Linear Space UI does not respect Additive Blend Mode

gunspike

📌 Issue Summary: Spine SkeletonGraphic in Unity URP Linear Space UI does not respect Additive Blend Mode
🔧 Environment
Unity Version: [예: 2022.3.14f1 LTS]

Spine Unity Runtime: spine-unity 4.1 (latest as of Aug 2025)

Render Pipeline: URP

Color Space: Linear

Usage Context: SkeletonGraphic used in UI Canvas (Screen Space)

Export Settings:

Exported with Straight Alpha

Blend Mode: Additive used on specific slots

Used SkeletonGraphicDefault-Straight material

🐞 Problem
When using SkeletonGraphic (UI) in Unity URP Linear color space, slots with Additive blend mode from Spine do not render as intended.

All slots (normal + additive) are rendered using a single material, so additive blending does not appear correctly.

In contrast, SkeletonRenderer (for world space) automatically detects blend modes and separates materials accordingly. It renders correctly.

The same Spine asset shows visibly brighter/more intense alpha when using SkeletonGraphic, particularly with additive slots.

This makes additive effects like glows appear incorrect or overly dark in UI context.

🎯 Expected Behavior
When using SkeletonGraphic, each slot’s blend mode (Normal, Additive, Multiply, etc.) should be respected, just like in SkeletonRenderer.

Additive slots should render with correct alpha blending in linear color space using URP.

🔍 Attempts Made to Resolve
Verified export alpha mode (Straight Alpha + premultiplied also tested)

Verified Unity texture import settings (sRGB, Alpha Is Transparency, etc.)

Applied both SkeletonGraphicDefault-Straight and -PMATexture materials

Enabled Multiple Canvas Renderers, tested with separator slots

Tried custom SkeletonGraphic.CustomSlotMaterials with additive shaders

Additive slots still do not render correctly — they appear normal or have incorrect intensity.

❗ Root Cause (Suspected)
SkeletonGraphic uses only one material at runtime, so multiple blend modes cannot be expressed simultaneously.

Additive blending needs a dedicated material or render pass, which SkeletonRenderer provides, but SkeletonGraphic does not.

💡 Suggested Feature
Provide a mechanism for SkeletonGraphic to:

Automatically split slots based on blend mode (like SkeletonRenderer)

OR allow assigning multiple materials (per blend mode) to SkeletonGraphic, either via slot detection or material override.

📷 Screenshots & Files
[Attached texture import settings, shader settings, and visual comparison between SkeletonRenderer vs SkeletonGraphic]

[Optional: Unity scene files or GIFs showing difference in additive blending]

Harald

gunspike All slots (normal + additive) are rendered using a single material, so additive blending does not appear correctly.

This is a false assumption, when Advanced - PMA Vertex Colors is enabled, an additive slot does not require separate materials. This also applies to SkeletonRenderer. What is your SkeletonDataAsset Blend Mode Materials configuration regarding Additive blend mode?

gunspike Root Cause (Suspected)
SkeletonGraphic uses only one material at runtime, so multiple blend modes cannot be expressed simultaneously.

Additive blending needs a dedicated material or render pass, which SkeletonRenderer provides, but SkeletonGraphic does not.

See above. In general, in the most recent spine-unity 4.1, blend modes should be covered automatically at SkeletonGraphic already. What are your SkeletonGraphic Inspector component settings under Advanced with Multiple CanvasRenderers enabled under Blend Mode Materials? Do you see a Blend Mode Materials subsection at all when enabling Multiple CanvasRenderers?

gunspike Suggested Feature
Provide a mechanism for SkeletonGraphic to:

Automatically split slots based on blend mode (like SkeletonRenderer)

OR allow assigning multiple materials (per blend mode) to SkeletonGraphic, either via slot detection or material override.

No need to suggest a feature that already exists for quite some time 🙂. If blend mode detection fails for some reason, that would be a bug then.

gunspike

시도한 해결 방법 (Tried Workarounds)
Multiple Canvas Renderers 옵션 활성화
→ UI 렌더러가 분할되지 않음 (Renderer1, Renderer2 등 생성 안 됨)

Separator Slot Names 지정
→ GameObject가 물리적으로 분리되나 Blend Mode에 따른 자동 분리는 아님

Tried Workarounds
Enabled Multiple Canvas Renderers option
→ UI renderers (e.g., Renderer1, Renderer2) are not automatically split based on blend modes.

Set Separator Slot Names
→ GameObjects are physically split, but this does not separate slots by their blend modes automatically.

gunspike

After switching Spine's color space to Linear, I noticed that the animation looks different compared to when it was in Gamma.
I'll revise the animation accordingly.

Harald

gunspike After switching Spine's color space to Linear, I noticed that the animation looks different compared to when it was in Gamma.
I'll revise the animation accordingly.

Yes, if it looked the same there would be no such viewport option. 😉

Please note that alpha blending in Linear color space behaves quite different compared to Gamma space, some might say it behaves in a rather unintuitive way. That is why we suggest using Gamma space in Unity unless you have good reasons to use Linear color space.

gunspike

The image above is from Unity, and the one below is from Spine.
In Unity, the alpha appears noticeably stronger.
I’ve tried many different settings and approaches to resolve this issue,
but I haven’t been able to find a proper solution yet.

I’m looking forward to your response.

Harald

gunspike You can send us a minimal Unity project with just the relevant parts to reproduce your issue, then we can have a look at what's going wrong. You can send it as a zip file to contact@esotericsoftware.com, briefly mentioning this forum URL so that we know the context. Then we can have a look at it. Please also include the original Spine project (including the attachment image directory) in the package so that we can compare it more easily.

gunspike

감사합니다. 지금 답장을 확인했습니다. 내일 메일로 답장을 드리도록 하겠습니다. 항상 친절한 답변에 감사드립니다.

Misaki

gunspike Thank you for your email! We received it, but it only contained the Spine project and skeleton data files, not the minimum Unity project. In order to reproduce your issue, we need to check the settings in the Unity project, so could you please prepare and send us the minimum Unity project that can reproduce the issue?