I have wanted to make an FBX Exporter to convert FBX files to my own format for a while. The entire process is not very smooth, mainly because FBX's official documentation is not very clear. Plus, since FBX format is utilized by a number of applications, rather than just game engines, the sample code provided is not using the argots we use in game development.
I have searched almost all the corners on the Internet to clarify things so that I can have a clear mapping from FBX SDK's data to what I need in a game engine. Since I don't think anyone has ever posted a clear and thorough tutorial on how to convert FBX files to custom formats, I will do it. I hope this will help people.
This tutorial would be specifically about game engines. Basically I will tell the reader how to get the data they need for their game engine. For things like "how to initialize FBX SDK", please check the sample code yourself, the "ImportScene" sample would be very useful in this respect.
The first thing you want to do is to get the mesh data; it already feels pretty damn good if you can import your static mesh into your engine.
First please let me explain how FBX stores all its information about a mesh. In FBX we have the term "Control Point", basically a control point is a physical vertex. For example, you have a cube, then you have 8 vertices. These 8 vertices are the only 8 "control points" in the FBX file. As a result, if you want, you can use "Vertex" and "Control Point" interchangeably. The position information is stored in the control points.
The following code would get you the positions of all the vertices of your mesh:
Then you ask "how the hell can I get the UVs, Normals, Tangents, Binormals?" Well, please think of a mesh like this for a moment: You have this body of the mesh, but this is only the geometry, the shape of it. This body does not have any information about its surface. In other words, you have this shape, but you don't have any information on how the surface of this shape looks.
FBX introduces this sense of "Layer", which covers the body of the mesh. It is like you have a box, and you wrap it with gift paper. This gift paper is the layer of the mesh in FBX. In the layer, you can acquire the information of UVs, Normals, Tangents, Binormals.
However, you might have already asked me. How can I relate the Control Points to the information in the layer? Well, this is the pretty tricky part and please let me show you some code and then explain it line by line. Without loss of generality, I will use Binormal as an example:
Well, this is pretty damn long but please don't be scared. Actually it is very simple. One thing to keep in mind is that outside of this function, we are using a loop to traverse all the vertices of all the triangles in this mesh. That is why we can have parameters like inCtrlPointIndex and inVertexCounter
The parameters of this function:
FbxMesh* inMesh: the mesh that we are trying to export
int inCtrlPointIndex: the index of the Control Point. We need this because we want to relate our layer information with our vertices (Control Points)
int inVertexCounter: this is the index of the current vertex that we are processing. This might be confusing. Ignore this for now.
XMFLOAT3& outNormal: the output. This is trivial to explain
This gets us the normal information in the layer
The first switch statement is about MappingMode(). For a game engine, I think we only need to worry about FbxGeometryElement::eByControlPoint and FbxGeometryElement::eByPolygonVertex. Let me explain the 2 modes. As I said, Control Points are basically the vertices. However, there is a problem. Although a cube has 8 vertices, it will have more than 8 normals if you want your cube to look correct. The reason is if you have a sharp edge, we have to assign more than one normal to the same control point (vertex) to guarantee that feeling of sharpness.
As a result, FbxGeometryElement::eByControlPoint is when you don't have sharp edged situations so each control point only has one normal. FbxGeometryElement::eByPolygonVertex is when you have sharp edges and you need to get the normals of each vertex on each face because each face has a different normal assigned for the same control point. So FbxGeometryElement::eByControlPoint means we can pinpoint the normal of a control point by the index of the control point while FbxGeometryElement::eByPolygonVertex means we cna pinpoint the normal of a vertex on a face by the index of the vertex
This is why in the above code I passed in both inCtrlPointIndex and inVertexCounter. Because we don't know which one we need to get the information we need, we better pass in both.
Now we have another switch statement nested inside, and we are "switching" on ReferenceMode(). This is some kind of optimization FBX is doing, same idea like index buffer in computer graphics. You don't want to have the same Vector3 many times; instead, you refer to it using its index.
