在DirectX 12中使用tessellation

DirectX 12支持tessellation，我们可以在vertex shader后加入hull shader和domain shader，来对顶点进行动态细分，以实现不同的效果。在CPU层面上，我们传入的顶点信息需要设置为patchlist，DirectX 12最多支持32个控制点的patchlist：

object->mPrimitiveTopology = D3D_PRIMITIVE_TOPOLOGY_4_CONTROL_POINT_PATCHLIST;

mCommandList->IASetPrimitiveTopology(object->mPrimitiveTopology);

另外，在创建pipeline state object时，也需要进行一下调整：

psoDesc.PrimitiveTopologyType = D3D12_PRIMITIVE_TOPOLOGY_TYPE_PATCH;

hull shader的定义如下：

struct HullOut
{
	float3 PosL : POSITION;
};
    
[domain("quad")]
[partitioning("integer")]
[outputtopology("triangle_cw")]
[outputcontrolpoints(4)]
[patchconstantfunc("ConstantHS")]
[maxtessfactor(64.0f)]
HullOut HS(InputPatch<VertexOut, 4> p, 
           uint i : SV_OutputControlPointID,
           uint patchId : SV_PrimitiveID)
{
	HullOut hout;
	
	hout.PosL = p[i].PosL;
	
	return hout;
}

domain表示输入到hull shader的primitive格式，partitioning表示硬件tessellation的方式，outputtopology表示hull shader输出的primitive格式和绕序，outputcontrolpoints表示输出控制点的数量，这个数量决定了hull shader执行的次数。patchconstantfunc表示具体如何对一个primitive进行partition的函数，maxtessfactor表示最多tessellation的数量。

patchconstantfunc的定义如下：

struct PatchTess
{
	float EdgeTess[4]   : SV_TessFactor;
	float InsideTess[2] : SV_InsideTessFactor;
};

PatchTess ConstantHS(InputPatch<VertexOut, 4> patch, uint patchID : SV_PrimitiveID)
{
	PatchTess pt;
	
	float3 centerL = 0.25f*(patch[0].PosL + patch[1].PosL + patch[2].PosL + patch[3].PosL);
	float3 centerW = mul(float4(centerL, 1.0f), gWorld).xyz;
	
	float d = distance(centerW, gEyePosW);

	// Tessellate the patch based on distance from the eye such that
	// the tessellation is 0 if d >= d1 and 64 if d <= d0.  The interval
	// [d0, d1] defines the range we tessellate in.
	
	const float d0 = 20.0f;
	const float d1 = 100.0f;
	float tess = 64.0f*saturate( (d1-d)/(d1-d0) );

	// Uniformly tessellate the patch.

	pt.EdgeTess[0] = tess;
	pt.EdgeTess[1] = tess;
	pt.EdgeTess[2] = tess;
	pt.EdgeTess[3] = tess;
	
	pt.InsideTess[0] = tess;
	pt.InsideTess[1] = tess;
	
	return pt;
}

EdgeTess控制边tessellation的数量，InsideTess控制primitive内部tessellation的数量。

tessellation的结果会以uv的方式传递给domain shader。domain shader的定义如下：

struct DomainOut
{
	float4 PosH : SV_POSITION;
};

// The domain shader is called for every vertex created by the tessellator.  
// It is like the vertex shader after tessellation.
[domain("quad")]
DomainOut DS(PatchTess patchTess, 
             float2 uv : SV_DomainLocation, 
             const OutputPatch<HullOut, 4> quad)
{
	DomainOut dout;
	
	// Bilinear interpolation.
	float3 v1 = lerp(quad[0].PosL, quad[1].PosL, uv.x); 
	float3 v2 = lerp(quad[2].PosL, quad[3].PosL, uv.x); 
	float3 p  = lerp(v1, v2, uv.y); 
	
	// Displacement mapping
	p.y = 0.3f*( p.z*sin(p.x) + p.x*cos(p.z) );
	
	float4 posW = mul(float4(p, 1.0f), gWorld);
	dout.PosH = mul(posW, gViewProj);
	
	return dout;
}

最后运行的结果如下：

如果你觉得我的文章有帮助，欢迎关注我的微信公众号（大龄社畜的游戏开发之路）^-