SkeletonRenderTexture is not working correctly on web

ClamariCL

I'm using the sample SkeletonRenderTexture to fade in and fade out a spine skeleton animation. In play mode in Unity IDE it works without any issues, but when run in the web version, the sprite stays invisible while fading in/out with the SkeletonRenderTexture script enabled

Versions used:
Unity 6.3
spine-unity 4.3

Spinebot

This sounds like a platform-specific issue with how Unity handles RenderTextures in WebGL builds. Here are some things to investigate and try:

RenderTexture format compatibility

WebGL has more limited RenderTexture format support than the desktop platforms. The RenderTexture created internally by SkeletonRenderTexture may be using a format that isn't supported in WebGL. Check the Unity console for any warnings or errors related to RenderTexture format when running the WebGL build.

Premultiplied alpha considerations

WebGL rendering pipelines can behave differently with premultiplied alpha. Make sure your skeleton's materials and the SkeletonRenderTexture component are configured consistently regarding premultiplied alpha. If your atlas was exported with premultiplied alpha, ensure the corresponding material and the RenderTexture blending are set up to match.

Alternative fade approaches for WebGL

As noted in the spine-unity rendering documentation, RenderTextures are a costly solution and there are other ways to achieve a fade effect that may work more reliably across platforms:

Tinting with a solid color: gradually tint the skeleton toward black or white using the skeleton's color property instead of adjusting alpha directly.
Decreasing scale: a scale-down effect can simulate a fade-out without transparency issues.
Shader-based approach: write a custom shader that handles the transparency compositing in a single pass, avoiding the RenderTexture entirely.

Debugging steps

Open the browser's developer console and check for any WebGL-related errors or warnings when the fade is triggered.
Try reducing the RenderTexture depth buffer setting or changing the color format to ARGB32 if it isn't already.
Test with a very simple scene containing only the skeleton and the SkeletonRenderTexture to rule out other scene elements interfering.

The example scene Spine Examples/Other Examples/RenderTexture FadeOut Transparency included with the spine-unity package may also be worth building and testing directly for WebGL to see if the issue is isolated to your setup or is more general.

ClamariCL

I don't think this automated answer is useful. Is there any evidence of the SkeletonRenderTexture working on web version?

Harald

@ClamariCL Sorry to hear you're having troubles. By "when run in the web version" do you mean a WebGL build? If so, what are your project configurations regards to the player, and where do you test?

Also, which render pipeline (Built-In Render Pipeline, URP, URP 2D) are you using?
Also please be sure that all required materials and shaders are included in the build (like the render quad shader).

ClamariCL

I mean Build profile Web, the one you play in your browser and can upload to Unity Play. So yes, I understand that's WebGL. I’m using URP 17.3.0 with the 2D Renderer. The WebGL quality level is High and uses that URP asset; the project uses Linear color space.
I reproduce it both in a local Development Build served from localhost and in the build hosted on Unity Play. It works correctly in Editor Play Mode. The local browser reports a WebGL 2.0 context.
I’m using the sample SkeletonRenderTexture with shader pass 0. Diagnostics show a nonempty Spine mesh and a created R8G8B8A8_SRGB RenderTexture. The quad is active and references that texture, but reading it back during both spawn and death fade (during spawn and death animations I use a coroutine to fade in/out the sprite) gives zero pixels with nonzero alpha. So it appears the mesh isn’t being drawn into the RenderTexture in WebGL.

ClamariCL

My camera preserves a 9:16 view by adding side bars in a wide window. During one failing Web test, the screen was 1500×938, while targetCamera.pixelRect was approximately (486, 0, 528, 938). Enabling SkeletonRenderTexture made the creature invisible during both spawn and death fades; disabling it showed the normal Spine mesh.

Diagnostics showed that the source mesh and shader pass 0 were valid. The RenderTexture existed, and a manual magenta clear could be read back, but the sample’s draw produced zero pixels with alpha. Drawing the same mesh into that texture with a simple projection produced visible pixels.

The issue is in SkeletonRenderTextureBase.CalculateOrthoMatrix(): it divides full-screen coordinates by pixelRect.size without first subtracting pixelRect.position. I changed the normalization to:

Vector2 viewportOrigin = targetCamera.pixelRect.position;
Vector2 min = (new Vector2(screenSpaceMin.x, screenSpaceMin.y) - viewportOrigin) / fullSizePixels;
Vector2 max = (new Vector2(screenSpaceMax.x, screenSpaceMax.y) - viewportOrigin) / fullSizePixels;

With that change, the diagnostic Web build showed both fades correctly. RenderTexture readback then found nontransparent pixels during spawn and death. This appears to be a viewport-origin issue, rather than a WebGL texture-format issue.

Harald

@ClamariCL Thanks for the detailed report, much appreciated! We've just pushed a bugfix for this issue, a new spine-unity 4.3 unitypackage is available here as usual:
https://esotericsoftware.com/spine-unity-download

Issue ticket for later reference:
EsotericSoftware/spine-runtimes3183