# DX11 GPU Skinning Problem

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Hello,

I have a Problem with GPU Skinning, I load from a COLLADA File my object with the vertices and the weights and bone indices for it and the bones with the matrices.

For every vertex I choose 4 weights and 4 bone indices.

For every non skin vertex i choose by the weights 1 0 0 0 and bone indices 0 0 0 0 (In the Bone Matrices Array is index 0 a Matrix Idetity)

And i check up if all weights values together is always 1 or i calculate it to 1.

So far so good, my Shader looks like this:

bool HasBones;
matrix BoneMatrices[256];

struct Vertex
{
float3 Position  : POSITION;
float3 Normal    : NORMAL;
float2 UV        : TEXCOORD0;
float3 Tangent   : TANGENT;
float4 Weights   : WEIGHTS;
int4 BoneIndices : BONEINDICES;
};

float4 ApplyBoneTransform(Vertex input, float4 value)
{
if(HasBones)
{
float4x4 skinTransform = (float4x4)0;
skinTransform += BoneMatrices[input.BoneIndices.x] * input.Weights.x;
skinTransform += BoneMatrices[input.BoneIndices.y] * input.Weights.y;
skinTransform += BoneMatrices[input.BoneIndices.z] * input.Weights.z;
skinTransform += BoneMatrices[input.BoneIndices.w] * input.Weights.w;

float4 position = mul(value, skinTransform);

return position;
}
else
return value;
}

{
Pixel result = (Pixel) 0;

float4 posWorld = mul(ApplyBoneTransform(input, float4(input.Position.xyz, 1.0f)), World);
result.Position = mul(mul(posWorld, View), Projection);
result.Normal = normalize(mul(ApplyBoneTransform(input, float4(input.Normal.xyz, 1.0f)), WorldIT));
result.UV = input.UV;
result.View = ViewInverse[3] - mul(float4(input.Position.xyz, 1.0f), World);
result.Tangent = normalize(mul(ApplyBoneTransform(input, float4(input.Tangent.xyz, 1.0f)), WorldIT).xyz);
result.Binormal = normalize(cross(input.Normal, input.Tangent));

return result;
}

And if i set HasBones to true, my object will not draw right anymore, i only see two dark triangles

I believe it depends on the bone matrices i load from the controller_lib of the COLLADA File and send it to the BoneMatrices in the shader + at the index 0 the Matrix Idetity.

Has anyone an Idea what I make wrong and could help and explain me it?

And i upload to this post the Collada file and images of the object draw in HasBones = false and HasBones = true

Greets

Benajmin

Model.dae

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You do linear interpolation of multiple matrices which makes no sense here.

You should transform the vertex by each matrix, and lerp the resulting vectors instead.

But to do so, you first need to transform the vertex into the local bone space instead, so you typically store 2 matrices for each bone. One (static) that transforms to bone space, and another (animated) that transforms the result back to world space.

Something like this:

EDIT: Hey - who upvoted this nonsense? Maybe crossing out helps

	vec os = currentVertexPosInObjectSpace;
vec ws (0); // result
for each affecting bone {
int i = bone.matrixIndex;
vec v = ObjectSpaceToBoneMatrices[i].Transform(os); // sadly this is necessary so the vertex knows it position relative to each bone
v = AnimatedBoneInWorlsSpaceMatricws[i].Transform(v); // but knowing that, we can transform now to world space for this bone
ws += v * bone.weight; // sum up weighted results
}

Edited by JoeJ

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Unfortunately this problem needs to be debugged. What helped me is loading a very simple model (I see you are already using a box so that should be fine) and very simple bone structure. Debug that your bone matrices are correct by drawing simple lines with each transform matrix. the lines should form a skeleton when correct.

The most common mistakes done is messing up the order of matrix multiplications of Scale*Rotate*Translate as well as bone hierarchy order, and there is also the need of multiplying with a t-pose relative matrix (inverse t-pose), and all these need to be in the correct order. Also HLSL expects matrices to be in column major order, while your application side math library might produce row-major matrices, so maybe they should either be pre-transposed, or reverse the multiplication order.

The shader you posted doesn't seem too bad at first look, though normal and tangent vectors probably shouldn't be set their w component to avoid applying translations to them. Also I noticed that you are adding your matrices together and multiplying by the bone weights. That should produce correct results, but less operations are performed if you would perform vector transformations first and then multiplying the vector with the weight, then finally adding the vectors together. The result should be the about same (if we don't consider floating point accuracy) but achieved faster. You also don't have to use a 4x4 matrix, 3x4 is perfectly enough, but set the 4th row to (0,0,0,1).

You can check out my skinning shader for reference: https://github.com/turanszkij/WickedEngine/blob/master/WickedEngine/skinningCS.hlsl

Good luck, it is a very rewarding experience once you manage to correct it!

17 minutes ago, JoeJ said:

You do linear interpolation of multiple matrices which makes no sense here.

