o3de/Gems/AtomTressFX/Assets/Shaders/HairSimulationCompute.azsl

/*
* Modifications 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
*
*/

//---------------------------------------------------------------------------------------
// Shader code related to simulating hair strands in compute.
//-------------------------------------------------------------------------------------
//
// Copyright (c) 2019 Advanced Micro Devices, Inc. All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
//--------------------------------------------------------------------------------------
#include <HairSimulationComputeSrgs.azsli>
#include <HairSimulationCommon.azsli>

//--------------------------------------------------------------------------------------
//
// IntegrationAndGlobalShapeConstraints
//
// Compute shader to simulate the gravitational force with integration and to maintain the
// global shape constraints.
//
// One thread computes one vertex.
//
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void IntegrationAndGlobalShapeConstraints(
    uint GIndex : SV_GroupIndex,
    uint3 GId : SV_GroupID,
    uint3 DTid : SV_DispatchThreadID)
{
    uint globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType;
    CalcIndicesInVertexLevelMaster(GIndex, GId.x, globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType);

    // Copy data from init rest data to be used to set updated shared memory
    float4 initialPos = float4(CM_TO_METERS,CM_TO_METERS,CM_TO_METERS,1.0) * g_InitialHairPositions[globalVertexIndex]; // rest position

    // Apply bone skinning to initial position
    BoneSkinningData skinningData = g_BoneSkinningData[globalStrandIndex];
    float4 bone_quat;
    initialPos.xyz = ApplyVertexBoneSkinning(initialPos.xyz, skinningData, bone_quat);

    // position when this step starts. In other words, a position from the last step.
    sharedPos[indexForSharedMem] = GetSharedPosition(globalVertexIndex);
    float4 currentPos = sharedPos[indexForSharedMem];
    //    float4 currentPos = sharedPos[indexForSharedMem] = g_HairVertexPositions[globalVertexIndex];

    GroupMemoryBarrierWithGroupSync();

    // Integrate
    float dampingCoeff = GetDamping(strandType);
    float4 oldPos = g_HairVertexPositionsPrev[globalVertexIndex];

    // reset if we got teleported
    if (g_ResetPositions != 0.0f)
    {   // Originally part of the data here was NaN as the original TressFX code wrote number 
        // vertices including the follow hair although the shader accounts for that, hence 
        // memory was overwriten.  In our implementation the memory resides all within
        // a single buffer and this would actively overwrite the rest of the buffer hence
        // destroying the original contexnt. 
        currentPos = initialPos;
        g_HairVertexPositions[globalVertexIndex] = initialPos;
        g_HairVertexPositionsPrev[globalVertexIndex] = initialPos;
        g_HairVertexPositionsPrevPrev[globalVertexIndex] = initialPos;
        oldPos = initialPos;
    }

    // skipping all the physics simulation in between
    if ( IsMovable(currentPos) )
        sharedPos[indexForSharedMem].xyz = Integrate(currentPos.xyz, oldPos.xyz, initialPos.xyz, dampingCoeff);
    else 
        sharedPos[indexForSharedMem] = initialPos;

    // Global Shape Constraints
    float stiffnessForGlobalShapeMatching = GetGlobalStiffness(strandType);
    float globalShapeMatchingEffectiveRange = GetGlobalRange(strandType);

    if ( stiffnessForGlobalShapeMatching > 0 && globalShapeMatchingEffectiveRange )
    {
        if ( IsMovable(sharedPos[indexForSharedMem]) )
        {
            if ( (float)localVertexIndex < globalShapeMatchingEffectiveRange * (float)numVerticesInTheStrand )
            {
                float factor = stiffnessForGlobalShapeMatching;
                float3 del = factor * (initialPos - sharedPos[indexForSharedMem]).xyz;
                sharedPos[indexForSharedMem].xyz += del;
            }
        }
    }

