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GPU Skinning problem (MD5 Doom)

2 posts in this topic


I'm trying to implement GPU skinning using md5 format. But after many hours of trying to figure out what's wrong I still get incorrect results for animation.

Firstly, I calculate Bind Pose and Inverse Bind Pose matrices for each bone:
[source lang="cpp"]Mat4x4 trans; trans.BuildTranslation(joints[i].posObjectSpace); //build translation from joint position in object space
Mat4x4 rot = joints[i].rot.BuildMatrix(); //joints[i].rot is unit quaternion
Mat4x4 bindPoseTransform = rot *trans;
(**ppBones)[i].bindPoseTransform = rot * trans;
(**ppBones)[i].inverseBindPoseTransform = bindPoseTransforms.Inverse();[/source]

After that I calculate vertex positions in object space and fill vertex buffer with positions, indices and weights
[source lang="cpp"]
for (int j = 0; j < mesh.numVertices; j++)
//calculate position
Vec3 pos(0.0f, 0.0f, 0.0f);
md5_vertex vertex = mesh.vertices[j];
for (int k = 0; k < vertex.numWeights; k++)
md5_weight weight = mesh.weights[vertex.startWeight+k];
md5_joint joint = joints[weight.jointId];
//calculate vertex position in object space

temp = Vec4(weight.posJointSpace); temp.w = 0.0f;
Quaternion q = joint.rot;
Quaternion revq = joint.rot.Inverse();

Vec3 temp3 = q * temp * revq;
temp3 = temp3 + joint.posObjectSpace;
temp3 *= weight.weight;
pos += temp3;// * weight.weight;
//finally fill vertex structure with data
v[j].pos = pos;
v[j].texCoord = vertex.texCoords;
for (int k = 0; k < vertex.numWeights; k++)
v[j].bonesIndices[k] = mesh.weights[vertex.startWeight+k].jointId;

v[j].bonesWeights = Vec4(
vertex.numWeights > 0 ? mesh.weights[vertex.startWeight].weight : 0.0f,
vertex.numWeights > 1 ? mesh.weights[vertex.startWeight+1].weight : 0.0f,
vertex.numWeights > 2 ? mesh.weights[vertex.startWeight+2].weight : 0.0f,
vertex.numWeights > 3 ? mesh.weights[vertex.startWeight+3].weight : 0.0f);
assert( fabs(v[j].bonesWeights.x + v[j].bonesWeights.y + v[j].bonesWeights.z + v[j].bonesWeights.w - 1.0f) < 0.001f);
Next step is to calculate joints transforms in object space for each frame in animation:
[source lang="cpp"]//pos - current joint position relative to the parent (if have any)
//rot - quaternion representng rotation of current joint
//if we have parent joint...
if (joint.id >= 0)
md5_joint parent = joints[joint.id];

//calculate joint pos
Quaternion parentOrient = parent.rot;
Vec4 temp( pos.x, pos.y, pos.z, 0.0f);

pos = parentOrient * temp * parentOrient.Inverse();
pos = pos + parent.posObjectSpace;

//concatenate rotation
rot = parentOrient * rot';

//update joint position and orientation
joints[j].posObjectSpace = pos;
joints[j].rot = rot;

Mat4x4 trans;
Mat4x4 rotation = rot.BuildMatrix();
Mat4x4 transform = rotation * trans;

newFrame.m_transforms.push_back(transform); //use just fixed frames for debugging, no interpolation for now...
//Finally I render animation!
[source lang="cpp"]
SkinningBuffer skinningBuffer;

for (int j = 0; j < m_numBones; j++)
Mat4x4 world = pScene->GetWorld(); //world transform. identity for now
Mat4x4 inverseBindPose = m_pBones[j].m_inverseBindPose; //inverse bind pose matrix

Mat4x4 curObjectSpace = m_pAnimations[0].GetTransform(j); //current object space transform for given bone
skinningBuffer.bonesTransforms[j] = Transpose(inverseBindPose * curObjectSpace * world);

//next frame...
//update constant buffer for shader
m_pSkinningBuffer->UpdateAndSet(&amp;amp;skinningBuffer, ST_Vertex, 3);

Here is my vertex shader code:
[source lang="cpp"]////////////////////////////////////////////////
//Constant Buffers
cbuffer MatrixBuffer : register(b0)
float4x4 m_ViewProjection;

cbuffer LightBuffer : register(b1)
float4 ambientColor : packoffset(c0);
float4 diffuseColor : packoffset(c1);
float3 lightDir : packoffset(c2.x);
float padb1 : packoffset(c2.w);

cbuffer CameraBuffer : register(b2)
float3 eyePos : packoffset(c0.x);
float padb2 : packoffset(c0.w);

cbuffer SkinningBuffer : register(b3)
float4x4 m_BoneWorld[50];

//Texture Resources and Sampler States
Texture2D objTexture : register(t0);
Texture2D bumpMap : register(t1);
Texture2D heightMap : register(t2);

SamplerState LinearSampler : register(s0);

