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E.g. I have a picture with print size 10x20 cm and resolution 300 PPI. How can I change PPI to 200 and keep the print size 10x20 cm? Size in pixels should also change of course. GIMP always does vice versa. It preserves the size in pixels and changes the printing size. Very annoying behavior by default and no obvious way to change it. It drives me mad.

In GIMP GUI terms in "Scale Image" window I want to be able to change "X resolution" and "Y resolution" (pixels/in) and have "Width" and "Height" (px) changed respectively. Or in "Print Size" window change "X resolution" and "Y resolution" (pixels/in) without changing "Width" and "Height" (mm). In fact none of the windows does the thing. What's the purpose of having two different windows "Print Size" and "Scale Image" if they do pretty much the same?!

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    The reason image editors default to preserving the pixels is because that's your image. If you want to change your image, you have the tools to do it, but the image software isn't going to modify the content on its own. PPI is just a setting, and some image formats don't even preserve it in the metadata. If you want to change the print size, changing the PPI does it without losing information or degrading the image. Why do you even care what the PPI is? – fixer1234 Aug 21 '19 at 4:16
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    @fixer1234 Strange question. Say I want to reduce the file size (in MB) and send it by email. Yes, I know I can use JPEG compression. But what if I want to reduce the size even more? Another example - I want my PPI be multiple of the printer's resolution (DPI). Or I want decrease PPI to get more colors when printing. Photoshop and Paint.NET can do such things easily. Why can't GIMP do it? The fact is it does support changing the image itself (not the metadata, see "Interpolation" drop-down) but there's no convenient way to do it - I can't do it by changing PPI. – Mike Siomkin Aug 21 '19 at 8:38
  • That's modifying the image. Yes, there are other reasons you might want to do that, like you explained in the comment, and yes, GIMP provides the tools to do it. What you describe in the question is expecting GIMP to automatically modify your image content for the purpose of printing at certain dimensions. Printing doesn't require changing the content, and no image editor will do that without your explicit instructions. You just need to use the built-in tools to do what you want with the image. The question doesn't mention any of what's in your comment, which is really a different question. – fixer1234 Aug 21 '19 at 15:39
  • @fixer1234 "Printing doesn't require changing the content" - OK let "Print Size" dialog not modify the image. But if I open "Scale Image" isn't it obvious I am going to modify it? Why not let me do it in several ways (not just by directly changing the size in pixels)? – Mike Siomkin Aug 22 '19 at 14:44
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It looks like the only way is to do some simple calculations myself. GIMP is a bit lazy. OK, I'll do the dirty work. I have 300 PPI and want 200. So the factor is 200/300 = 2/3. I multiply the dimensions of the picture (height and width in pixels) by the factor and press "Scale" in "Scale Image" window. Here GIMP performs resampling - it doesn't have other choice :) Print size also changes. OK, let it be. Than I have two options: I can change PPI to 200 and get the print size close to original or change the print size to original and have PPI close to 200. And if I'm lucky (a good factor/dimensions combination) I get both things sharp: the original size and the required PPI. That's it: I changed PPI and kept the print size.

And yes, "Print Size" dialog is not needed. "Scale Image" can do everything "Print Size" does plus image resampling.

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