22. URP Lit Call Flow & Edit Points

This chapter is a practical guide to end “I studied URP Lit, but I don’t know where to fix it.”

• Lit Forward (UniversalForward) Fixes the actual flow of the fragment to the “correct reference” and
• Where to put customization so as not to break compatibility (=edit points)
• What has a high risk of breaking immediately when touched (=risk points)?

We organize it step by step.

22.0 Accurate Reference (IDE)

The symbols/definitions/references in this chapter refer to the IDE references below as the primary source.

• Lit core symbol xref: @@TOK_0_8394c928@@
• Function signature: @@TOK_1_20e1594e@@
• Structure/Field: @@TOK_2_11a7626e@@
• Lit Pass/Include Map: @@TOK_3_14164784@@

22.1 Fixed Lit Forward entry (definition location/signature)

As of URP 17.3.0:

• LitPassVertex
  • Definition: <URP>/Shaders/LitForwardPass.hlsl:158
  • Signature: Varyings LitPassVertex(Attributes input)
• LitPassFragment
  • Definition: <URP>/Shaders/LitForwardPass.hlsl:223
  • Core: void + out SV_Target* output

Output structure (summary) of LitPassFragment:

void LitPassFragment(
    Varyings input,
    out half4 outColor : SV_Target0
    // + optional: outRenderingLayers : SV_Target1
)

Why not return half4?
For extending features such as MRT/Rendering Layers, the “keep output contract as out parameter” structure is safer.
Related: 16. URP Lit include 체인 맵

22.2 Actual call flow of Lit Forward (URP 17.3.0)

The core flow of URP 17.3.0 LitForwardPass is summarized below (exact call points are in xref):

1. (Optional) Parallax / LODFade
2. Configure SurfaceData with InitializeStandardLitSurfaceData
3. Configure InputData with InitializeInputData
4. (Optional) Decal(DBuffer)
5. Prepare baked GI with InitializeBakedGIData etc.
6. Lighting synthesis with UniversalFragmentPBR
7. Output after Fog/Alpha processing

Core function definition location (URP 17.3.0):

Fix the flow with Mermaid:

22.3 Edit Points (Safe Customization Points) — “Where can I change something to make it less broken?”

URP Lit is tightly coupled with surrounding features (Forward+/Decal/Rendering Layers/GI/ShadowMixing, etc.). Therefore, the safest way to “customize while maintaining compatibility” is to adjust internal values ​​while maintaining the light loop/pass contract.

22.3.1 Edit Point A: Adjust SurfaceData (most secure)

Target:

• surfaceData.albedo, surfaceData.normalTS, surfaceData.smoothness, surfaceData.emission, etc.

Advantages:

• Material model can be changed while maintaining Pass contract/Forward+ loop/Shadow/GI coupling

Risk:

• Low (however, when combined with alpha/_ALPHATEST_ON, ShadowCaster/DepthOnly needs to be checked)

Verification:

• Check if ShadowCaster/DepthOnly/DepthNormals are normal together in Frame Debugger
• Check whether SRP Batcher (material constant) is maintained (Chapter 09)

22.3.2 Edit Point B: Adjust InputData (medium difficulty)

Target:

• inputData.normalWS, inputData.viewDirectionWS, inputData.shadowCoord, inputData.normalizedScreenSpaceUV, etc.

Advantages:

• Easy to add view/space based effects (e.g. custom fresnel, view-dependent term)

Risk:

• medium
  • In particular, messing with normalizedScreenSpaceUV may break the Forward+ cluster loop.

Verification:

• Check if additional light is normal in Forward+ ON (Chapter 07)

22.3.3 Edit Point C: UniversalFragmentPBR Adjust color before/after (only when the purpose is clear)

Target:

• Post-process the half4 color = UniversalFragmentPBR(...) results (rim, toon, custom tone) or
• Expression for normalizing PBR input (e.g. roughness clamp)

Risk:

• Medium to high
  • Physics-based assumptions may be broken and unexpected results may occur when combined with Debug Display/Decal/Rendering Layers.

Verification:

• Check with Debug Display/Post Processing/Decal turned on

22.3.4 Risk Point: “Implement additional light loops yourself” (most risky)

In Forward+, there is a contract that GetAdditionalLightsCount() can be 0.

• Definition: <URP>/ShaderLibrary/RealtimeLights.hlsl:271

So if you need to implement additional light loops yourself, at least use URP’s macros.

uint count = GetAdditionalLightsCount();
LIGHT_LOOP_BEGIN(count)
    Light light = GetAdditionalLight(lightIndex, inputData, shadowMask, aoFactor);
    // accumulate...
LIGHT_LOOP_END

More details: 07. Forward/Forward+/Lights

22.4 Edit Point Matrix (Summary)

22.5 Verification routine (required)

1. Fix the definition position as ide: @@TOK_4_8394c928@@
2. Frame Debugger: Check with which LightMode Pass my object is drawn.
3. Compare Forward+ ON/OFF: Check whether the additional light is normal in both modes
4. DepthNormals/MotionVectors: If the project has functions such as SSAO/TAA turned on, check whether the pass is actually consumed.

Troubleshooting Playbook: @@TOK_5_a2e9de9a@@