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Basic Scaling(?) Question

Dec 7, 2011, 22:33
Post: #1
Basic Scaling(?) Question
Hello,

I'm new here though I've been enjoying The GIMP for many years--many thanks to the developers.

There's a basic feature that I've somehow never been able to get my head around--i.e., scaling(?). I've searched Google, GIMP help and the threads here and have found many references but am still unable to figure it out, so sorry if this post is redundant.

From Gimp.org:

Resolution applies to printing only and has no effect on the image's size when it is displayed on a monitor or a mobile device.

OK, got it. Sort of. Since 'Resolution applies to printing only' I've changed the 'print size' in Image > Scale to 7" x 5", hoping to display the image at that size on a web page. As you probably know, this didn't work. It's a big image--260 x 260 pixels/in--and shows up huge on the page (or when opened in Firefox). I can't figure out how to reduce the display size on the web page. Apparently 'Scale Image' isn't the answer, or I'm missing something essential (and probably obvious).

I don't wan't this huge image on the web, partly because it's original art and I don't want to make it too easy to make high quality reproductions from the posted image.

I suspect my problem lies in not really understanding ppi vs dpi etc.,--which misunderstanding I've gathered from my reading is very common--in spite of having read a number of elementary explanations. Can anyone explain to me what it is that I'm missing and/or where to find instructions I might be able to comprehend?

Any help would be greatly appreciated.
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Dec 8, 2011, 01:51 (This post was last modified: Dec 8, 2011 01:55 by ofnuts.)
Post: #2
RE: Basic Scaling(?) Question
(Dec 7, 2011 22:33)mlnease Wrote:  Hello,

I'm new here though I've been enjoying The GIMP for many years--many thanks to the developers.

There's a basic feature that I've somehow never been able to get my head around--i.e., scaling(?). I've searched Google, GIMP help and the threads here and have found many references but am still unable to figure it out, so sorry if this post is redundant.

From Gimp.org:

Resolution applies to printing only and has no effect on the image's size when it is displayed on a monitor or a mobile device.

OK, got it. Sort of. Since 'Resolution applies to printing only' I've changed the 'print size' in Image > Scale to 7" x 5", hoping to display the image at that size on a web page. As you probably know, this didn't work. It's a big image--260 x 260 pixels/in--and shows up huge on the page (or when opened in Firefox). I can't figure out how to reduce the display size on the web page. Apparently 'Scale Image' isn't the answer, or I'm missing something essential (and probably obvious).

I don't wan't this huge image on the web, partly because it's original art and I don't want to make it too easy to make high quality reproductions from the posted image.

I suspect my problem lies in not really understanding ppi vs dpi etc.,--which misunderstanding I've gathered from my reading is very common--in spite of having read a number of elementary explanations. Can anyone explain to me what it is that I'm missing and/or where to find instructions I might be able to comprehend?

Any help would be greatly appreciated.
Image "size" is a misnomer. Let's call that image detail instead. Image detail is measured in pixels. This is the only important measure. If you reduce the size in pixels, you lose detail. If you increase the size in pixels you need more detail. Since usually there is nothing to add even fake detail to the image in this case, the picture looks like it has less detail than it should and looks blurry.

Now, we want to show all this detail...

A computer display is made of tiny square dots (or things that look a lot like tiny square dots...). There are between 75 (the old CRTs) and 120-150 (modern LCD screens) such dots per inch of physical display size. It is still rather easy to distinguish pixel-size details. When we display an image on that there are three
cases:
  • the size of the image is such that we have exactly one pixel for each dot: thinks look perfect.
  • we have more pixels than necessary for the display dots: some downscaling is done on the fly. This is usually not too much of a problem for an untrained eye, but it is not done as nicely as one would using Gimp (control over the algorithm used, re-sharpening, anti-aliasing)
  • we have less pixels than necessary for the display dots: the image needs to be upscaled and it usually ugly, even to the untrained eye.
For these reasons, and because in many cases everything around the images is defined in display dots anyway, most uses of an image on a computer display will assume a 1:1 mapping between pixels and display dots; in other words, to insure maximum detail, you relinquish control on the physical size, which will depend on the screen definition. The screen definition is measured in dots per inch (DPI) and since we use a 1:1 mapping, the image is displayed at the same value, expressed in pixels per inch (PPI).

On a printer, the case is a bit different. The naked eye won't see dots smaller than 1/600th of an inch, and the printer has a DPI rating which is even smaller (2400DPI or more). Furthermore you often want some control over the physical size of the print. So, between a fixed size in pixels and a physical size, the adjustment variable is a print resolution expressed in pixels per inch. 150PPI is draft quality, 300PPI is adequate, 600PPI is perfect. Even at 600PPI the printer will use several dots to produce the pixel (in fact it uses dots of different colors to produce the color tone). But there is no free lunch. If your image is 1000x1500 pixels, at 300PPI you get a 3.33"x5" print out, and if you want a 6.7"x10" print you need to decrease the print resolution to 150PPI.

