o3de/Gems/Atom/Feature/Common/Assets/Materials/Pipelines/Common/MeshMotionVector.azsli

/*
 * Copyright (c) Contributors to the Open 3D Engine Project.
 * For complete copyright and license terms please see the LICENSE at the root of this distribution.
 *
 * SPDX-License-Identifier: Apache-2.0 OR MIT
 *
 */
 

#define MATERIALPIPELINE_SHADER_HAS_PIXEL_STAGE 0 // Well, it does have a pixel stage, but not one that can be customized.
#define MATERIALPIPELINE_USES_PREV_VERTEX_POSITION 1

//////////////////////////////////////////////////////////////////////////////////////////////////

#include MATERIAL_TYPE_AZSLI_FILE_PATH

//////////////////////////////////////////////////////////////////////////////////////////////////

#include <scenesrg_all.srgi>
#include <viewsrg_all.srgi>

struct PsOutput
{
    float2 m_motion : SV_Target0;
};

VsOutput VertexShader(VsInput IN, uint instanceId : SV_InstanceID)
{
    VsSystemValues SV;
    SV.m_instanceId = instanceId;
    VsOutput OUT = EvaluateVertexGeometry(IN, SV);
    return OUT;
}

PsOutput PixelShader(VsOutput IN)
{
    PsOutput OUT;

    // Current clip position
    float4 clipPos = mul(ViewSrg::m_viewProjectionMatrix, float4(IN.m_worldPos, 1.0));
    
    // Reprojected last frame's clip position, for skinned mesh it also implies last key frame
    float4 clipPosPrev = mul(ViewSrg::m_viewProjectionPrevMatrix, float4(IN.m_worldPosPrev, 1.0));

    float2 motion = (clipPos.xy / clipPos.w - clipPosPrev.xy / clipPosPrev.w) * 0.5;

    OUT.m_motion = motion;
    
    // Flip y to line up with uv coordinates
    OUT.m_motion.y = -OUT.m_motion.y;
    
    return OUT;
}