BlueRoseNote/03-UnrealEngine/Rendering/RenderingPipeline/Materials&MeshDraw/材质中PassSwitch节点实现方式.md

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title	date	excerpt	tags	rating
Untitled	2024-09-23 19:47:03			⭐

ShadowPassSwitch

Cpp

MaterialExpressionShadowReplace.h

MaterialExpressions.cpp

int32 UMaterialExpressionShadowReplace::Compile(class FMaterialCompiler* Compiler, int32 OutputIndex)
{
	if (!Default.GetTracedInput().Expression)
	{
		return Compiler->Errorf(TEXT("Missing input Default"));
	}
	else if (!Shadow.GetTracedInput().Expression)
	{
		return Compiler->Errorf(TEXT("Missing input Shadow"));
	}
	else
	{
		const int32 Arg1 = Default.Compile(Compiler);
		const int32 Arg2 = Shadow.Compile(Compiler);
		return Compiler->ShadowReplace(Arg1, Arg2);
	}
}

MaterialCompiler.h => HLSLMaterialTranslator.cpp

int32 FHLSLMaterialTranslator::ShadowReplace(int32 Default, int32 Shadow)  
{  
    return GenericSwitch(TEXT("GetShadowReplaceState()"), Shadow, Default);  
}

int32 FHLSLMaterialTranslator::GenericSwitch(const TCHAR* SwitchExpressionText, int32 IfTrue, int32 IfFalse)
{
	if (IfTrue == INDEX_NONE || IfFalse == INDEX_NONE)
	{
		return INDEX_NONE;
	}

	// exactly the same inputs on both sides - no need to generate anything extra
	if (IfTrue == IfFalse)
	{
		return IfTrue;
	}

	FMaterialUniformExpression* IfTrueExpression = GetParameterUniformExpression(IfTrue);
	FMaterialUniformExpression* IfFalseExpression = GetParameterUniformExpression(IfFalse);
	if (IfTrueExpression &&
		IfFalseExpression &&
		IfTrueExpression->IsConstant() &&
		IfFalseExpression->IsConstant())
	{
		FMaterialRenderContext DummyContext(nullptr, *Material, nullptr);
		FLinearColor IfTrueValue;
		FLinearColor IfFalseValue;
		IfTrueExpression->GetNumberValue(DummyContext, IfTrueValue);
		IfFalseExpression->GetNumberValue(DummyContext, IfFalseValue);
		if (IfTrueValue == IfFalseValue)
		{
			// If both inputs are wired to == constant values, avoid adding the runtime switch
			// This will avoid breaking various offline checks for constant values
			return IfTrue;
		}
	}

	// Both branches of '?:' need to be the same type
	const EMaterialValueType ResultType = GetArithmeticResultType(IfTrue, IfFalse);
	const FString IfTrueCode = CoerceParameter(IfTrue, ResultType);
	const FString IfFalseCode = CoerceParameter(IfFalse, ResultType);
	
	if (IsLWCType(ResultType))
	{
		AddLWCFuncUsage(ELWCFunctionKind::Other);
		return AddCodeChunk(ResultType, TEXT("LWCSelect(%s, %s, %s)"), SwitchExpressionText, *IfTrueCode, *IfFalseCode);
	}
	else
	{
		return AddCodeChunk(ResultType, TEXT("(%s ? (%s) : (%s))"), SwitchExpressionText, *IfTrueCode, *IfFalseCode);
	}
}

可以看得出最终会编译成(%s ? (%s) : (%s))，也就是 GetShadowReplaceState() ? True的Shader ： False时的Shader。

Shader

Common.ush

// NOTE: The raytraced implementation of the ShadowPassSwitch node is kept in RayTracingShaderUtils.ush as it needs to access per ray information.  
#if RAYHITGROUPSHADER == 0  
// Experimental way to allow adjusting the OpacityMask for shadow map rendering of masked materials.  
// This is exposed via the ShadowPassSwitch material node. This can also be accessed with a Custom  
// material node. If this turns out to be very useful we can expose as MaterialFunction  
// and potentially expose other queries as well (e.g. SkeletalMesh, HitProxy, ).  
// @return 0:no, 1:yes  
bool GetShadowReplaceState()  
{  
#if SHADOW_DEPTH_SHADER  
    return true;  
#else  
    return false;  
#endif  
}  
  
float IsShadowDepthShader()  
{  
    return GetShadowReplaceState() ? 1.0f : 0.0f;  
}  
  
#endif // RAYHITGROUPSHADER == 0

可以看得出主要通过SHADOW_DEPTH_SHADER宏进行判断，而渲染阴影的Shader ShadowDepthVertexShader.usf&ShadowDepthPixelShader.usf都定义该宏为1。

NaniteRasterizer.usf

// This must be defined before including Common.ush (see GetShadowReplaceState)  
#define SHADOW_DEPTH_SHADER DEPTH_ONLY

在NaniteCullRaster.cpp中DEPTH_ONLY设置为1。

RayTracingShaderUtils.ush

光追相关

#ifndef RAYHITGROUPSHADER  
#error "This header should only be included in raytracing contexts"  
#endif  
  
#ifndef PATH_TRACING // Path Tracing has a similar implemental with a slightly different set of flags  
  
#include "RayTracingCommon.ush"  
  
static int CurrentPayloadInputFlags = 0;  
  
bool GetShadowReplaceState()  
{  
    return (CurrentPayloadInputFlags & RAY_TRACING_PAYLOAD_INPUT_FLAG_SHADOW_RAY) != 0;  
}  
  
float IsShadowDepthShader()  
{  
    return GetShadowReplaceState() ? 1.0f : 0.0f;  
}  
  
#endif