FbxGeometryElement::eDirect means you can refer to our normal using the index of control point or index of face-vertex directly
FbxGeometryElement::eIndexToDirect means using the index of control point or index of face-vertex would only gives us an index pointing to the normal we want, we have to use this index to find the actual normal.
This line of code gives us the index we need
So these are the main steps to extract position and "layer" information of a mesh. Below is how I traverse the triangles in a mesh.
Note that there is some code related to blending info for animation. You can ignore it for now. We will come back to it later.
Now we move onto animation and this is the hard part of FBX exporting.
So let's think about what we need from FBX to make animation work in our renderer (game engine).
To get the skeleton hierarchy is pretty easy: basically we perform a recursive DFS from the root node of the scene and we go down levels. If a node is of skeleton type, we add it into our list of joints and its index will just be the size of the list. Therefore, we can guarantee that the index of the parent is always going to be less than that of the child. This is necessary if you want to store local transform and calculate the transformation of a child at time t manually. But if you are using global transformation like I do, you don't necessary need it like this.
Now we need to get the SkinningWeight-JointIndex pairs of each vertex. Unfortunately, my code is not very clean on animation so the function below does steps 2,3,4 all at once. I will go over the code so please do not lose patience. This is mainly because the way FBX stores information prevents me from getting data in separate functions efficiently. I need to traverse the same data in multiple-passes if I want to separate my code.
Before seeing any code, please let me explain the terms used in FBX SDK. This is the part where I think most people get confused because FBX SDK's keywords do not match ours (game developers).
In FBX, there is such a thing called a "Deformer". I see a deformer as a way to deform a mesh. You know in Maya, you can have skeletal deformers but you can also have "contraints" to deform your mesh. I think you can think of "Deformers" as the entire skeleton of a mesh. Inside each "Deformer" (I think usually a mesh only has one), you have "Clusters". Each cluster is and is not a joint...... You can see a cluster as a joint, but actually, inside each cluster, there is a "link". This "link" is actually the real joint, and it contains the useful information I need.
Now we delve into the code:
At the beginning I have this:
Well, this is what I saw on the FBX SDK Forum. The officials there told us we should take into account the "GeometricTransform". But according to my experience, most of the times, this "GeometricTransform" is just an identity matrix. Anyways, to get this "GeometricTransform", use this function:
The very most important thing in this code is how I get the inverse of global bind pose of each joint. This part is very tricky and screwed up many people. I will explain this in details.
So let's start from this GetTransformMatrix. The TransformMatrix is actually a legacy thing. It is the Global Transform of the entire mesh at binding time and all the clusters have exactly the same TransformMatrix. This matrix would not be needed if your artists have good habits and before they rig the model, they "Freeze Transformations" on all channels of the model. If your artists do "Freeze Transformations", then this matrix would just be an identity matrix.
Now we go on to GetTransformLinkMatrix. This is the very essence of the animation exporting code. This is the transformation of the cluster (joint) at binding time from joint space to world space in Maya.
So now we are all set and we can get our inverse of global bind pose of each joint. What we want eventually is the InverseOfGlobalBindPoseMatrix in VertexAtTimeT = TransformationOfPostAtTimeT * InverseOfGlobalBindPoseMatrix * VertexAtBindingTime
To get this, we do this: transformLinkMatrix.Inverse() * transformMatrix * geometryTransform
Now we are 2 steps away from animation. We need to get the SkinningWeight-JointIndex pair for each vertex and we still need to get the transformations at different times in the animation
Let's deal with SkinningWeight-JointIndex pair first.
In our game engine, we have this relationship: Vertex -> 4 SkinningWeight-JointIndex pairs. However, in FBX SDK the relationship is inverted. Each cluster has a list of all the control points (vertices) it affects and how much it affects. The code below gets the relationship in the format we favor but please recall that when I process control points, I stored all the control points into a map based on their indices. This is where we can profit. With this map, here we can lookup and update the control point a cluster affects in O(1).
