The compositor backdrop in certain files always have a size of 256x256
regardless of the actual size of the viewer image. That's because the
compositor writes its result to a different image buffer than the one
the image engine reads its image buffer from. And the image engine
assumes a default size of 256x256. The reason is a bit involved.
For non multi-view images, the image module uses the special cache index
value of IMA_NO_INDEX for the compositor backdrop, which works fine if
the image was detected as a non multi-view image in the first place.
However, this detection fails because the compositor may still write
multi-view images even for non multi-view renders.
In particular, before the compositor writes its viewer image, it ensures
correct views by calling BKE_image_ensure_viewer_views, which first
checks if we need to recreate the views of the viewer image if they
don't match the render views. And in the case of non multi-view image,
that check fails in one case.
Functions like BKE_image_is_multiview checks if a single unnamed view
exists in the image, unnamed being the keyword here. The root issue is
that BKE_image_ensure_viewer_views only checks that a single view
exists, while it should also check that it is unnamed. Which happens
when the user enabled multi-view, added only one view, then disabled
multi-view again.
To fix this, we add a check for the name of the view in case of non
multi-view images. And additionally pull the view matching code into its
own documented utility function for clarity.
Pull Request: https://projects.blender.org/blender/blender/pulls/132348
d4008562dd