SkeletonGraphic renders incorrect when using etc compression

Fish

In some devices I have to use ETC compression for images. I'm using SkeletonGraphic in Canvas.
I know ETC compression doesn't have alpha channel. So I'm trying to modify the Spine-Skeleton.shader to get the AlphaTexture. But I found it's not enough. When I use "multiple images", or "Additive-Materials", I have to modify too many scripts.
Is there any existing solutions to use for ETC compression?

Spinebot

I understand you're facing issues with ETC compression when using SkeletonGraphic, since ETC doesn't support alpha channels.

Important: You should not use the Spine-Skeleton.shader with SkeletonGraphic. Only use Materials with special CanvasRenderer compatible shaders at SkeletonGraphic components, such as the Spine/SkeletonGraphic* shaders. Using non-UI shaders like Spine/Skeleton with a SkeletonGraphic component can cause incorrect rendering, especially on target mobile devices, even if it appears to work in the Unity Editor.

For ETC compression support:

Unfortunately, there isn't a built-in solution in the spine-unity runtime specifically designed for ETC compression with separate alpha textures. You would need to:

Use the appropriate Spine/SkeletonGraphic* shaders as your base (not Spine/Skeleton)
Modify these UI-compatible shaders to sample from a separate alpha texture
If using multiple textures (via Advanced - Multiple CanvasRenderers), you'll need to modify the blend mode materials as well (SkeletonGraphicAdditive, SkeletonGraphicMultiply, SkeletonGraphicScreen)

Keep in mind that SkeletonGraphic is limited to a single texture by default due to CanvasRenderer limitations. If you're using multiple atlas pages, you'll need to enable Advanced - Multiple CanvasRenderers, which generates child CanvasRenderer GameObjects for each submesh. For better performance, consider packing your skeleton to a single atlas page texture where possible.

Fish

Spinebot I modified the SkeletonGraphic.cs and add canvasRenderer.SetAlphaTexture. Also I modify the Spine-SkeletonGraphic-NormalPass.cginc and add color.a *= tex2D(_AlphaTex, IN.texcoord).r;.
It works all right now.
Is there anything else I should consern?

Harald

@Fish Glad to hear you've figured it out, thanks for getting back to us. FYI: Spinebot only replies once for each thread.

Fish Also I modify the Spine-SkeletonGraphic-NormalPass.cginc and add color.a *= tex2D(_AlphaTex, IN.texcoord).r;.

Is there anything else I should consern?

Depending on which shader you're modifying and where you place a statement like color.a = .., you just need to make sure that the statement is either going before alpha is pre-multiplied onto the color.rgb values like color.rgb *= color.a, or if not possible, be sure to apply that pre-multiplication yourself. In the case of SkeletonGraphic shaders it's usually automatically done in the last few lines of the shader before returning the result color.

In general, whenever you see Blend One OneMinusSrcAlpha in the .shader file, the shader is using premultiplied alpha output blend mode.

For more info on what is relevant in writing (or modifying) your own shaders see the documentation here:
https://esotericsoftware.com/spine-unity-rendering#Writing-your-own-shaders