Now we only need the last piece in the puzzle: the Transformations at time t in the animation. Note that this part is something I did not do well, my way is not very optimized since I get every keyframe. What should ideally be done is to get the keys and interpolate between them, but I guess this is a trade-off between space and speed. Also, I did not get down to my feet and study the animation hierarchy of FBX. There is actually an animation curve stored inside FBX file and with some work, you can access it and get lean and clean what you need.
This part is pretty straightforward - the only thing to be noted is that Maya currently does not support multi-take animations (Perhaps MotionBuilder does). I will decide if I write about exporting materials based on how many people read this article, but it is pretty easy and can be learnt through the "ImportScene" example
FBX uses the same coordinate system as OpenGL does, which is (X-Right, Y-Up, Z-Out). However, what if we want to use our animations in DirectX?
Basically:
Position, Normal, Binormal, Tangent -> we need to negate the Z component of the Vector
UV -> we need to make V = 1.0f - V
Vertex order of a triangle -> change from Vertex0, Vertex1, Vertex2 to Vertex0, Vertex2, Vertex1 (Basically invert the culling order)
Matrices:
Well, I am actually quite a noob on FBXSDK and game programming in general. If you see any mistakes, or you see any space for improvements, please comment on this article and help me get better. I know there are a lot of pros on this forum, they just don't have enough time to write an article like this to teach people step by step.
Well, FBXSDK can be pretty nasty to work with. However, once you know what data in FBX means, it is actually very easy to use. I think my article is enough to get people started on using FBXSDK. Please leave a comment if you have any questions.
2.19.2014 First Version Submitted
I have searched almost all the corners on the Internet to clarify things so that I can have a clear mapping from FBX SDK's data to what I need in a game engine. Since I don't think anyone has ever posted a clear and thorough tutorial on how to convert FBX files to custom formats, I will do it. I hope this will help people.
This tutorial would be specifically about game engines. Basically I will tell the reader how to get the data they need for their game engine. For things like "how to initialize FBX SDK", please check the sample code yourself, the "ImportScene" sample would be very useful in this respect.
Mesh Data(position, UV, normal, tangent, binormal)
The first thing you want to do is to get the mesh data; it already feels pretty damn good if you can import your static mesh into your engine.
First please let me explain how FBX stores all its information about a mesh. In FBX we have the term "Control Point", basically a control point is a physical vertex. For example, you have a cube, then you have 8 vertices. These 8 vertices are the only 8 "control points" in the FBX file. As a result, if you want, you can use "Vertex" and "Control Point" interchangeably. The position information is stored in the control points.
The following code would get you the positions of all the vertices of your mesh:
// inNode is the Node in this FBX Scene that contains the mesh
// this is why I can use inNode->GetMesh() on it to get the mesh
void FBXExporter::ProcessControlPoints(FbxNode* inNode)
{
FbxMesh* currMesh = inNode->GetMesh();
unsigned int ctrlPointCount = currMesh->GetControlPointsCount();
for(unsigned int i = 0; i < ctrlPointCount; ++i)
{
CtrlPoint* currCtrlPoint = new CtrlPoint();
XMFLOAT3 currPosition;
currPosition.x = static_cast<float>(currMesh->GetControlPointAt(i).mData[0]);
currPosition.y = static_cast<float>(currMesh->GetControlPointAt(i).mData[1]);
currPosition.z = static_cast<float>(currMesh->GetControlPointAt(i).mData[2]);
currCtrlPoint->mPosition = currPosition;
mControlPoints[i] = currCtrlPoint;
}
}
Then you ask "how the hell can I get the UVs, Normals, Tangents, Binormals?" Well, please think of a mesh like this for a moment: You have this body of the mesh, but this is only the geometry, the shape of it. This body does not have any information about its surface. In other words, you have this shape, but you don't have any information on how the surface of this shape looks.
FBX introduces this sense of "Layer", which covers the body of the mesh. It is like you have a box, and you wrap it with gift paper. This gift paper is the layer of the mesh in FBX. In the layer, you can acquire the information of UVs, Normals, Tangents, Binormals.
