传统的蒙皮骨骼动画混合方法易于理解,但是在SM 2.0的256常量限制下,骨骼数保守计算最多50根骨头,因此对美术的工作流程以及模型渲染方法造成了很大的障碍
float4x4 matBoneArray[40]; // 这是传输的瓶颈
VS_OUTPUT vs_main( SkinnedVS_INPUT In )
{
VS_OUTPUT Out = (VS_OUTPUT)0;
float4x4 skinTransform = 0;
skinTransform += matBoneArray[In.BoneIndices.x] * In.BoneWeights.x;
skinTransform += matBoneArray[In.BoneIndices.y] * In.BoneWeights.y;
skinTransform += matBoneArray[In.BoneIndices.z] * In.BoneWeights.z;
skinTransform += matBoneArray[In.BoneIndices.w] * In.BoneWeights.w;
float4 localpos = mul(In.Position, skinTransform);
Out.Position = mul( localpos, matViewProj );
Out.TexCoord = In.TexCoord;
return Out;
}
matBoneArray这个数组是骨骼的LocalRot和LocalTranslation 通过以下函数build出来
Matrix4& Matrix4::FromTranslationRotation( const Vector3& translation, const Quaternion& rotation )
{ float xx = rotation.x * rotation.x * 2.0f, yy = rotation.y * rotation.y * 2.0f, zz = rotation.z * rotation.z * 2.0f;
float xy = rotation.x * rotation.y * 2.0f, zw = rotation.z * rotation.w * 2.0f, xz = rotation.x * rotation.z * 2.0f;
float yw = rotation.y * rotation.w * 2.0f, yz = rotation.y * rotation.z * 2.0f, xw = rotation.x * rotation.w * 2.0f;
m[0][0] = 1.0f - yy - zz; m[0][1] = xy + zw; m[0][2] = xz - yw; m[0][3] = 0.0f;
m[1][0] = xy - zw; m[1][1] = 1.0f - xx - zz; m[1][2] = yz + xw; m[1][3] = 0.0f;
m[2][0] = xz + yw; m[2][1] = yz - xw; m[2][2] = 1.0f - xx - yy; m[2][3] = 0.0f;
m[3][0] = translation.x; m[3][1] = translation.y; m[3][2] = translation.z; m[3][3] = 1.0f;
return *this;
}
从这里你可以发现, 本来每根骨头只需要2个float4 传递变换信息的,现在却需要4个float4,也就是一个矩阵来传递,矩阵中还有很多不使用的变量也被传输到GPU中,这里就是优化的点.
重新调整后的Shader代码:
float4x4 BuildFromTransRot( float4 translation, float4 rot )
{ float4 rotation = rot;
float xx = rotation.x * rotation.x * 2.0f, yy = rotation.y * rotation.y * 2.0f, zz = rotation.z * rotation.z * 2.0f;
float xy = rotation.x * rotation.y * 2.0f, zw = rotation.z * rotation.w * 2.0f, xz = rotation.x * rotation.z * 2.0f;
float yw = rotation.y * rotation.w * 2.0f, yz = rotation.y * rotation.z * 2.0f, xw = rotation.x * rotation.w * 2.0f;
float4x4 m = { {1.0f - yy - zz, xy + zw, xz - yw, 0}, {xy - zw, 1.0f - xx - zz, yz + xw, 0}, {xz + yw, yz - xw, 1.0f - xx - yy, 0}, {translation.x, translation.y, translation.z, 1}
};
return m;
}
float4x4 GetBoneElement( float index )
{ return BuildFromTransRot( vecBoneLocalTrans[index], vecBoneLocalRot[index] );
}
VS_OUTPUT vs_main( SkinnedVS_INPUT In )
{
VS_OUTPUT Out = (VS_OUTPUT)0;
float4x4 skinTransform = 0;
skinTransform += GetBoneElement(In.BoneIndices.x) * In.BoneWeights.x;
skinTransform += GetBoneElement(In.BoneIndices.y) * In.BoneWeights.y;
skinTransform += GetBoneElement(In.BoneIndices.z) * In.BoneWeights.z;
skinTransform += GetBoneElement(In.BoneIndices.w) * In.BoneWeights.w;
float4 localpos = mul(In.Position, skinTransform);
Out.Position = mul( localpos, matViewProj );
Out.TexCoord = In.TexCoord;
return Out;
}
我们将骨头的local旋转及偏移传递至GPU,然后在GPU内重组,虽然对GPU性能计算有部分损耗,但是骨骼数量就能保守提高到100个.