UnrealEngineUWP/Engine/Shaders/ScreenSpaceRayCast.usf

// Copyright 1998-2015 Epic Games, Inc. All Rights Reserved.

#pragma once

#include "Common.usf"	

// 0:reference (slower, potentially higher quality) 1:use mips of the HZB depth (better performance)
#define USE_HZB		1

#define SCALAR_BRANCHLESS		0
#define VECTORIZED_BRANCHLESS	0
#define VECTORIZED_EARLY_OUT	1

float4 SampleDepthTexture( Texture2D Texture, SamplerState Sampler, float Level, float4 SampleUV0, float4 SampleUV1 )
{
	float4 SampleDepth;
#if USE_HZB
	SampleDepth.x = Texture2DSampleLevel( Texture, Sampler, SampleUV0.xy, Level ).r;
	SampleDepth.y = Texture2DSampleLevel( Texture, Sampler, SampleUV0.zw, Level ).r;
	SampleDepth.z = Texture2DSampleLevel( Texture, Sampler, SampleUV1.xy, Level ).r;
	SampleDepth.w = Texture2DSampleLevel( Texture, Sampler, SampleUV1.zw, Level ).r;
#else
	SampleDepth.x = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV0.xy, 0 ).r;
	SampleDepth.y = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV0.zw, 0 ).r;
	SampleDepth.z = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV1.xy, 0 ).r;
	SampleDepth.w = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV1.zw, 0 ).r;
#endif
	return SampleDepth;
}

float4 RayCast( Texture2D Texture, SamplerState Sampler, float3 R, float Roughness, float SceneDepth, float3 PositionTranslatedWorld, int NumSteps, float StepOffset )
{
	// TODO provide RayStartUVz

	// NOTE could clip ray against frustum planes

	// TODO use screen position and skip matrix mul
	float4 RayStartClip	= mul( float4( PositionTranslatedWorld, 1 ), View.TranslatedWorldToClip );
	float4 RayEndClip	= mul( float4( PositionTranslatedWorld + R * SceneDepth, 1 ), View.TranslatedWorldToClip );

	float3 RayStartScreen = RayStartClip.xyz / RayStartClip.w;
	float3 RayEndScreen = RayEndClip.xyz / RayEndClip.w;

	// Normalize 2D length
	float3 RayStepScreen = ( RayEndScreen - RayStartScreen ) / length( RayEndScreen.xy - RayStartScreen.xy );
	RayStepScreen *= 1.5;

#if USE_HZB
	float3 RayStartUVz = float3( RayStartScreen.xy * float2( 0.5, -0.5 ) + 0.5, RayStartScreen.z );
	float3 RayStepUVz = float3( RayStepScreen.xy * float2( 0.5, -0.5 ), RayStepScreen.z );
#else
	float3 RayStartUVz = float3( RayStartScreen.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz, RayStartScreen.z );
	float3 RayStepUVz = float3( RayStepScreen.xy * View.ScreenPositionScaleBias.xy, RayStepScreen.z );
#endif
	
	const float Step = 1.0 / (NumSteps + 1);

	// *2 to get less morie pattern in extreme cases, larger values make object appear not grounded in reflections
	const float CompareTolerance = abs( RayStepUVz.z ) * Step * 2;

	// avoid bugs with early returns inside of loops on certain platform compilers.
	float4 Result = float4( 0, 0, 0, 1 );

#if SCALAR_BRANCHLESS

	float MinHitTime = 1;
	float LastDiff = 0;

	float SampleTime = StepOffset * Step + Step;

	UNROLL
	for( int i = 0; i < NumSteps; i++ )
	{
		float3 SampleUVz = RayStartUVz + RayStepUVz * SampleTime;
		
		// Use lower res for farther samples
		float Level = Roughness * (i * 4.0 / NumSteps);
		float SampleDepth = Texture.SampleLevel( Sampler, SampleUVz.xy, Level ).r;

		float DepthDiff = SampleUVz.z - SampleDepth;
		bool Hit = abs( DepthDiff + CompareTolerance ) < CompareTolerance;

		// Find more accurate hit using line segment intersection
		float TimeLerp = saturate( LastDiff / (LastDiff - DepthDiff) );
		float IntersectTime = SampleTime + TimeLerp * Step - Step;
		float HitTime = Hit ? IntersectTime : 1;
		MinHitTime = min( MinHitTime, HitTime );