However, you might have already asked me. How can I relate the Control Points to the information in the layer? Well, this is the pretty tricky part and please let me show you some code and then explain it line by line. Without loss of generality, I will use Binormal as an example:
void FBXExporter::ReadNormal(FbxMesh* inMesh, int inCtrlPointIndex, int inVertexCounter, XMFLOAT3& outNormal)
{
if(inMesh->GetElementNormalCount() < 1)
{
throw std::exception("Invalid Normal Number");
}
FbxGeometryElementNormal* vertexNormal = inMesh->GetElementNormal(0);
switch(vertexNormal->GetMappingMode())
{
case FbxGeometryElement::eByControlPoint:
switch(vertexNormal->GetReferenceMode())
{
case FbxGeometryElement::eDirect:
{
outNormal.x = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inCtrlPointIndex).mData[0]);
outNormal.y = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inCtrlPointIndex).mData[1]);
outNormal.z = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inCtrlPointIndex).mData[2]);
}
break;
case FbxGeometryElement::eIndexToDirect:
{
int index = vertexNormal->GetIndexArray().GetAt(inCtrlPointIndex);
outNormal.x = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[0]);
outNormal.y = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[1]);
outNormal.z = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[2]);
}
break;
default:
throw std::exception("Invalid Reference");
}
break;
case FbxGeometryElement::eByPolygonVertex:
switch(vertexNormal->GetReferenceMode())
{
case FbxGeometryElement::eDirect:
{
outNormal.x = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inVertexCounter).mData[0]);
outNormal.y = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inVertexCounter).mData[1]);
outNormal.z = static_cast<float>(vertexNormal->GetDirectArray().GetAt(inVertexCounter).mData[2]);
}
break;
case FbxGeometryElement::eIndexToDirect:
{
int index = vertexNormal->GetIndexArray().GetAt(inVertexCounter);
outNormal.x = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[0]);
outNormal.y = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[1]);
outNormal.z = static_cast<float>(vertexNormal->GetDirectArray().GetAt(index).mData[2]);
}
break;
default:
throw std::exception("Invalid Reference");
}
break;
}
}
Well, this is pretty damn long but please don't be scared. Actually it is very simple. One thing to keep in mind is that outside of this function, we are using a loop to traverse all the vertices of all the triangles in this mesh. That is why we can have parameters like inCtrlPointIndex and inVertexCounter
The parameters of this function:
FbxMesh* inMesh: the mesh that we are trying to export
int inCtrlPointIndex: the index of the Control Point. We need this because we want to relate our layer information with our vertices (Control Points)
int inVertexCounter: this is the index of the current vertex that we are processing. This might be confusing. Ignore this for now.
XMFLOAT3& outNormal: the output. This is trivial to explain
This gets us the normal information in the layer
FbxGeometryElementNormal* vertexNormal = inMesh->GetElementNormal(0);
The first switch statement is about MappingMode(). For a game engine, I think we only need to worry about FbxGeometryElement::eByControlPoint and FbxGeometryElement::eByPolygonVertex. Let me explain the 2 modes. As I said, Control Points are basically the vertices. However, there is a problem. Although a cube has 8 vertices, it will have more than 8 normals if you want your cube to look correct. The reason is if you have a sharp edge, we have to assign more than one normal to the same control point (vertex) to guarantee that feeling of sharpness.
As a result, FbxGeometryElement::eByControlPoint is when you don't have sharp edged situations so each control point only has one normal. FbxGeometryElement::eByPolygonVertex is when you have sharp edges and you need to get the normals of each vertex on each face because each face has a different normal assigned for the same control point. So FbxGeometryElement::eByControlPoint means we can pinpoint the normal of a control point by the index of the control point while FbxGeometryElement::eByPolygonVertex means we cna pinpoint the normal of a vertex on a face by the index of the vertex
This is why in the above code I passed in both inCtrlPointIndex and inVertexCounter. Because we don't know which one we need to get the information we need, we better pass in both.
Now we have another switch statement nested inside, and we are "switching" on ReferenceMode(). This is some kind of optimization FBX is doing, same idea like index buffer in computer graphics. You don't want to have the same Vector3 many times; instead, you refer to it using its index.
