// Copyright 1998-2014 Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	DistortApplyScreenPixelShader.usf: Pixel shader for rendering screen distortion pass
=============================================================================*/

#include "Common.usf"

static const half InvDistortionScaleBias = 1 / 4.0f;

/** 
 * contains accumulated distortion values as 
 * R=positive horizontal offset
 * G=positive vertical offset
 * B=negative horizontal offset
 * A=negative vertical offset 
 */
Texture2D		DistortionTexture;
SamplerState	DistortionTextureSampler;


/**
 * Contains the valid region in the scenecolor texture (in UV space).
 * Trying to sample outside this region will yield garbage colors.
 * X,Y = Upper-left corner
 * Z,W = Lower-left corner
 */
float4		SceneColorRect;

/** distorts screen texture using accumulated distortion offsets */
void Main(
	in float4 TexCoord: TEXCOORD0,
	out float4 OutColor : SV_Target0
	)
{
	// hack
//	OutColor = Texture2DSample(SceneColorTexture, SceneColorTextureSampler, TexCoord) * float4(1,0,0,0);
//	return;

	// sample accumulated distortion and apply inverse scale
	half4 AccumDist = Texture2DSample(DistortionTexture,DistortionTextureSampler,TexCoord.xy);

	// offset = [R-B,G-A] 
	half2 DistOffset = (AccumDist.rg - AccumDist.ba);
	//Scale by the screen size and a fudge factor to come close to the offset values we would have had under normal circumstances before my changes. Also flip Y and invert the precision bias scale.
	DistOffset *= InvDistortionScaleBias;
	
	
	float2 NewTexCoord = TexCoord.xy + DistOffset;

	// If we're about to sample outside the valid SceneColorRect, set to 0 distortion.
	FLATTEN if ( NewTexCoord.x < SceneColorRect.x || NewTexCoord.x > SceneColorRect.z ||
				 NewTexCoord.y < SceneColorRect.y || NewTexCoord.y > SceneColorRect.w )
	{
		NewTexCoord = TexCoord.xy;
	}

	// sample screen using offset coords
	half4 DistColor = Texture2DSample(SceneColorTexture, SceneColorTextureSampler, NewTexCoord);
	OutColor = DistColor;
}