this in Indexed mode, so that the pixels don't blur.
Really, it looks identical to the original screenshot, except the 1x1 pixels
in the screenshot have been expanded into little WxH rectangles... something
their page layout program should be able to do FOR them using the original
150x200 screenshot! Sheesh!
It just ends up being a waste of my time (having to scale the screenshot to
fit into their 'requirement' of 300dpi), and a waste of space and bandwith.
(A 2KB screenshot bloats up into 10s or 100s of KBs :^) )
Sorry... just ranting.
-bill!
bi...@newbreedsoftware.com Have I been helpful?
http://newbreedsoftware.com/ http://svcs.affero.net/rm.php?r=billkendrick
Peter Heckert
William Kendrick wrote:
> Doctor J. Frink <fr...@homer.cmp.liv.ac.uk> wrote:
>> When it comes to monitors choose your size in pixels, that's all you
>> have to do and that's all graphics cards are aware of. The "use 72dpi
>> for web images" idea is total rubbish.
>
> Yeah, I'm often asked to produce screenshots or mockups of games I'm
> developing for a wireless phone carrier, so they can add it to a brochure
> or something.
>
> They want it in 300dpi. WTF does that even MEAN? I'm taking a
> SCREENSHOT. It will be EXACTLY X pixels wide by Y pixels tall! When they
> ask for a file at a particular dpi, they should ALSO be specifying the
> width and height, in inches! Sheesh!
>
There are applications in graphics industry that import images and scale
them according to the dpi setting.
Mostly these are DTP Programs or Programs with WYSIWYG features
(typesetting software,web-page generators,.word processors) and and the
designers don't know about this.
AFAIK Adobe Acrobat belongs to this category of software.
These designers dont think in technical
terms of pixels, they are artists and dont know what's happening at low
level, when an image is scaled.
So, if they want 300dpi, then you should deliver 300 dpi.
If they dont like the size, then they will change it themselves or they
will phone to you for a different size or better resolution. ;-)
regard,
peter