FbxGeometryElement::eDirect means you can refer to our normal using the index of control point or index of face-vertex directly
FbxGeometryElement::eIndexToDirect means using the index of control point or index of face-vertex would only gives us an index pointing to the normal we want, we have to use this index to find the actual normal.
This line of code gives us the index we need
int index = vertexNormal->GetIndexArray().GetAt(inVertexCounter);
So these are the main steps to extract position and "layer" information of a mesh. Below is how I traverse the triangles in a mesh.
void FBXExporter::ProcessMesh(FbxNode* inNode)
{
FbxMesh* currMesh = inNode->GetMesh();
mTriangleCount = currMesh->GetPolygonCount();
int vertexCounter = 0;
mTriangles.reserve(mTriangleCount);
for (unsigned int i = 0; i < mTriangleCount; ++i)
{
XMFLOAT3 normal[3];
XMFLOAT3 tangent[3];
XMFLOAT3 binormal[3];
XMFLOAT2 UV[3][2];
Triangle currTriangle;
mTriangles.push_back(currTriangle);
for (unsigned int j = 0; j < 3; ++j)
{
int ctrlPointIndex = currMesh->GetPolygonVertex(i, j);
CtrlPoint* currCtrlPoint = mControlPoints[ctrlPointIndex];
ReadNormal(currMesh, ctrlPointIndex, vertexCounter, normal[j]);
// We only have diffuse texture
for (int k = 0; k < 1; ++k)
{
ReadUV(currMesh, ctrlPointIndex, currMesh->GetTextureUVIndex(i, j), k, UV[j][k]);
}
PNTIWVertex temp;
temp.mPosition = currCtrlPoint->mPosition;
temp.mNormal = normal[j];
temp.mUV = UV[j][0];
// Copy the blending info from each control point
for(unsigned int i = 0; i < currCtrlPoint->mBlendingInfo.size(); ++i)
{
VertexBlendingInfo currBlendingInfo;
currBlendingInfo.mBlendingIndex = currCtrlPoint->mBlendingInfo[i].mBlendingIndex;
currBlendingInfo.mBlendingWeight = currCtrlPoint->mBlendingInfo[i].mBlendingWeight;
temp.mVertexBlendingInfos.push_back(currBlendingInfo);
}
// Sort the blending info so that later we can remove
// duplicated vertices
temp.SortBlendingInfoByWeight();
mVertices.push_back(temp);
mTriangles.back().mIndices.push_back(vertexCounter);
++vertexCounter;
}
}
// Now mControlPoints has served its purpose
// We can free its memory
for(auto itr = mControlPoints.begin(); itr != mControlPoints.end(); ++itr)
{
delete itr->second;
}
mControlPoints.clear();
}
Note that there is some code related to blending info for animation. You can ignore it for now. We will come back to it later.
Now we move onto animation and this is the hard part of FBX exporting.
Animation Data
So let's think about what we need from FBX to make animation work in our renderer (game engine).
- The skeleton hierarchy. Which joint is which joint's parent
- For each vertex, we need 4 SkinningWeight-JointIndex pairs
- The Bind pose matrix for each joint to calculate the inverse of global bind pose
- The transformation matrix at time t so that we can transform our mesh to that pose to achieve animation
To get the skeleton hierarchy is pretty easy: basically we perform a recursive DFS from the root node of the scene and we go down levels. If a node is of skeleton type, we add it into our list of joints and its index will just be the size of the list. Therefore, we can guarantee that the index of the parent is always going to be less than that of the child. This is necessary if you want to store local transform and calculate the transformation of a child at time t manually. But if you are using global transformation like I do, you don't necessary need it like this.
