GNOME Bugzilla – Bug 556417
Interpolation modes do not work (scaling)
Last modified: 2008-10-15 22:41:54 UTC
When scaling images or layers the options linear, cubic and Lanczos3 interpolation have no effect. The results are the same as if none of them is chosen.
Created attachment 120662 [details] example for the malfunction (cubic interpolation) Here is an example: I scaled a screen shot of the GIMP website at 50% using cubic interpolation. The original is on the left, the scaled image on the right. The results of linear and Lanczos3 interpolation are completely identical.
Hi and thanks for the bug report! I can clearly see differences between the interpolation modes. Please provide a detailed step-by-step on how to reproduce the problem.
Created attachment 120673 [details] test images step-by-step reproduction: 1. Ctrl-T or Image -> Scale Image or Layer -> Scale Layer 2. Type in a resolution, for example 50% 3. Choose cubic interpolation mode 4. Click on 'Scale' Furthermore I just have remarked something weird: The error occurs only when you shrink images by a power of 2 factor. I made out some examples here. As one can see, the image is scaled without interpolation when I shrink it to 50% or 25%, but if I choose 50% minus 1 pixel (45 px instead of 46 px), scaling works well. 75% gives good results, too. By the way, the error not only occurs if I type in percentages. The same happens if I do it with pixels, inches and so on.
What exactly is the original size of the image you are using as an example? Can you please start gimp with the environment variable GIMP_LOG set to "scale" and redo your experiments? The output should give us a hint on what might be going wrong.
The original size of the example image (the three circles) is 92 x 92 pixels, but the error occurs with other resolutions, too. As you demanded it from me, I tried to activate the GIMP log. I entered 'GIMP_LOG' with the value 'scale' both as a user and a system variable (in the Windows environment variables control panel), but when I start GIMP and scale an image, no console and no file (which I expected to contain the log) appears ... Since I am not very used to debugging GIMP and did not find out anything about GIMP_LOG neither on the Internet nor on Bugzilla, it would be nice if you could explain it to me in detail.
This is a duplicate of bug #556248. There's a patch attached there that fixes your problem. *** This bug has been marked as a duplicate of 556248 ***