    // update global position buffers
    UpdateFinalVertexPositions(currentPos, sharedPos[indexForSharedMem], globalVertexIndex);
}


//--------------------------------------------------------------------------------------
//
// Calculate Strand Level Data
//
// Propagate velocity shock resulted by attached based mesh
//
// One thread computes two vertices within a strand.
//
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void CalculateStrandLevelData(
    uint GIndex : SV_GroupIndex,
    uint3 GId : SV_GroupID,
    uint3 DTid : SV_DispatchThreadID)
{
    uint local_id, group_id, globalStrandIndex, numVerticesInTheStrand, globalRootVertexIndex, strandType;
    CalcIndicesInStrandLevelMaster(GIndex, GId.x, globalStrandIndex, numVerticesInTheStrand, globalRootVertexIndex, strandType);

    // Accounting for the right and left side of the strand.
    float4 pos_old_old[2];  // previous previous positions for vertex 0 (root) and vertex 1.
    float4 pos_old[2];      // previous positions for vertex 0 (root) and vertex 1.
    float4 pos_new[2];      // current positions for vertex 0 (root) and vertex 1.

    pos_old_old[0] = g_HairVertexPositionsPrevPrev[globalRootVertexIndex];
    pos_old_old[1] = g_HairVertexPositionsPrevPrev[globalRootVertexIndex + 1];

    pos_old[0] = g_HairVertexPositionsPrev[globalRootVertexIndex];
    pos_old[1] = g_HairVertexPositionsPrev[globalRootVertexIndex + 1];

    pos_new[0] = g_HairVertexPositions[globalRootVertexIndex];
    pos_new[1] = g_HairVertexPositions[globalRootVertexIndex + 1];

    float3 u = normalize(pos_old[1].xyz - pos_old[0].xyz);
    float3 v = normalize(pos_new[1].xyz - pos_new[0].xyz);

    // Compute rotation and translation which transform pos_old to pos_new. 
    // Since the first two vertices are immovable, we can assume that there is no scaling during tranform. 
    float4 rot = QuatFromTwoUnitVectors(u, v);
    float3 trans = pos_new[0].xyz - MultQuaternionAndVector(rot, pos_old[0].xyz);

    float vspCoeff = GetVelocityShockPropogation();
    float restLength0 = g_HairRestLengthSRV[globalRootVertexIndex];
    float vspAccelThreshold  = GetVSPAccelThreshold();

    // Increase the VSP coefficient by checking pseudo-acceleration to handle over-stretching when the character moves very fast 
    float accel = length(pos_new[1] - 2.0 * pos_old[1] + pos_old_old[1]);

    if (accel > vspAccelThreshold)
        vspCoeff = 1.0f;
    g_StrandLevelData[globalStrandIndex].vspQuat = rot;
    g_StrandLevelData[globalStrandIndex].vspTranslation = float4(trans, vspCoeff);

    // Skinning

    // Copy data from init rest data to be used to set updated shared memory
    float4 initialPos = float4(CM_TO_METERS,CM_TO_METERS,CM_TO_METERS,1.0) * g_InitialHairPositions[globalRootVertexIndex]; // rest position

    // Apply bone skinning to initial position
    BoneSkinningData skinningData = g_BoneSkinningData[globalStrandIndex];
    float4 bone_quat;
    initialPos.xyz = ApplyVertexBoneSkinning(initialPos.xyz, skinningData, bone_quat);
    g_StrandLevelData[globalStrandIndex].skinningQuat = bone_quat;
}


//--------------------------------------------------------------------------------------
//
// VelocityShockPropagation
//
// Propagate velocity shock resulted by attached based mesh
//
// One thread computes a vertex in a strand.
//
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void VelocityShockPropagation(
    uint GIndex : SV_GroupIndex,
    uint3 GId : SV_GroupID,
    uint3 DTid : SV_DispatchThreadID)
{
    uint globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType;
    CalcIndicesInVertexLevelMaster(GIndex, GId.x, globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType);