void FBXExporter::ProcessSkeletonHierarchy(FbxNode* inRootNode)
{
for (int childIndex = 0; childIndex < inRootNode->GetChildCount(); ++childIndex)
{
FbxNode* currNode = inRootNode->GetChild(childIndex);
ProcessSkeletonHierarchyRecursively(currNode, 0, 0, -1);
}
}
// inDepth is not needed here, I used it for debug but forgot to remove it
void FBXExporter::ProcessSkeletonHierarchyRecursively(FbxNode* inNode, int inDepth, int myIndex, int inParentIndex)
{
if(inNode->GetNodeAttribute() && inNode->GetNodeAttribute()->GetAttributeType() && inNode->GetNodeAttribute()->GetAttributeType() == FbxNodeAttribute::eSkeleton)
{
Joint currJoint;
currJoint.mParentIndex = inParentIndex;
currJoint.mName = inNode->GetName();
mSkeleton.mJoints.push_back(currJoint);
}
for (int i = 0; i < inNode->GetChildCount(); i++)
{
ProcessSkeletonHierarchyRecursively(inNode->GetChild(i), inDepth + 1, mSkeleton.mJoints.size(), myIndex);
}
}
Now we need to get the SkinningWeight-JointIndex pairs of each vertex. Unfortunately, my code is not very clean on animation so the function below does steps 2,3,4 all at once. I will go over the code so please do not lose patience. This is mainly because the way FBX stores information prevents me from getting data in separate functions efficiently. I need to traverse the same data in multiple-passes if I want to separate my code.
Before seeing any code, please let me explain the terms used in FBX SDK. This is the part where I think most people get confused because FBX SDK's keywords do not match ours (game developers).
In FBX, there is such a thing called a "Deformer". I see a deformer as a way to deform a mesh. You know in Maya, you can have skeletal deformers but you can also have "contraints" to deform your mesh. I think you can think of "Deformers" as the entire skeleton of a mesh. Inside each "Deformer" (I think usually a mesh only has one), you have "Clusters". Each cluster is and is not a joint...... You can see a cluster as a joint, but actually, inside each cluster, there is a "link". This "link" is actually the real joint, and it contains the useful information I need.
Now we delve into the code:
void FBXExporter::ProcessJointsAndAnimations(FbxNode* inNode)
{
FbxMesh* currMesh = inNode->GetMesh();
unsigned int numOfDeformers = currMesh->GetDeformerCount();
// This geometry transform is something I cannot understand
// I think it is from MotionBuilder
// If you are using Maya for your models, 99% this is just an
// identity matrix
// But I am taking it into account anyways......
FbxAMatrix geometryTransform = Utilities::GetGeometryTransformation(inNode);
// A deformer is a FBX thing, which contains some clusters
// A cluster contains a link, which is basically a joint
// Normally, there is only one deformer in a mesh
for (unsigned int deformerIndex = 0; deformerIndex < numOfDeformers; ++deformerIndex)
{
// There are many types of deformers in Maya,
// We are using only skins, so we see if this is a skin
FbxSkin* currSkin = reinterpret_cast<FbxSkin*>(currMesh->GetDeformer(deformerIndex, FbxDeformer::eSkin));
if (!currSkin)
{
continue;
}
unsigned int numOfClusters = currSkin->GetClusterCount();
for (unsigned int clusterIndex = 0; clusterIndex < numOfClusters; ++clusterIndex)
{
FbxCluster* currCluster = currSkin->GetCluster(clusterIndex);
std::string currJointName = currCluster->GetLink()->GetName();
unsigned int currJointIndex = FindJointIndexUsingName(currJointName);
FbxAMatrix transformMatrix;
FbxAMatrix transformLinkMatrix;
FbxAMatrix globalBindposeInverseMatrix;
currCluster->GetTransformMatrix(transformMatrix); // The transformation of the mesh at binding time
currCluster->GetTransformLinkMatrix(transformLinkMatrix); // The transformation of the cluster(joint) at binding time from joint space to world space
