ScriptableRenderPipeline/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitTessellation.hlsl

float3 GetVertexDisplacement(float3 positionWS, float3 normalWS, float2 texCoord0, float2 texCoord1, float2 texCoord2, float2 texCoord3, float4 vertexColor)
{
    // This call will work for both LayeredLit and Lit shader
    LayerTexCoord layerTexCoord;
    ZERO_INITIALIZE(LayerTexCoord, layerTexCoord);
    GetLayerTexCoord(texCoord0, texCoord1, texCoord2, texCoord3, positionWS, normalWS, layerTexCoord);

    // TODO: do this algorithm for lod fetching as lod not available in vertex/domain shader
    // http://www.sebastiansylvan.com/post/the-problem-with-tessellation-in-directx-11/
    float lod = 0.0;
    return ComputePerVertexDisplacement(layerTexCoord, vertexColor, lod) * normalWS;
}

void ApplyVertexModification(AttributesMesh input, float3 normalWS, inout float3 positionWS)
{
#if defined(_VERTEX_DISPLACEMENT)

    positionWS += GetVertexDisplacement(positionWS, normalWS,
    #ifdef ATTRIBUTES_NEED_TEXCOORD0
        input.uv0,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef ATTRIBUTES_NEED_TEXCOORD1
        input.uv1,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef ATTRIBUTES_NEED_TEXCOORD2
        input.uv2,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef ATTRIBUTES_NEED_TEXCOORD3
        input.uv3,
    #else
        float2(0.0, 0.0),
    #endif            
    #ifdef ATTRIBUTES_NEED_COLOR
        input.color
    #else
        float4(0.0, 0.0, 0.0, 0.0)
    #endif      
        );
#endif
    
#ifdef _VERTEX_WIND
    float3 rootWP = mul(GetObjectToWorldMatrix(), float4(0, 0, 0, 1)).xyz;
    ApplyWindDisplacement(positionWS, normalWS, rootWP, _Stiffness, _Drag, _ShiverDrag, _ShiverDirectionality, _InitialBend, input.color.a, _Time);
#endif
}

#ifdef TESSELLATION_ON

float4 GetTessellationFactors(float3 p0, float3 p1, float3 p2, float3 n0, float3 n1, float3 n2)
{
    float maxDisplacement = GetMaxDisplacement();


    // For tessellation we want to process tessellation factor always from the point of view of the camera (to be consistent and avoid Z-fight).
    // For the culling part however we want to use the current view (shadow view).
    // Thus the following code play with both.

#if defined(SHADERPASS) && (SHADERPASS != SHADERPASS_SHADOWS)
    bool frustumCulledCurrentView = WorldViewFrustumCull(p0, p1, p2, maxDisplacement, (float4[4])_FrustumPlanes); // _FrustumPlanes are primary camera planes
    bool frustumCulledMainView = false;
#else
    bool frustumCulledCurrentView = WorldViewFrustumCull(p0, p1, p2, maxDisplacement, (float4[4])unity_CameraWorldClipPlanes); // unity_CameraWorldClipPlanes is set by legacy Unity in case of shadow and contain shadow view plan
    // In the case of shadow, we don't want to tessellate anything that is not seen by the main view frustum. It can result in minor popping of tessellation into a shadow but we can't afford it anyway.
    bool frustumCulledMainView = WorldViewFrustumCull(p0, p1, p2, maxDisplacement, (float4[4])_FrustumPlanes);
#endif

    bool faceCull = false;

#ifndef _DOUBLESIDED_ON
    if (_TessellationBackFaceCullEpsilon > -0.99) // Is backface culling enabled ?
    {
        // Handle transform mirroring (like negative scaling)
        // Caution: don't change p1/p2 directly as it is use later
        float3 backfaceP1 = unity_WorldTransformParams.w < 0.0 ? p2 : p1;
        float3 backfaceP2 = unity_WorldTransformParams.w < 0.0 ? p1 : p2;

        faceCull = BackFaceCullTriangle(p0, backfaceP1, backfaceP2, _TessellationBackFaceCullEpsilon, GetCurrentViewPosition()); // Use shadow view
    }
#endif

    if (frustumCulledCurrentView || faceCull)
    {
        // Settings factor to 0 will kill the triangle
        return float4(0.0, 0.0, 0.0, 0.0);
    }
    
    // See comment above:
    // During shadow passes, we decide that anything outside the main view frustum should not be tessellated.
    if (frustumCulledMainView)
    {
        return float4(1.0, 1.0, 1.0, 1.0);
    }

    // We use the parameters of the primary (scene view) camera in order
    // to have identical tessellation levels for both the scene view and
    // shadow views. Otherwise, depth comparisons become meaningless!
    float3 tessFactor = float3(1.0, 1.0, 1.0);

    // Adaptive screen space tessellation
    if (_TessellationFactorTriangleSize > 0.0)
    {
        // return a value between 0 and 1
        tessFactor *= GetScreenSpaceTessFactor( p0, p1, p2, _ViewProjMatrix, _ScreenSize, _TessellationFactorTriangleSize); // Use primary camera view
    }

    // Distance based tessellation
    if (_TessellationFactorMaxDistance > 0.0)
    {
        float3 distFactor = GetDistanceBasedTessFactor(p0, p1, p2, GetPrimaryCameraPosition(), _TessellationFactorMinDistance, _TessellationFactorMaxDistance);  // Use primary camera view
        // We square the disance factor as it allow a better percptual descrease of vertex density.
        tessFactor *= distFactor * distFactor;
    }

    tessFactor *= _TessellationFactor;

    // TessFactor below 1.0 have no effect. At 0 it kill the triangle, so clamp it to 1.0
    tessFactor.xyz = float3(max(1.0, tessFactor.x), max(1.0, tessFactor.y), max(1.0, tessFactor.z));

    return CalcTriEdgeTessFactors(tessFactor);
}

// tessellationFactors
// x - 1->2 edge
// y - 2->0 edge
// z - 0->1 edge
// w - inside tessellation factor
void ApplyTessellationModification(VaryingsMeshToDS input, float3 normalWS, inout float3 positionWS)
{
#if defined(_TESSELLATION_DISPLACEMENT)

    positionWS += GetVertexDisplacement(positionWS, normalWS,
    #ifdef VARYINGS_DS_NEED_TEXCOORD0
        input.texCoord0,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef VARYINGS_DS_NEED_TEXCOORD1
        input.texCoord1,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef VARYINGS_DS_NEED_TEXCOORD2
        input.texCoord2,
    #else
        float2(0.0, 0.0),
    #endif
    #ifdef VARYINGS_DS_NEED_TEXCOORD3
        input.texCoord3,
    #else
        float2(0.0, 0.0),
    #endif            
    #ifdef VARYINGS_DS_NEED_COLOR
        input.color
    #else
        float4(0.0, 0.0, 0.0, 0.0)
    #endif      
        );
#endif // _TESSELLATION_DISPLACEMENT
}

#endif // #ifdef TESSELLATION_ON