You should transform the vertex by each matrix, and lerp the resulting vectors instead.

But to do so, you first need to transform the vertex into the local bone space instead, so you typically store 2 matrices for each bone. One (static) that transforms to bone space, and another (animated) that transforms the result back to world space.

Something like this:


vec os = currentVertexPosInObjectSpace;
vec ws (0); // result
for each affecting bone {
int i = bone.matrixIndex;
vec v = ObjectSpaceToBoneMatrices[i].Transform(os); // sadly this is necessary so the vertex knows it position relative to each bone
v = AnimatedBoneInWorlsSpaceMatricws[i].Transform(v); // but knowing that, we can transform now to world space for this bone
ws += v * bone.weight; // sum up weighted results
}


You can do the linear interpolation of matrices fine, the result is the same, just results in more operations. Also, you can premultiply the bone matrices with the relative "objectspaceToBoneMatrix" matrix on the application side and only send one bone matrix to the shader.

Edited by turanszkij

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1 minute ago, turanszkij said:

there is also the need of multiplying with a t-pose relative matrix (inverse t-pose)

This is what i mean with ObjectSpaceToBoneMatrices (to avoid any confusion)

You calc that by the inverse transform of the bone in rest pose

4 minutes ago, turanszkij said:

You can do the linear interpolation of matrices fine, the result is the same, just results in more operations. Also, you can premultiply the bone matrices with the relative "objectspaceToBoneMatrix" matrix on the application side and only send one bone matrix to the shader.

Ooops - you're right. And i've forgotten about the 'trick' to premultiply... embrassing

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@turanszkij With Draw the Bones to see the Skeleton, i did this already, to check if i load the bone matrices correct. see the image.

I take the postion of each Bone and draw a line to the next child postion.

But i see only a right skeleton result if i invert every single bone matrix.

And do you mean with t-pose the bind pose matrix?

And befor i send the boneMatrices Array to the Shader, here is the code how i collect them

                public Matrix CalculateMatrixFromParents(Joint joint, Matrix world)
{
if (joint.Parent != null)
{
world *= CalculateMatrixFromParents(joint.Parent, joint.Parent.Matrix);
return world;
}
else
return joint.Matrix;
}

public List<Matrix> GetBoneMatrices()
{
List<Matrix> boneMatrices = new List<Matrix>();

foreach (Joint joint in this.bones)
{
Matrix m = this.CalculateMatrixFromParents(joint, Matrix.Identity);
}

return boneMatrices;
}

I am not really math genius , but i hope i will understand your explain?

Greets

Benjamin

Edited by B. /

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You could try inspecting the shader bone data in a graphics debugger, if the contents of it match the bone data on the application side. Nvidia Nsight or the visual studio graphics debugger are good choices.

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11 hours ago, B. / said:

foreach (Joint joint in this.bones) { Matrix m = this.CalculateMatrixFromParents(joint, Matrix.Identity); boneMatrices.Add(m); }

To me it looks like you do not take the rest/t-pose/bind-pose (however we call it) into account here.

It might look somehow like this (assuming the matrices in your current code are the animated ones):

foreach (Joint joint in this.bones) {

Matrix m = this.CalculateMatrixFromParents(joint, Matrix.Identity* joint.bindPoseWorldSpaceMatrix.Inversed();

You should get this right after some trial and error usually.

Your shader however is potentially inefficient, because each thread may store 4 matrices in registers, that's 4*16 = 64 alone for that, which is a lot.

To get 100% occupancy on AMD you should only use 24 IIRC, NV varies but is similar.

To achieve this, you should transform the position by each matrix in order (as already said), so the compiler has the option to have just 1 or 2 matrices in registers at the same time. And you should transform position and normal in one go of course, otherwise the compiler will likely decide to store them all to have them available for the normal. (Also, using subfunctions almost always has a cost the last time i checked - it seems compilers are too stupid to inline the code, better do it yourself.)

This is said just to be nit-picking. The matrices likely end up in fast constant ram, so storing all 4 in registers might not be necessary and my proposed optimization would have no effect in practice. But you never really know how different GPUs / compilers handle this and thinking of it is often no extra work. (optimizing for low register usage is usually also better than optimizing for less instructions.)

Edited by JoeJ

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Hi Guys,

thank you for your answers, today i had also test loading weights/matrices per vertex in assimp, to test, if i dont make a mistake by loading the datas. The matrices was right, but the weights per vertex was different and the draw result was still wrong, but very better as mine.

The strange thing only was in the collada file say vcount, the first vertex has 2 weights and the second has 3 weights ...

But assimp load for the fist vertex 3 weights and for the second 2 weights. So i thought, maybe i dont right understand the docu of the collada file, to set the right weights and bone indices to the right vertex maybe?