    // The first two vertices are the ones attached to the skin
    if (localVertexIndex < 2)
        return;

    float4 vspQuat = g_StrandLevelData[globalStrandIndex].vspQuat;
    float4 vspTrans = g_StrandLevelData[globalStrandIndex].vspTranslation;
    float vspCoeff = vspTrans.w;

    float4 pos_new_n = g_HairVertexPositions[globalVertexIndex];
    float4 pos_old_n = g_HairVertexPositionsPrev[globalVertexIndex];

    pos_new_n.xyz = (1.f - vspCoeff) * pos_new_n.xyz + vspCoeff * (MultQuaternionAndVector(vspQuat, pos_new_n.xyz) + vspTrans.xyz);
    pos_old_n.xyz = (1.f - vspCoeff) * pos_old_n.xyz + vspCoeff * (MultQuaternionAndVector(vspQuat, pos_old_n.xyz) + vspTrans.xyz);

    g_HairVertexPositions[globalVertexIndex].xyz = pos_new_n.xyz;
    g_HairVertexPositionsPrev[globalVertexIndex].xyz = pos_old_n.xyz;
}


//--------------------------------------------------------------------------------------
//
// LocalShapeConstraints
//
// Compute shader to maintain the local shape constraints.
//
// One thread computes one strand.
//
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void LocalShapeConstraints(
    uint GIndex : SV_GroupIndex,
    uint3 GId : SV_GroupID,
    uint3 DTid : SV_DispatchThreadID)
{
    uint local_id, group_id, globalStrandIndex, numVerticesInTheStrand, globalRootVertexIndex, strandType;
    CalcIndicesInStrandLevelMaster(GIndex, GId.x, globalStrandIndex, numVerticesInTheStrand, globalRootVertexIndex, strandType);

    // stiffness for local shape constraints
    float stiffnessForLocalShapeMatching = GetLocalStiffness(strandType);

    // Going beyond the TH will create less stability in convergence
    const float stabilityTH = 0.95f;
    stiffnessForLocalShapeMatching = 0.5f * min(stiffnessForLocalShapeMatching, stabilityTH);

    //--------------------------------------------
    // Local shape constraint for bending/twisting
    //--------------------------------------------
    {
        float4 boneQuat = g_StrandLevelData[globalStrandIndex].skinningQuat;

        // vertex 1 through n-1
        for (uint localVertexIndex = 1; localVertexIndex < numVerticesInTheStrand - 1; localVertexIndex++)
        {
            uint globalVertexIndex = globalRootVertexIndex + localVertexIndex;

            float4 pos = g_HairVertexPositions[globalVertexIndex];
            float4 pos_plus_one = g_HairVertexPositions[globalVertexIndex + 1];
            float4 pos_minus_one = g_HairVertexPositions[globalVertexIndex - 1];

            float3 bindPos = MultQuaternionAndVector(boneQuat, g_InitialHairPositions[globalVertexIndex].xyz * CM_TO_METERS);
            float3 bindPos_plus_one = MultQuaternionAndVector(boneQuat, g_InitialHairPositions[globalVertexIndex + 1].xyz * CM_TO_METERS);
            float3 bindPos_minus_one = MultQuaternionAndVector(boneQuat, g_InitialHairPositions[globalVertexIndex - 1].xyz * CM_TO_METERS);

            float3 lastVec = pos.xyz - pos_minus_one.xyz;

            float3 vecBindPose = bindPos_plus_one - bindPos;
            float3 lastVecBindPose = bindPos - bindPos_minus_one;
            float4 rotGlobal = QuatFromTwoUnitVectors(normalize(lastVecBindPose), normalize(lastVec));

            float3 orgPos_i_plus_1_InGlobalFrame = MultQuaternionAndVector(rotGlobal, vecBindPose) + pos.xyz;
            float3 del = stiffnessForLocalShapeMatching * (orgPos_i_plus_1_InGlobalFrame - pos_plus_one.xyz);

            if (IsMovable(pos))
                pos.xyz -= del.xyz;

            if (IsMovable(pos_plus_one))
                pos_plus_one.xyz += del.xyz;


            g_HairVertexPositions[globalVertexIndex].xyz = pos.xyz;
            g_HairVertexPositions[globalVertexIndex + 1].xyz = pos_plus_one.xyz;
        }
    }
}

