Now this pull request is draft because we have to add tests for Disposal Method = 3.

kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv_extra4/testdata/highgui/gifsuite$ git show
commit 727770994fe49acca31b8d7ffe8653febed67cb5 (HEAD -> 4.x, origin/master, origin/HEAD, origin/4.x)
Merge: eddc7acc 8ff6cb8a
Author: Alexander Smorkalov <2536374+asmorkalov@users.noreply.github.com>
Date:   Fri Feb 14 19:40:11 2025 +0300
    Merge pull request #1236 from asmorkalov:as/openvx_hal_features2d
    Regression data for FAST OpenVX case.
kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv_extra4/testdata/highgui/gifsuite$ gifbuild -d *.gif | grep disposal | sort | uniq -c
      7         disposal mode 0
     87         disposal mode 1
     10         disposal mode 2

i can confirm the PR seems fixed the bug.

I feel more refactoring seems be needed from current GifEncoder/Decoder.
Here are some points I noticed. I think these should be fixed separately from this fix.

• Member variables in Encoder/Decoder should be started "m_" prefix. Now it is hard to know which is it function local variable or not.
• readExtensions() should be rename readGraphiControlExtension(). Now it doesn't handle other extensions.
• uchar should be uint8_t.
• "flags" variable is used for "Packed Fields" for some Descriptor and Extensions. But it is hard to know what does means.
• GifEncoder has both (A) m_height/m_width which means "Logical Screen Width/Height" in "Logical Screen Descriptor" , and (B) height/width which means "Image Width/Height" in "Image Descriptor". I feel it is natural to introduce innner strcutors to split information.

class GifEncoder {
 + struct ImageDescriptor {
    + imageWidth;
    + imageHeight; };
 + struct LogicalScreenDescriptor {
    + logicalScreenWidth;
    + logicalScreenHeight; };
}

Sorry I test with ./bin/opencv_test_imgcodecs --gtest_filter="*Gif* command, so I miss this error.

     const uint8_t gcePackedFields = (GIF_DISPOSE_RESTORE_PREVIOUS << GIF_DISPOSE_METHOD_SHIFT) |
-                                    (criticalTransparency) ? GIF_TRANSPARENT_INDEX_GIVEN : GIF_TRANSPARENT_INDEX_NOT_GIVEN;
+                                    (criticalTransparency ? GIF_TRANSPARENT_INDEX_GIVEN : GIF_TRANSPARENT_INDEX_NOT_GIVEN);

It seems this code works following.

const uint8_t gcePackedFields =
  ( (GIF_DISPOSE_RESTORE_PREVIOUS << GIF_DISPOSE_METHOD_SHIFT) | criticalTransparency) )
  ? 
  GIF_TRANSPARENT_INDEX_GIVEN : 
  GIF_TRANSPARENT_INDEX_NOT_GIVEN;
- ```