//Shaders' input/output layouts
struct vs_input
float3 pos : POSITION;
uint4 bones : BONEIDS;
float4 weights : BONEWEIGHTS;
float2 texCoord : TEXCOORD;
float3 normal : NORMAL;
float3 tangent : TANGENT;
float3 binormal : BINORMAL;

struct ps_input
float4 pos : SV_POSITION;
float2 texCoord : TEXCOORD;
float4 color : COLOR;
float3 normal : NORMAL;
float3 tangent : TANGENT;
float3 binormal : BINORMAL;

//Vertex Shader
ps_input VS(vs_input input)
ps_input output;

//apply bone transforms to position
output.pos = mul(float4(input.pos, 1.0f), m_BoneWorld[input.bones.x] * input.weights.x);
output.pos += mul(float4(input.pos, 1.0f), m_BoneWorld[input.bones.y] * input.weights.y);
output.pos += mul(float4(input.pos, 1.0f), m_BoneWorld[input.bones.z] * input.weights.z);
output.pos += mul(float4(input.pos, 1.0f), m_BoneWorld[input.bones.w] * input.weights.w);

output.pos = mul(output.pos, m_ViewProjection);

//apply bone transforms to normal
output.normal = mul(float4(input.normal, 1.0f), m_BoneWorld[input.bones.x]) * input.weights.x;
output.normal += mul(float4(input.normal, 1.0f), m_BoneWorld[input.bones.y]) * input.weights.y;
output.normal += mul(float4(input.normal, 1.0f), m_BoneWorld[input.bones.z]) * input.weights.z;
output.normal += mul(float4(input.normal, 1.0f), m_BoneWorld[input.bones.w]) * input.weights.w;

//apply bone transforms to tangent
output.tangent = mul(float4(input.tangent, 1.0f), m_BoneWorld[input.bones.x]) * input.weights.x;
output.tangent += mul(float4(input.tangent, 1.0f), m_BoneWorld[input.bones.y]) * input.weights.y;
output.tangent += mul(float4(input.tangent, 1.0f), m_BoneWorld[input.bones.z]) * input.weights.z;
output.tangent += mul(float4(input.tangent, 1.0f), m_BoneWorld[input.bones.w]) * input.weights.w;

//apply bone transforms to normal
output.binormal = mul(float4(input.binormal, 1.0f), m_BoneWorld[input.bones.x]) * input.weights.x;
output.binormal += mul(float4(input.binormal, 1.0f), m_BoneWorld[input.bones.y]) * input.weights.y;
output.binormal += mul(float4(input.binormal, 1.0f), m_BoneWorld[input.bones.z]) * input.weights.z;
output.binormal += mul(float4(input.binormal, 1.0f), m_BoneWorld[input.bones.w]) * input.weights.w;

//transfer texture coordinates
output.texCoord = input.texCoord;

//output result
return output;
I'm pretty sure, that vertex object space positions are calculated correctly and shaders code works ok as I get correct results for Bind Pose. But when animation starts it doesn't work:


The model is taken from http://www.katsbits.com/download/models/ .

To simplify debugging, I created simple model from 2 joints and 4 vertices with two frames of animation and still it doesn't work, but it seems that i checked everything...

This is capture of the second frame(of two). The first frame is rendering correctly showing this black plane lies straight on the floor. In the second frame I just have left part lifted up a bit. Obvisously, I renders incorrectly for me. I attached file with this plane to my post.

Could you give me some ideas why I get wrong result? If you already have implemented GPU skinning could you type your matrices for inverseBindPose of each bone and curObjectTransform of bones for each frame, so that i can compare them with mine(for that black plane model)? I would really appreciate any help!

Thanks! Edited by DgekGD

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As I've implemented MD5 skinning on GPU, I can give you few tips (I don't have time right now to move through your code as I'm being quite in time-pressure till thursday)...

1.) Start from simple MD5 file, that contains only translation and single bone. If it works, create only rotation, then combine them together, then build double-bone - e.g. trace where the problems lie
2.) Double check your math. I was sitting for few hours on problem with inverse matrices, then I found that VS actually *removes* few instructions from my SSE2 matrix inversion code (... forgot volatiles *huh* .. problem hadn't appeared on my main dev platform - linux & gcc)
3.) And triple check your quaternion <-> matrices. Often the problem lies only in rotations, and doing row-major matrix out of quaternion instead of column-major matrix is totally wrong (because of nature of 3x3 rotational matrix, you will actually get inverse rotation, and this can create mess you see).

And last point - try your skinning (e.g. inverse-bind-pose and matrix transformation) on CPU first, it's a lot easier to debug there, than on GPU. [img]http://public.gamedev.net//public/style_emoticons/default/smile.png[/img]

If you'll still have troubles implement it, I can post fragments of my code (with little how-to) here, although not till Thursday evening.

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Thanks for your reply, [b]Vilem Otte[/b]!

[quote]3.) And triple check your quaternion <-> matrices. Often the problem lies only in rotations, and doing row-major matrix out of quaternion instead of column-major matrix is totally wrong (because of nature of 3x3 rotational matrix, you will actually get inverse rotation, and this can create mess you see).[/quote]

This is where the problem was coming from! So, i changed my code to transpose rotation matrices after they were built from quaternios and it works perfectly!
[source lang="cpp"]Mat4x4 rotation = rot.BuildMatrix();[/source]
[source lang="cpp"]Mat4x4 rotation = rot.BuildMatrix();
Thanks again for your help, you saved me from many hours of trying to solve this problem!

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