//--------------------------------------------------------------------------------------
//
// LengthConstriantsWindAndCollision
//
// Compute shader to move the vertex position based on wind, maintain the lenght constraints
// and handles collisions.
//
// One thread computes one vertex.
//
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void LengthConstriantsWindAndCollision(uint GIndex : SV_GroupIndex,
                  uint3 GId : SV_GroupID,
                  uint3 DTid : SV_DispatchThreadID)
{
    uint globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType;
    CalcIndicesInVertexLevelMaster(GIndex, GId.x, globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType);

    uint numOfStrandsPerThreadGroup = g_NumOfStrandsPerThreadGroup;

    //------------------------------
    // Copy data into shared memory
    //------------------------------
    sharedPos[indexForSharedMem] = g_HairVertexPositions[globalVertexIndex];
    sharedLength[indexForSharedMem] = g_HairRestLengthSRV[globalVertexIndex] * CM_TO_METERS;

    GroupMemoryBarrierWithGroupSync();

/*
    //------------
    // Wind - does not work yet and requires some LTC
    //------------
    if (any(g_Wind.xyz))    // g_Wind.w is the current frame
    {
        float4 force = float4(0, 0, 0, 0);

        if ( localVertexIndex >= 2 && localVertexIndex < numVerticesInTheStrand-1 )
        {
            // combining four winds.
            float a = ((float)(globalStrandIndex % 20))/20.0f;
            float3  w =  a* g_Wind.xyz + (1.0f - a) * g_Wind1.xyz + a * g_Wind2.xyz + (1.0f - a) * g_Wind3.xyz;
//            float3  w = float3(5.2, 0, 0);

            uint sharedIndex = localVertexIndex * numOfStrandsPerThreadGroup + localStrandIndex;

            float3 v = sharedPos[sharedIndex].xyz - sharedPos[sharedIndex+numOfStrandsPerThreadGroup].xyz;
            float3 force = -cross(cross(v, w), v);
            sharedPos[sharedIndex].xyz += force*g_TimeStep*g_TimeStep;
        }
    }

    GroupMemoryBarrierWithGroupSync();
*/
    //----------------------------
    // Enforce length constraints
    //----------------------------
    uint a = numVerticesInTheStrand/2.0f;
    uint b = (numVerticesInTheStrand-1)/2.0f;

    int lengthContraintIterations = GetLengthConstraintIterations();

    for ( int iterationE=0; iterationE < lengthContraintIterations; iterationE++ )
    {
        uint sharedIndex = 2 * localVertexIndex * numOfStrandsPerThreadGroup + localStrandIndex;

        // Notice that the following is based on the fact that we are dealing here with two vertices 
        // one at each side of the central control point and each should extend towards its side only.
        if( localVertexIndex < a )
            ApplyDistanceConstraint(sharedPos[sharedIndex], sharedPos[sharedIndex+numOfStrandsPerThreadGroup], sharedLength[sharedIndex].x);

        GroupMemoryBarrierWithGroupSync();

        if( localVertexIndex < b )
            ApplyDistanceConstraint(sharedPos[sharedIndex+numOfStrandsPerThreadGroup], sharedPos[sharedIndex+numOfStrandsPerThreadGroup*2], sharedLength[sharedIndex+numOfStrandsPerThreadGroup].x);