		LastDiff = DepthDiff;

		SampleTime += Step;
	}

	float3 HitUVz = RayStartUVz + RayStepUVz * MinHitTime;
#if USE_HZB
	HitUVz.xy = HitUVz.xy * float2( 2, -2 ) + float2( -1, 1 );
	HitUVz.xy = HitUVz.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;
#endif
	Result = float4( HitUVz, MinHitTime );

#elif VECTORIZED_BRANCHLESS

	float MinHitTime = 1;
	float LastDiff = 0;
	float Level = 0;
	
	// Vectorized to group fetches
	float4 SampleTime = ( StepOffset + float4( 1, 2, 3, 4 ) ) * Step;
	float4 SampleUV0 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.xxyy;
	float4 SampleUV1 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.zzww;
	float4 SampleZ   = RayStartUVz.zzzz + RayStepUVz.zzzz * SampleTime;

	LOOP
	for( int i = 0; i < NumSteps; i += 4 )
	{
		// Use lower res for farther samples
		float4 SampleDepth = SampleDepthTexture( Texture, Sampler, Level, SampleUV0, SampleUV1 );

		#if 1
			float4 DepthDiff = SampleZ - SampleDepth;
			float4 IntersectTime = ( SampleDepth - RayStartUVz.zzzz ) / RayStepUVz.zzzz;
		
			bool4 Hit = abs( DepthDiff + CompareTolerance ) < CompareTolerance;
	
			// If hit set to intersect time. If missed set to 1, beyond end of ray
			float4 HitTime = Hit ? IntersectTime : 1;

			// Take closest hit
			HitTime.xy = min( HitTime.xy, HitTime.zw );
			MinHitTime = min( HitTime.x, HitTime.y );
		#else
			// Line segment intersection
			float4 DepthDiff1 = SampleZ - SampleDepth;
			float4 DepthDiff0 = float4( LastDiff, DepthDiff1.xyz );
			float4 TimeLerp = saturate( DepthDiff0 / (DepthDiff0 - DepthDiff1) );
			float4 IntersectTime = SampleTime + (TimeLerp - 1) / (NumSteps + 1);
		
			bool4 Hit = abs( DepthDiff1 + CompareTolerance ) < CompareTolerance;
	
			// If hit set to intersect time. If missed set to 1, beyond end of ray
			float4 HitTime = Hit ? IntersectTime : 1;

			// Take closest hit
			HitTime.xy = min( HitTime.xy, HitTime.zw );
			MinHitTime = min( MinHitTime, min( HitTime.x, HitTime.y ) );

			LastDiff = DepthDiff1.w;
		#endif
		Level += Roughness * (16.0 / NumSteps);

		SampleTime	+= 4.0 / (NumSteps + 1);
		SampleUV0	+= RayStepUVz.xyxy * 4.0 / (NumSteps + 1);
		SampleUV1	+= RayStepUVz.xyxy * 4.0 / (NumSteps + 1);
		SampleZ		+= RayStepUVz.zzzz * 4.0 / (NumSteps + 1);
	}

	float3 HitUVz = RayStartUVz + RayStepUVz * MinHitTime;
#if USE_HZB
	HitUVz.xy = HitUVz.xy = HitUVz.xy * float2( 2, -2 ) + float2( -1, 1 );
	HitUVz.xy = HitUVz.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;
#endif
	Result = float4( HitUVz, MinHitTime );

#else // VECTORIZED_EARLY_OUT

	float LastDiff = 0;
	float Level = 0;

	float4 SampleTime = ( StepOffset + float4( 1, 2, 3, 4 ) ) * Step;

	LOOP
	for( int i = 0; i < NumSteps; i += 4 )
	{
		// Vectorized to group fetches
		float4 SampleUV0 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.xxyy;
		float4 SampleUV1 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.zzww;
		float4 SampleZ   = RayStartUVz.zzzz + RayStepUVz.zzzz * SampleTime;
		
		// Use lower res for farther samples
		float4 SampleDepth = SampleDepthTexture( Texture, Sampler, Level, SampleUV0, SampleUV1 );

		float4 DepthDiff1 = SampleZ - SampleDepth;
		bool4 Hit = abs( -DepthDiff1 - CompareTolerance ) < CompareTolerance;

