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[Assimp] Build the skeleton from the scene

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How to construct the 'global' skeleton from a scene imported by the Open Asset Import Library (Assimp) ?


In Assimp, a scene is essentially a hierarchy of nodes, each node can possibly have meshes, each mesh can have a set of bones so it's easy to build a joint hierarchy for a single mesh, but I need to build the joint tree for the whole mesh (as in Doom 3).


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yes, after a week of painful debugging and studying tens of skeletal anim samples i'm able to build a basic skeleton, interpolate and display it correctly (one of the problems was with my quaternion math - needed to multiply the .w component by -1).


I'm going to abandon mSkinOffset and instead calculate inverse bind pose matrices myself (after building the 'whole mesh' skeleton with absolute joint positions and orientations in object space).


I will post my code in the hope that it will be useful.

Here's how I build the skeleton: I gather all bone names - I use them to prune the skeleton - the resulting joint tree will be same as originally defined in md5 mesh file.

// set of all animated bones/nodes
typedef std::set< std::string >	BoneNameSetT;

static void GatherBoneNames( const aiScene* scene, const NodeMapT& nodes, BoneNameSetT &bones )
	// Loop through each bone of each mesh.
    for( int meshIndex = 0; meshIndex < scene->mNumMeshes; meshIndex++ ) 
		const aiMesh* mesh = scene->mMeshes[ meshIndex ];
		for( int boneIndex = 0; boneIndex < mesh->mNumBones; boneIndex++ )
			const aiBone* bone = mesh->mBones[ boneIndex ];
			const aiNode* node = FindNodeByName( nodes, bone->mName.C_Str() );
			while( node )
				bones.insert( node->mName.C_Str() );
				node = node->mParent;
				if( node && node->mNumChildren == 1 ) {
					break;	// don't chase up until the scene root, if possible

struct BoneDesc
	const aiNode *	node;
	std::string	name;	
	int		parentIndex;
	aiMatrix4x4	globalTransform;// absolute (world => node) transform

static void BuildSkeleton(const aiNode* node,
						  const BoneNameSetT& boneNames,
						  const aiMatrix4x4& parentTransform,
						  const int parentBoneIndex,
						  TArray< BoneDesc* > &skeleton)
	// accumulated parents => node transform
	const aiMatrix4x4 globalTransform = node->mTransformation * parentTransform;	// P * B

	int newBoneIndex = parentBoneIndex;

	BoneNameSetT::const_iterator boneIt = boneNames.find( node->mName.C_Str() );
	if( boneIt != boneNames.end() )
		newBoneIndex = skeleton.Num();

		BoneDesc* newBone = new BoneDesc();

		newBone->node = node;
		newBone->name = node->mName.C_Str();		
		newBone->globalTransform = globalTransform;
		newBone->parentIndex = parentBoneIndex;
	for( int childIndex = 0; childIndex < node->mNumChildren; childIndex++ )
		const aiNode* childNode = node->mChildren[ childIndex ];
		BuildSkeleton(childNode, boneNames, globalTransform, newBoneIndex, skeleton);

Here's how I calculate local joint transforms:

		// The bone's transformation in the skeleton space,
		// aka the bind matrix - the bone's parent's local matrices concatenated with the bone's local matrix.
		const aiMatrix4x4 nodeGlobalTransform = boneDesc->globalTransform;
		//Assert(nodeGlobalTransform == CalculateGlobalTransform(node));

		// The transformation relative to the bone's parent (parent => bone space).
		aiMatrix4x4 nodeLocalTransform;
		if( boneDesc->parentIndex != -1 ) {
			const BoneDesc* parentBone = skeleton[ boneDesc->parentIndex ];
			aiMatrix4x4 parentGlobalTransform = parentBone->globalTransform;
			aiMatrix4x4 inverseParentGlobalTransform(parentGlobalTransform);
			nodeLocalTransform = nodeGlobalTransform * inverseParentGlobalTransform;	// N = P^-1 * B
		} else {
			nodeLocalTransform = node->mTransformation;

		aiVector3D	scaling;
		aiQuaternion	rotation;
		aiVector3D	translation;
		nodeLocalTransform.Decompose(scaling, rotation, translation);

		Vector3D	jointTranslation = ToMyVec3D(translation);
		Quaternion	jointOrientation = ToMyQuat(rotation);

I studied the samples very closely: scenes are animated by interpolating node movement, but I don't want to have the full node hierarchy at run-time with redundant bones (referring to the same node, but having different mOffsetMatrix matrices, as mentioned.


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