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Dot for dot
Gimp doc says "When view>dot for dot is unchecked the image should be the real size if the image resolution is the same as the screen resolution.", according to this attachment.

I measured my monitor vertically. I divided it by 1440p, which is my display setting. The result ppi is approximately 109 ppi.

I opened a blank new image sized 640x480 with 109 ppi resolution.

Then I unchecked the "dot for dot" in view menu.

I measured the vertical size of the image. The size is smaller than 480/109. It should be 480/109.

So, I checked the "dot for dot" in view menu. Then the size is 480/109.

Why is this happen?
This is all about printing and print resolutions.

Two images both 300 x 300 pix no.1 @ 300 ppi and no.2 @ 150 ppi. with dot-for-dot off (default setting) both obviously show the same size (zoom being equal)

With dot-for-dot off their printed size will display
no.1 = 1 inch (300 pix/300 ppi)
no.2 = 2 inch (300 pix/150 ppi)

comparison see:

If your monitor is correctly set up, then you can measure these values on the screen.

This is my laptop, has a HD display, and a pixel resolution of about 157 ppi

Edit -> Preferences -> Display see:

There is a calibration page to assist, not very good in Gimp 2.8. Automatic does not always work, a know problem that has been fixed in Gimp 2.10. Try it but you might have to do a bit of measuring on the screen and arithmetic to get the correct screen pixel density.

calibration looks like:
Yeah, like you said the automatic ppi is not correct. I've decided I trust what I measured. I manually changed the ppi in preferences. Then all works fine.

I haven't upgraded gimp to 2.10. I'm planning to upgrade it after reading all the doc pages.

Thanks for the help.

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