		BRANCH if( any( Hit ) )
		{
			// Find more accurate hit using line segment intersection
			float4 DepthDiff0 = float4( LastDiff, DepthDiff1.xyz );
			float4 TimeLerp = saturate( DepthDiff0 / (DepthDiff0 - DepthDiff1) );
			float4 IntersectTime = SampleTime + (TimeLerp - 1) / (NumSteps + 1);
			float4 HitTime = Hit ? IntersectTime : 1;

			// Take closest hit
			HitTime.xy = min( HitTime.xy, HitTime.zw );
			float MinHitTime = min( HitTime.x, HitTime.y );
			
			float3 HitUVz = RayStartUVz + RayStepUVz * MinHitTime;
#if USE_HZB
			HitUVz.xy = HitUVz.xy * float2( 2, -2 ) + float2( -1, 1 );
			HitUVz.xy = HitUVz.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;
#endif
			Result = float4( HitUVz, MinHitTime );
			break;
		}

		LastDiff = DepthDiff1.w;
		Level += Roughness * (16.0 / NumSteps);

		SampleTime += 4.0 / (NumSteps + 1);
	}

#endif

	return Result;
}

float4 RayCastCheap( Texture2D Texture, SamplerState Sampler, float3 R, float SceneDepth, float3 PositionTranslatedWorld, float StepOffset )
{
	// TODO provide RayStartUVz

	// NOTE could clip ray against frustum planes

	// TODO use screen position and skip matrix mul
	float4 RayStartClip	= mul( float4( PositionTranslatedWorld, 1 ), View.TranslatedWorldToClip );
	float4 RayEndClip	= mul( float4( PositionTranslatedWorld + R * SceneDepth, 1 ), View.TranslatedWorldToClip );

	float3 RayStartScreen = RayStartClip.xyz / RayStartClip.w;
	float3 RayEndScreen = RayEndClip.xyz / RayEndClip.w;

	// Normalize 2D length
	float3 RayStepScreen = ( RayEndScreen - RayStartScreen ) / length( RayEndScreen.xy - RayStartScreen.xy );
	RayStepScreen *= 0.75;

#if USE_HZB
	float3 RayStartUVz = float3( RayStartScreen.xy * float2( 0.5, -0.5 ) + 0.5, RayStartScreen.z );
	float3 RayStepUVz = float3( RayStepScreen.xy * float2( 0.5, -0.5 ), RayStepScreen.z );
#else
	float3 RayStartUVz = float3( RayStartScreen.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz, RayStartScreen.z );
	float3 RayStepUVz = float3( RayStepScreen.xy * View.ScreenPositionScaleBias.xy, RayStepScreen.z );
#endif
	
	const float Step = 1.0 / (4 + 1);

	// *2 to get less morie pattern in extreme cases, larger values make object appear not grounded in reflections
	const float CompareTolerance = abs( RayStepUVz.z ) * Step * 2;

	// Vectorized to group fetches
	float4 SampleTime = ( StepOffset + float4( 1, 2, 3, 4 ) ) * Step;
	float4 SampleUV0 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.xxyy;
	float4 SampleUV1 = RayStartUVz.xyxy + RayStepUVz.xyxy * SampleTime.zzww;
	float4 SampleZ   = RayStartUVz.zzzz + RayStepUVz.zzzz * SampleTime;

	float4 SampleDepth;
#if USE_HZB
	SampleDepth.x = Texture2DSampleLevel( Texture, Sampler, SampleUV0.xy, 0 ).r;
	SampleDepth.y = Texture2DSampleLevel( Texture, Sampler, SampleUV0.zw, 0 ).r;
	SampleDepth.z = Texture2DSampleLevel( Texture, Sampler, SampleUV1.xy, 1 ).r;
	SampleDepth.w = Texture2DSampleLevel( Texture, Sampler, SampleUV1.zw, 1 ).r;
#else
	SampleDepth.x = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV0.xy, 0 ).r;
	SampleDepth.y = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV0.zw, 0 ).r;
	SampleDepth.z = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV1.xy, 0 ).r;
	SampleDepth.w = Texture2DSampleLevel( SceneDepthTexture, SceneDepthTextureSampler, SampleUV1.zw, 0 ).r;
#endif