As an aside, for photography it's not as bad as it looks. The bigger the physical size of the picture, the farther you have to hold it to see it as a whole (unlike text that you literally "scan"). So the size of the smallest detail you can see is proportional to the physical size. In photography this is considered to be around 1/1800 of the diagonal. If you do the reverse math, you find that this is the diagonal of a 1440x1080 image. In other words a 1.5Mpix photo will always look acceptable whatever the size/print resolution (*). So in the example above, if it is a photo, the 6.7"x10" print will still look good despite being printed at 150PPI.

In Gimp, when you create an image, you can specify pixels, inches, and PPI. But you cannot pick the three independently.

(*) Digital cameras got a foothold in the professional market when they reached the 6Mpix mark, thus producing details at 1/3600th of the diagonal. This sets the "professional quality" print criteria...

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Dec 8, 2011, 02:44
Post: #3
RE: Basic Scaling(?) Question
Ofnuts,

Thanks for the speedy reply. I can see that it's rather dense (with information, I mean)--I have yet to make useful sense of it but look forward to pondering it at greater length. I'll get back to you when I've done so.

Cheers,

mn
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Dec 9, 2011, 00:46
Post: #4
RE: Basic Scaling(?) Question
Thanks very much again for your detailed response. May I ask a practical question? Attached is the image in question as displayed in GIMP.

   

Is it possible to configure it, using GIMP (or in any other way, for that matter) so that it will display at a particular size (in inches) when uploaded to the web (in this case, Wordpress)?

Thanks for your time and patience.

p.s. I'm new to the forum so sorry If I've failed to upload the screen print correctly. I can't tell from the preview. If such is the case, I'll try again after viewing the post.
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Dec 9, 2011, 01:15
Post: #5
RE: Basic Scaling(?) Question
(Dec 9, 2011 00:46)mlnease Wrote:  Thanks very much again for your detailed response. May I ask a practical question? Attached is the image in question as displayed in GIMP.



Is it possible to configure it, using GIMP (or in any other way, for that matter) so that it will display at a particular size (in inches) when uploaded to the web (in this case, Wordpress)?

Thanks for your time and patience.

p.s. I'm new to the forum so sorry If I've failed to upload the screen print correctly. I can't tell from the preview. If such is the case, I'll try again after viewing the post.
No... because web browsers stick to the 1:1 mapping so the physical size will depend on the screen definition of the viewer. And as far as I can tell, HTML/Javascript cannot tell the screen resolution or physical size either, so you can't use HTML or Javascript to resize the picture on the fly for this.

On the whole, you shouldn't expect any specific appearance on the user's navigator (and modern navigators will downscale pictures that don't fit in the window...)

PS: the resize dialog in your screenshot shows that you are using Cubic interpolation. This is the default, but unless you have a very slow computer you should be using the Sinc/Lanczos one (you can also make it the default in Edit/Preferences/Tool options).

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Dec 9, 2011, 01:24
Post: #6
RE: Basic Scaling(?) Question
Thanks again for your detailed--and obviously expert--advice. I guess ultimately, web design isn't for amateurs--at least not those without a great deal of time to learn the craft. Since there is plainly no practical solution to my problem I guess I'm done with the web as a venue for selling my art. Disappointing, but then, what isn't these days?

Thanks (yet) again for your time and patience.
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Dec 9, 2011, 07:22 (This post was last modified: Dec 9, 2011 07:42 by ofnuts.)
Post: #7
RE: Basic Scaling(?) Question
(Dec 9, 2011 01:24)mlnease Wrote:  Thanks again for your detailed--and obviously expert--advice. I guess ultimately, web design isn't for amateurs--at least not those without a great deal of time to learn the craft. Since there is plainly no practical solution to my problem I guess I'm done with the web as a venue for selling my art. Disappointing, but then, what isn't these days?
When it comes to art on the web, the size problem is completely minor compared to the color problem. And that doesn't prevent most people from selling their art (see deviantart.com). The computer screen isn't the final media. What you sell are physical prints, not bitmaps.

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Dec 9, 2011, 18:21
Post: #8
RE: Basic Scaling(?) Question
Yes, of course. I think I have the print part managed (though I won't be certain till I've sent a CD to the printers for a proof); also the color quality displayed has been more than adequate. Simply getting the images displayed at a reasonable size is my challenge. I'll keep tinkering. Thanks again for your time and expertise.
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Dec 9, 2011, 20:33
Post: #9
RE: Basic Scaling(?) Question
The average screen on the web is 1024*768pix or more according to this site (unless you also take in acccount the smartphones). So a 800x600 picture would fit in about any navigator window.

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Dec 9, 2011, 20:34
Post: #10
RE: Basic Scaling(?) Question
(Dec 9, 2011 20:33)ofnuts Wrote:  The average screen on the web is 1024*768pix or more according to this site (unless you also take in acccount the smartphones). So a 800x600 picture would fit in about any navigator window.
Thanks again!
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