@Jeroen Bakker (jbakker) Ah, that's from the section that I removed from OVERLAY_grid_cache_init; I didn't realize it was used for the background like that. I may need some help/hints now then.

That opaque background quad draws over the images, defeating the purpose of this patch. I have not been able to get it not to do that. I'm a first-time RenderDoc user here and all I've been able to determine is that these overlays are drawn after the pass that draws all the images - which makes sense - but this particular background quad shader doesn't have any configurable parameters to do a proper depth test(?) and so isn't very nice as an "overlay".

Is there are set of opengl draw states I can use or does the background need to be split into a separate pass/shader that occurs before image drawing?

I added back in the removed code but I could not get any of the existing fragment shaders to do what I needed so I had to make another one. This seems to work but now leaves a weird 1 pixel border around the render window's output:

Not really sure why that might be the case.

Although this works, it seems that the functionality isn't clear to the end user.
An image can have a UDIM Grid, but can add tiles outside this grid (this was always the case, but at that point the grid wasn't shown anymore). I do think that showing the UDIM grid would need more design

• When does the user want to see the grid?
• What happens when the user adds a tile outside the grid?

Best way to move forward is to collect feedback from users and steer the patch in that direction.
Think it is good to involve @Julien Kaspar (JulienKaspar) to give a well thought out idea.

I think as a quick fix for the overlay this goes in the right direction. This is useful for:

• Showing and using the Grid on UDIM tiles that are still missing or not using any images. This also highlights the disconnect between the UV Grids and UDIM image tiles.
• Showing the boundary of each UDIM tile even when images are used. This can also be really useful when when "Repeat Image" is toggled on

For that alone I think this is worth committing.

This goes a bit out of topic but I think in the long-term it would be best if the Grid is less static but automatically extends whenever a UV island exceeds the original 1:1 space and moves to another tile.
This would make it visually obvious when the UVs are (intentional or unintentionally) going out of bounds.
It would also make the Grid automatically correct itself for every individual UV map instead of having it as a global setting. Each UV map can have a different UDIM count after all.

Based on the feedback I've received so far on the DevTalk thread, there seems like good consensus for following through with this current patch as-is right now. The design task will track the other issues we've come up with so far(no strong consensus for some of the other topics yet). Is there anything else you'd like me to do or check?

I will go over it one more time. Would be good to update the description of this change as that is normally the base for the commit message.