        <source id="pCube1Controller-Weights">
<float_array id="pCube1Controller-Weights-array" count="33">

1.000000 0.989534 0.009756 0.708022 0.289010 0.002968 0.989518 0.009771 0.708398 0.288669 0.002933 0.989525 0.009771 0.708446 0.288689 0.002866
0.989540 0.009756 0.708070 0.289030 0.002900 0.004697 0.497651 0.497651 0.003326 0.498337 0.498337 0.004704 0.497648 0.497648 0.003331 0.498334
0.498334</float_array>
<technique_common>
<accessor source="#pCube1Controller-Weights-array" count="33">
<param type="float"/>
</accessor>
</technique_common>
</source>

<vertex_weights count="12">
<input semantic="JOINT" offset="0" source="#pCube1Controller-Joints"/>
<input semantic="WEIGHT" offset="1" source="#pCube1Controller-Weights"/>
<vcount>2 3 2 3 2 3 2 3 3 3 3 3</vcount>
<v>0 1 1 2 0 3 1 4 2 5 0 6 1 7 0 8 1 9 2 10 0 11 1 12 0 13 1 14 2 15 0 16 1 17 0 18 1 19 2 20 0 21 1 22 2 23 0 24 1 25 2 26 0 27 1 28 2 29 0 30 1 31 2 32</v>
</vertex_weights>

Here is the code how i set the Weights and Bone Indices to the vertices:

InfluencesWeightsPerVertex = VCount List

1+ wCounterIndex, because the first Weight in the Weights Array is an extra Weights

RemoveRange, because i create a Vertex by default Weights 1 0 0 0, Bone Indices 0 0 0 0 (Index 0 = Matrix Idetity)

Insert Range because of insert the loaded values in the list

                        int vertexIndex = 0;
int wCounterIndex = 0;

// Set Vertex Weights and Bone Indices
foreach (int item in InfluencesWeightsPerVertex)
{
List<float> weights = skinClusterWeights.GetRange(1 + wCounterIndex, item);
List<int> boneIndices = new List<int>();

for (int i = 0; i < (item * 2); i += 2)

if (weights.Count > 4)
weights.RemoveRange(4, weights.Count - 4);

if (boneIndices.Count > 4)
boneIndices.RemoveRange(4, boneIndices.Count - 4);

// Normalize all weights to 1
float factor = 0;

foreach (float weight in weights)
factor += weight;

factor = 1.0f / factor;

for (int i = 0; i < weights.Count; i++ )
weights[i] = factor * weights[i];

Vertex vertex = geometry.Vertices[vertexIndex];
vertex.Weights.RemoveRange(0, weights.Count);
vertex.Weights.InsertRange(0, weights);
vertex.BoneIndices.RemoveRange(0, boneIndices.Count);
vertex.BoneIndices.InsertRange(0, boneIndices);

geometry.Vertices[vertexIndex] = vertex;

vertexIndex++;
wCounterIndex += item;
}

Can you check up my code and see, if my way to load the datas for skinning is right, that we can exclude this?

Greets

Benjamin

16 hours ago, turanszkij said:

You could try inspecting the shader bone data in a graphics debugger, if the contents of it match the bone data on the application side. Nvidia Nsight or the visual studio graphics debugger are good choices.

Hi,

if the Bone Data/Matrices would be wrong, than the draw of the skeleton lines would be wrong too, right?

Edited by B. /

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Maybe assimp is set up to optimize for better vertex caching and so changes the order of vertices?

When i came across collada, i noticed each app seems to have its own interpretation of the standard, and it was not really a good format to exchange things, especially when skinning is involved.

I would create a simple model like the box just proceduraly only from code to be sure the data is right. You could also do CPU skinning for reference. Some work, but such code may be reusable for the next issue and on the long run i prefer this against frustrating GPU debugging.

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3 hours ago, JoeJ said:

Maybe assimp is set up to optimize for better vertex caching and so changes the order of vertices?

When i came across collada, i noticed each app seems to have its own interpretation of the standard, and it was not really a good format to exchange things, especially when skinning is involved.

I would create a simple model like the box just proceduraly only from code to be sure the data is right. You could also do CPU skinning for reference. Some work, but such code may be reusable for the next issue and on the long run i prefer this against frustrating GPU debugging.

Hi Joe,

which Format would be the best for GPU Skinning, maybe FBX?

And I believe i know where my mistake is, i see in the collada file the count of vertex weights is 12, but my elbow cube has total 60 vertices

The other 48 vertices has only Weights 1 0 0 0 and Indices of 0 0 0 0 Matrix Idetity.

But thats wrong because i skin every single vertices on the 3 bones and so assimp also import weights on all vertices, but how do i calculate this from the only 12 given vcounts, that only say how many weights/bones has a vertex??????????????

And for CPU Skinning i would need a origin Vertex List and transform these and set it to the vertexbuffer for every fps?

Greets

Benjamin

Edited by B. /

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