        GroupMemoryBarrierWithGroupSync();
    }

    //------------------------------------------
    // Collision handling with capsule objects
    //------------------------------------------
    float4 oldPos = g_HairVertexPositionsPrev[globalVertexIndex];
    bool bAnyColDetected = false;       // Adi
//    bool bAnyColDetected = ResolveCapsuleCollisions(sharedPos[indexForSharedMem], oldPos);
    GroupMemoryBarrierWithGroupSync();

    //-------------------
    // Compute tangent
    //-------------------
    // If this is the last vertex in the strand, we can't get tangent from subtracting from the next vertex, need to use last vertex to current
    uint indexForTangent = (localVertexIndex == numVerticesInTheStrand - 1) ? indexForSharedMem - numOfStrandsPerThreadGroup : indexForSharedMem;
    float3 tangent = sharedPos[indexForTangent + numOfStrandsPerThreadGroup].xyz - sharedPos[indexForTangent].xyz;
    g_HairVertexTangents[globalVertexIndex].xyz = normalize(tangent);

    //---------------------------------------
    // clamp velocities, rewrite history
    //---------------------------------------
    float3 positionDelta = sharedPos[indexForSharedMem].xyz - oldPos;
    float speedSqr = dot(positionDelta, positionDelta);
    if (speedSqr > g_ClampPositionDelta * g_ClampPositionDelta) {
        positionDelta *= g_ClampPositionDelta * g_ClampPositionDelta / speedSqr;
        g_HairVertexPositionsPrev[globalVertexIndex].xyz = sharedPos[indexForSharedMem].xyz - positionDelta;
    }

    //---------------------------------------
    // update global position buffers
    //---------------------------------------
    g_HairVertexPositions[globalVertexIndex] = sharedPos[indexForSharedMem];

    if (bAnyColDetected)
        g_HairVertexPositionsPrev[globalVertexIndex] = sharedPos[indexForSharedMem];

    return;
}

//--------------------------------------------------------------------------------------
// 
// UpdateFollowHairVertices
// 
// Last stage update of the follow hair to follow their guide hair
// 
// One thread computes one vertex.
// 
//--------------------------------------------------------------------------------------
[numthreads(THREAD_GROUP_SIZE, 1, 1)]
void UpdateFollowHairVertices(
    uint GIndex : SV_GroupIndex,
    uint3 GId : SV_GroupID,
    uint3 DTid : SV_DispatchThreadID)
{
    uint globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType;
    CalcIndicesInVertexLevelMaster(GIndex, GId.x, globalStrandIndex, localStrandIndex, globalVertexIndex, localVertexIndex, numVerticesInTheStrand, indexForSharedMem, strandType);

    sharedPos[indexForSharedMem] = GetSharedPosition(globalVertexIndex); // g_HairVertexPositions[globalVertexIndex];
    sharedTangent[indexForSharedMem].xyz = GetSharedTangent(globalVertexIndex); // g_HairVertexTangents[globalVertexIndex];
    GroupMemoryBarrierWithGroupSync();

    for ( uint i = 0; i < g_NumFollowHairsPerGuideHair; i++ )
    {
        int globalFollowVertexIndex = globalVertexIndex + numVerticesInTheStrand * (i + 1);
        int globalFollowStrandIndex = globalStrandIndex + i + 1;
        float factor = g_TipSeparationFactor*((float)localVertexIndex / (float)numVerticesInTheStrand) + 1.0f;
        float3 followPos = sharedPos[indexForSharedMem].xyz + factor * CM_TO_METERS * g_FollowHairRootOffset[globalFollowStrandIndex].xyz;

        SetSharedPosition3(globalFollowVertexIndex, followPos); 
        //  g_HairVertexPositions[globalFollowVertexIndex].xyz = followPos;
        //-----------------------
        //        SetSharedTangent(globalFollowVertexIndex, sharedTangent[indexForSharedMem]); 
        g_HairVertexTangents[globalFollowVertexIndex] = sharedTangent[indexForSharedMem];
    }
    
    return;
}