Empty (UDIM) tiles where drawn with a transparency checkerboard. They
should be rendered with a border background. The cause is that the image
engine would select a single area that contained all tiles and draw them
as being part of an image.
The fix is to separate the color and depth part of the image engine
shader and only draw the depths of tiles that are enabled.
Adding better support for drawing huge images in the image/uv editor. Also solved tearing artifacts.
The approach is that for each image/uv editor a screen space gpu texture is created that only contains
the visible pixels. When zooming or panning the gpu texture is rebuild.
Although the solution isn't memory intensive other parts of blender memory usage scales together with
the image size.
* Due to complexity we didn't implement partial updates when drawing images tiled (wrap repeat).
This could be added, but is complicated as a change in the source could mean many different
changes on the GPU texture. The work around for now is to tag all gpu textures to be dirty when
changes are detected.
Original plan was to have 4 screen space images to support panning without gpu texture creation.
For now we don't see the need to implement it as the solution is already fast. Especially when
GPU memory is shared with CPU ram.
Reviewed By: fclem
Maniphest Tasks: T92525, T92903
Differential Revision: https://developer.blender.org/D13424
