I recently implemented vertex skinning in my own Vulkan engine. Pretty much all models render properly, however, I find that with Mixamo models, I get skinning artifacts. The main difference I found between "regular" glTF models and Mixamo models, is that the Mixamo models share inverse bind matrices between multiple meshes, but I highly doubt that this causes this issue.
Here you can see that for some reason the vertices are pulled towards one specific point which seems to be at (0, 0, 0). I know for sure that this is not caused by the vertex and index loading, as the model renders properly without vertex skinning.
Calculation of joint matrices
void Object::updateJointsByNode(Node *node) {
if (node->mesh && node->skinIndex > -1) {
auto inverseTransform = glm::inverse(node->getLocalMatrix());
auto skin = this->skinLookup[node->skinIndex];
auto numberOfJoints = skin->jointsIndices.size();
std::vector<glm::mat4> jointMatrices(numberOfJoints);
for (size_t i = 0; i < numberOfJoints; i++) {
jointMatrices[i] =
this->getNodeByIndex(skin->jointsIndices[i])->getLocalMatrix() * skin->inverseBindMatrices[i];
jointMatrices[i] = inverseTransform * jointMatrices[i];
}
this->inverseBindMatrices = jointMatrices;
}
for (auto &child : node->children) {
this->updateJointsByNode(child);
}
}
Calculation of vertex displacement in GLSL Vertex Shader
mat4 skinMat =
inWeight0.x * model.jointMatrix[int(inJoint0.x)] +
inWeight0.y * model.jointMatrix[int(inJoint0.y)] +
inWeight0.z * model.jointMatrix[int(inJoint0.z)] +
inWeight0.w * model.jointMatrix[int(inJoint0.w)];
localPosition = model.model * model.local * skinMat * vec4(inPosition, 1.0);
So basically the mistake I made was relatively simple. It was caused by casting the joints to the wrong type. When a glTF model uses a byte stride of size 4 for the vertex joints, the implicit type is uint8_t
(unsigned byte) and not the uint16_t
(unsigned short) which I was using.