URP下让美术同学困惑的多主光计算

float4 _MainLightPosition;
half4 _MainLightColor;

           Light sunLight = RenderSettings.sun;
            int brightestDirectionalLightIndex = -1;
            float brightestLightIntensity = 0.0f;
            for (int i = 0; i < totalVisibleLights; ++i)
            {
                VisibleLight currVisibleLight = visibleLights[i];
                Light currLight = currVisibleLight.light;

                // Particle system lights have the light property as null. We sort lights so all particles lights
                // come last. Therefore, if first light is particle light then all lights are particle lights.
                // In this case we either have no main light or already found it.
                if (currLight == null)
                    break;

                if (currVisibleLight.lightType == LightType.Directional)
                {
                    // Sun source needs be a directional light
                    if (currLight == sunLight)
                        return i;

                    // In case no sun light is present we will return the brightest directional light
                    if (currLight.intensity > brightestLightIntensity)
                    {
                        brightestLightIntensity = currLight.intensity;
                        brightestDirectionalLightIndex = i;
                    }
                }
            }

#ifdef _ADDITIONAL_LIGHTS
    uint pixelLightCount = GetAdditionalLightsCount();
    for (uint lightIndex = 0u; lightIndex < pixelLightCount; ++lightIndex)
    {
        Light light = GetAdditionalLight(lightIndex, inputData.positionWS, shadowMask);
        #if defined(_SCREEN_SPACE_OCCLUSION)
            light.color *= aoFactor.directAmbientOcclusion;
        #endif
        color += LightingPhysicallyBased(brdfData, brdfDataClearCoat,
                                         light,
                                         inputData.normalWS, inputData.viewDirectionWS,
                                         surfaceData.clearCoatMask, specularHighlightsOff);
    }
#endif

real4 unity_LightData;
real4 unity_LightIndices[2];

int GetAdditionalLightsCount()
{
    // TODO: we need to expose in SRP api an ability for the pipeline cap the amount of lights
    // in the culling. This way we could do the loop branch with an uniform
    // This would be helpful to support baking exceeding lights in SH as well
    return min(_AdditionalLightsCount.x, unity_LightData.y);
}

 uint pixelLightCount = GetAdditionalLightsCount();
    for (uint lightIndex = 0u; lightIndex < pixelLightCount; ++lightIndex)
    {
        Light light = GetAdditionalLight(lightIndex, inputData.positionWS, shadowMask);

int GetPerObjectLightIndex(uint index)
{
/////////////////////////////////////////////////////////////////////////////////////////////
// Structured Buffer Path                                                                   /
//                                                                                          /
// Lights and light indices are stored in StructuredBuffer. We can just index them.         /
// Currently all non-mobile platforms take this path :(                                     /
// There are limitation in mobile GPUs to use SSBO (performance / no vertex shader support) /
/////////////////////////////////////////////////////////////////////////////////////////////
#if USE_STRUCTURED_BUFFER_FOR_LIGHT_DATA
    uint offset = unity_LightData.x;
    return _AdditionalLightsIndices[offset + index];

/////////////////////////////////////////////////////////////////////////////////////////////
// UBO path                                                                                 /
//                                                                                          /
// We store 8 light indices in float4 unity_LightIndices[2];                                /
// Due to memory alignment unity doesn't support int[] or float[]                           /
// Even trying to reinterpret cast the unity_LightIndices to float[] won't work             /
// it will cast to float4[] and create extra register pressure. :(                          /
/////////////////////////////////////////////////////////////////////////////////////////////
#elif !defined(SHADER_API_GLES)
    // since index is uint shader compiler will implement
    // div & mod as bitfield ops (shift and mask).

    // TODO: Can we index a float4? Currently compiler is
    // replacing unity_LightIndicesX[i] with a dp4 with identity matrix.
    // u_xlat16_40 = dot(unity_LightIndices[int(u_xlatu13)], ImmCB_0_0_0[u_xlati1]);
    // This increases both arithmetic and register pressure.
    return unity_LightIndices[index / 4][index % 4];
#else
    // Fallback to GLES2. No bitfield magic here :(.
    // We limit to 4 indices per object and only sample unity_4LightIndices0.
    // Conditional moves are branch free even on mali-400
    // small arithmetic cost but no extra register pressure from ImmCB_0_0_0 matrix.
    half2 lightIndex2 = (index < 2.0h) ? unity_LightIndices[0].xy : unity_LightIndices[0].zw;
    half i_rem = (index < 2.0h) ? index : index - 2.0h;
    return (i_rem < 1.0h) ? lightIndex2.x : lightIndex2.y;
#endif
}