Transparency Level may unexpectedly affect rendered color under HDR

[shino-a]

Describe the bug
Changing the Transparency Level (_Tweak_transparency) may also affect the rendered color.
This seems to occur because saturate(x) may not be applied under HDR, which can cause alpha blend factors (SrcAlpha, OneMinusSrcAlpha) to become greater than 1 or negative values.
Is this intended behavior?

com.unity.toonshader/com.unity.toonshader/Runtime/Shaders/URP/UniversalToonBodyDoubleShadeWithFeather.hlsl

Lines 515 to 516 in 6eb6293

	float Set_Opacity = SATURATE_IF_SDR((_Inverse_Clipping_var + _Tweak_transparency));
	finalRGBA = fixed4(finalColor, Set_Opacity);

To Reproduce
Steps to reproduce the behavior:

1. Ensure HDR is enabled in both the RenderPipelineAsset and the camera.
2. Assign a Toon/Toon material with Clipping set to Transparent Clip to any model.
3. Render it overlapping with an opaque object in the scene.
4. Change Color or Transparency Level and observe the effect.

*Confirmed in URP.

Expected behavior
Transparency Level (_Tweak_transparency) should only affect transparency.

Screenshots

[sindharta-tanuwijaya]

Thanks for the detailed report and for pointing to the exact line.
To help us confirm this, could you share a minimal repro project?
That'll let us verify the exact conditions for this behavior.

[shino-a]

I've attached a minimal reproduction project.

Open Assets/ReproScene.unity and observe the rendering results of the GameObjects named ReproModel_*.
Also observe that the rendering results change when HDR is enabled or disabled in the Camera component.
issue-864-hdr-transparency-level-repro.zip

[sindharta-tanuwijaya]

Thanks for putting together the repro project.
That's really helpful. I'll take a look and follow up here.

[sindharta-tanuwijaya]

Hi @shino-a

Apologies for the late reply, and thanks again for the clear repro.
Your diagnosis was right: alpha should always be clamped to [0, 1] here.
A fix is on the way.
https://github.com/Unity-Technologies/com.unity.toonshader/pull/869/changes

Please note that Unity Toon Shader is an experimental package, so response times on issues may be slow.
Thanks for your understanding.