How to get a pixel's coordinates relative to attacment's bounding box?

ara

I'm trying to tint to my spine unity skeleton animation with overlay gray scale style color.
I'm using URP/Spine/Sprite shader.
And we want make tint color's alpha variable: alpha value based on different position on each attacment itself (not world position / not whole skeleton, something like Attachment Space reference coordinates)
The current problem is how to get a pixel's (x, y) position relative to its attachment's bounding box in fragment shader.
Is it possile to get this information through mesh data?
Or maybe instead of 2D only 1 dimension is also enough so that we could get a float(0₁) for lerp(t) .

Thanks for any advice!

Harald

@ara It's not a straightforward task unfortunately. You could use SkeletonRenderer.OnPostProcessVertices to e.g. encode the location data in the unused vertex normals, and access SkeletonRenderer.Skeleton.DrawOrder to know which vertex corresponds to which attachment. This will however fail if clipping is active and is removing vertices and adding new ones.

If modifying the spine-unity source code is an option, you could also modify the MeshGenerator code to directly add location information.

A different solution would be to add a second texture atlas layer (like a normal-map) which contains only the black & white gradient. You could do this either by programmatically packing your attachments in the same layout as normal attachment images (with disabled whitespace stripping, or identical transparent areas). Or alternatively you could draw over the atlas image after it has been exported. You then however need to redo or re-arrange parts of the atlas when added attachments changes the packing layout.

Harald

@ara My I ask what exactly you are planning to do with the gradient? If we know what you want to use it for, perhaps we can suggest a better alternative which does not need any gradients.