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I'm not understanding Gimp. Sometimes it works, sometimes it doesn't.

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  • I'm not understanding Gimp. Sometimes it works, sometimes it doesn't.

    I can't believe I'm having such a hard time using Gimp. I have already used it to make many lithophanes but sometimes it just doesn't work. To make my lithophanes I need a background that is grey and 102 mm x 156 and apply a photo of 92 x 144 (ideally) It can be shorter. I have a "canvass" set to the 102 x 156 in the grey colour I wish, When I drag a photo on to it that has been sized to 92 x 144 or less it suddenly becomes too small. But sometimes it works. For the life of me I can't tell what's going on.

  • #2
    I suspect I know exactly what's going on: pixels per inch. In my outline on how to set up for lithophanes I mentioned that you need to make sure the pixels per inch are exactly the same for both parts. Now, I do not know where you find out the number of pixels per inch in Gimp. In Photoshop it's in the Image Size area, so you might want to look there to start.

    In any case, you need to find out how many pixels per inch your template file (your canvas). Then, when you bring in a new image to put on top of the canvas, when you're setting up the 92x144, make sure it has the same pixels per inch as what's in the canvas.

    BTW, this is a classic issue when merging photos in editing software. You are by no means the first to run into it.

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    • #3
      I always change to mm instead of pixels. But I make sure that both are in mm, I just don't understand why it works sometime and not others. So Using the Canvass as the grey background is not wrong? Since I put both in mm why does it do that to me?

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      • #4
        I guess it is the aspect ratio that causes the issues.

        By default gimp is scaling with a locked aspect ratio, so an image does not get distorted.

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        • roon4660
          roon4660 commented
          Editing a comment
          I know that it keeps an aspect ratio and how to unlock it if I wish. I am just very puzzled. Years ago when I was making web sites I used photo shop with much less problem than I have with Gimp.

      • #5
        Originally posted by roon4660 View Post
        I always change to mm instead of pixels. But I make sure that both are in mm, I just don't understand why it works sometime and not others. So Using the Canvass as the grey background is not wrong? Since I put both in mm why does it do that to me?
        You need to check the pixels per inch. Let's make sure they're both the same before tackling other possibilities.

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        • #6
          I don't understand pixels per inch. I am working in mm. I have the choice of pixels, inches, mm etc. I set everything to mm.

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          • #7
            Next to the height and width gadgets in the UI of Gimp there is a lock symbol. If you click that you can scale the image to any size you want.

            In more modern versions of Gimp (I just tried 2.10) it looks like a junktion box with leads connecting the height and width gadget. Click on it and it opens.

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            • #8
              I am aware of it. My problem is keeping the canvass related to the photo I drag on top. I need a grey border around the photo so that the lithophane is 2 mm thick all around so that it will fit into the lamp. Something is happening even though I size the canvass and the photo in mm they do not keep the sizes when I drag the photo on top. Sometimes it works and sometimes it doesn't.

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              • #9
                Originally posted by roon4660 View Post
                I don't understand pixels per inch. I am working in mm. I have the choice of pixels, inches, mm etc. I set everything to mm.
                You need to ensure both images have the same resolution. AFAIK, you cannot do that with anything except pixels per inch. You don't have to understand pixels per inch, only make sure they are both the same. Once you have that, you can switch to mm to set the size to what you want.

                OK, I just checked in Photoshop. Notice in this image how the Width & Height are set to Millimeters, but the Resolution is still set to Pixels/Inch. That's because it isn't possible to set Resolution to mm. You have only 2 choices: pixels/inch, or pixels/centimeter.

                Click image for larger version

Name:	PSimageSize.JPG
Views:	23
Size:	45.8 KB
ID:	4253

                I'm not sure where you find the equivalent to this image in Gimp, but it's what you need in order to correct the issue. What you're trying to do will never work until you get the resolution exactly the same.
                Last edited by Ender5r; 10-01-2020, 09:40 AM.

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                • #10
                  Okay I shall check that. After my morning coffee.

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                  • #11
                    Its like with movies. If the aspect ratio is wrong you get borders or people with long heads

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                    • #12

                      In this case, I don't think aspect ratio is the issue. Even if the ratio is exactly the same for his background and foreground images, they still don't match up size-wise because, when he changes to mm measurements and sets his image to 92mm x 144mm, he expects the image to fit his 102mm x 156mm background properly. Unfortunately, if the background is set to 300 pixels/in and his foreground image is set to 200 pixels/in, the foreground & background will not line up as he expects.

                      I'll illustrate. The black background on the left is 102mm x 156mm x 300 pixels per inch. The orange foreground on top of the background is 92mm x 144mm x 200 pixels per inch. The black background on the right is also 102mm x156mm x 300 pixels per inch. However, the orange foreground on the right is 92mm x 144mm x 300 pixels per inch.

                      Click image for larger version  Name:	300x200pixelsPerInch.jpg Views:	0 Size:	41.2 KB ID:	4258Click image for larger version  Name:	300x300pixelsPerInch.jpg Views:	0 Size:	49.7 KB ID:	4257

                      Please note that the ratio is exactly the same in both images. Resolution is what makes the difference.

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                      • #13
                        Thank you for your help. I get the idea but haven't tried it out yet because my brain is giving me another bad day.

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                        • #14
                          Success. With your help I have solved my problem.

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                          • #15
                            Excellent. And now you have a formula for all future lithos.

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