	#if 1
		float4 DepthDiff = SampleZ - SampleDepth;
		float4 IntersectTime = ( SampleDepth - RayStartUVz.zzzz ) / RayStepUVz.zzzz;
		
		bool4 Hit = abs( DepthDiff + CompareTolerance ) < CompareTolerance;
	
		// If hit set to intersect time. If missed set to 1, beyond end of ray
		float4 HitTime = Hit ? IntersectTime : 1;

		// Take closest hit
		HitTime.xy = min( HitTime.xy, HitTime.zw );
		float MinHitTime = min( HitTime.x, HitTime.y );
	#else
		// Line segment intersection
		float4 DepthDiff1 = SampleZ - SampleDepth;
		float4 DepthDiff0 = float4( 0, DepthDiff1.xyz );
		float4 TimeLerp = saturate( DepthDiff0 / (DepthDiff0 - DepthDiff1) );
		float4 IntersectTime = SampleTime + (TimeLerp - 1) * Step;
		
		bool4 Hit = abs( DepthDiff1 + CompareTolerance ) < CompareTolerance;
	
		// If hit set to intersect time. If missed set to 1, beyond end of ray
		float4 HitTime = Hit ? IntersectTime : 1;

		// Take closest hit
		HitTime.xy = min( HitTime.xy, HitTime.zw );
		float MinHitTime = min( HitTime.x, HitTime.y );
	#endif

	float3 HitUVz = RayStartUVz + RayStepUVz * MinHitTime;

	BRANCH
	if( MinHitTime == 1 && RayStepUVz.z < 0 )
	{
		float4 RayInfClip	= mul( float4( PositionTranslatedWorld + R * 2000000, 1 ), View.TranslatedWorldToClip );
		float3 RayInfScreen = RayInfClip.xyz / RayInfClip.w;
		float3 RayInfUVz = float3( RayInfScreen.xy * float2( 0.5, -0.5 ) + 0.5, RayInfScreen.z );

		HitUVz = RayInfUVz;
		MinHitTime = 0.5;
	}

#if USE_HZB
	HitUVz.xy = HitUVz.xy = HitUVz.xy * float2( 2, -2 ) + float2( -1, 1 );
	HitUVz.xy = HitUVz.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;
#endif
	return float4( HitUVz, MinHitTime );
}

float4 SampleScreenColor( Texture2D Texture, SamplerState Sampler, float3 HitUVz )
{
	float4 OutColor;

#if PREV_FRAME_COLOR
	// Find previous screen position for hit since color buffer is from last frame
	// TODO combine to single matrix
	float4 HitClip = float4( (HitUVz.xy - View.ScreenPositionScaleBias.wz) / View.ScreenPositionScaleBias.xy, HitUVz.z, 1 );
	float4 HitTranslatedWorld = mul( HitClip, View.ClipToTranslatedWorld );
	HitTranslatedWorld /= HitTranslatedWorld.w;
		
	float3 PrevTranslatedWorld = HitTranslatedWorld.xyz + (View.PrevPreViewTranslation - View.PreViewTranslation);
	float4 PrevClip = mul( float4( PrevTranslatedWorld, 1 ), View.PrevTranslatedWorldToClip );
	float2 PrevScreen = PrevClip.xy / PrevClip.w;
	float2 PrevUV = PrevScreen.xy * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;

	OutColor.rgb = Texture.SampleLevel( Sampler, PrevUV, 0 ).rgb;
	OutColor.a = 1;

	// Off screen masking
	float2 Vignette = saturate( abs( PrevScreen ) * 5 - 4 );
#else
	OutColor.rgb = Texture.SampleLevel( Sampler, HitUVz.xy, 0 ).rgb;
	OutColor.a = 1;

	// Off screen masking
	float2 HitScreenPos = ( HitUVz.xy - View.ScreenPositionScaleBias.wz ) / View.ScreenPositionScaleBias.xy;
	float2 Vignette = saturate( abs( HitScreenPos ) * 5 - 4 );
#endif
		
	//PrevScreen sometimes has NaNs or Infs.  DX11 is protected because saturate turns NaNs -> 0.
	//Do a SafeSaturate so other platforms get the same protection.
	OutColor *= SafeSaturate( 1.0 - dot( Vignette, Vignette ) );

	// Transform NaNs to black, transform negative colors to black.
	OutColor.rgb = -min(-OutColor.rgb, 0.0);

	return OutColor;
}