UnrealEngineUWP/Engine/Shaders/VelocityCommon.usf

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

/*=============================================================================
	VelocityCommon.usf: Common functions for calculating velocity vectors.
=============================================================================*/

#pragma once

float2 Calculate2DVelocity(float4 PackedVelocityA, float4 PackedVelocityC)
{
	float2 ScreenPosition = PackedVelocityA.xy / PackedVelocityA.w - View.ViewToClip[2].xy;
	float2 PrevScreenPosition = PackedVelocityC.xy / PackedVelocityC.w - View.PrevProjection[2].xy;

	// 2d velocity, includes camera an object motion
	float2 Velocity = ScreenPosition - PrevScreenPosition;

	// Make sure not to touch 0,0 which is clear color
	return Velocity;
}