globalBindposeInverseMatrix = transformLinkMatrix.Inverse() * transformMatrix * geometryTransform;
// Update the information in mSkeleton
mSkeleton.mJoints[currJointIndex].mGlobalBindposeInverse = globalBindposeInverseMatrix;
mSkeleton.mJoints[currJointIndex].mNode = currCluster->GetLink();
// Associate each joint with the control points it affects
unsigned int numOfIndices = currCluster->GetControlPointIndicesCount();
for (unsigned int i = 0; i < numOfIndices; ++i)
{
BlendingIndexWeightPair currBlendingIndexWeightPair;
currBlendingIndexWeightPair.mBlendingIndex = currJointIndex;
currBlendingIndexWeightPair.mBlendingWeight = currCluster->GetControlPointWeights()[i];
mControlPoints[currCluster->GetControlPointIndices()[i]]->mBlendingInfo.push_back(currBlendingIndexWeightPair);
}
// Get animation information
// Now only supports one take
FbxAnimStack* currAnimStack = mFBXScene->GetSrcObject<FbxAnimStack>(0);
FbxString animStackName = currAnimStack->GetName();
mAnimationName = animStackName.Buffer();
FbxTakeInfo* takeInfo = mFBXScene->GetTakeInfo(animStackName);
FbxTime start = takeInfo->mLocalTimeSpan.GetStart();
FbxTime end = takeInfo->mLocalTimeSpan.GetStop();
mAnimationLength = end.GetFrameCount(FbxTime::eFrames24) - start.GetFrameCount(FbxTime::eFrames24) + 1;
Keyframe** currAnim = &mSkeleton.mJoints[currJointIndex].mAnimation;
for (FbxLongLong i = start.GetFrameCount(FbxTime::eFrames24); i <= end.GetFrameCount(FbxTime::eFrames24); ++i)
{
FbxTime currTime;
currTime.SetFrame(i, FbxTime::eFrames24);
*currAnim = new Keyframe();
(*currAnim)->mFrameNum = i;
FbxAMatrix currentTransformOffset = inNode->EvaluateGlobalTransform(currTime) * geometryTransform;
(*currAnim)->mGlobalTransform = currentTransformOffset.Inverse() * currCluster->GetLink()->EvaluateGlobalTransform(currTime);
currAnim = &((*currAnim)->mNext);
}
}
}
// Some of the control points only have less than 4 joints
// affecting them.
// For a normal renderer, there are usually 4 joints
// I am adding more dummy joints if there isn't enough
BlendingIndexWeightPair currBlendingIndexWeightPair;
currBlendingIndexWeightPair.mBlendingIndex = 0;
currBlendingIndexWeightPair.mBlendingWeight = 0;
for(auto itr = mControlPoints.begin(); itr != mControlPoints.end(); ++itr)
{
for(unsigned int i = itr->second->mBlendingInfo.size(); i <= 4; ++i)
{
itr->second->mBlendingInfo.push_back(currBlendingIndexWeightPair);
}
}
}
At the beginning I have this:
// This geometry transform is something I cannot understand // I think it is from MotionBuilder // If you are using Maya for your models, 99% this is just an // identity matrix // But I am taking it into account anyways...... FbxAMatrix geometryTransform = Utilities::GetGeometryTransformation(inNode);
Well, this is what I saw on the FBX SDK Forum. The officials there told us we should take into account the "GeometricTransform". But according to my experience, most of the times, this "GeometricTransform" is just an identity matrix. Anyways, to get this "GeometricTransform", use this function:
FbxAMatrix Utilities::GetGeometryTransformation(FbxNode* inNode)
{
if (!inNode)
{
throw std::exception("Null for mesh geometry");
}
const FbxVector4 lT = inNode->GetGeometricTranslation(FbxNode::eSourcePivot);
const FbxVector4 lR = inNode->GetGeometricRotation(FbxNode::eSourcePivot);
const FbxVector4 lS = inNode->GetGeometricScaling(FbxNode::eSourcePivot);
return FbxAMatrix(lT, lR, lS);
}
The very most important thing in this code is how I get the inverse of global bind pose of each joint. This part is very tricky and screwed up many people. I will explain this in details.
FbxAMatrix transformMatrix; FbxAMatrix transformLinkMatrix; FbxAMatrix globalBindposeInverseMatrix; currCluster->GetTransformMatrix(transformMatrix); // The transformation of the mesh at binding time currCluster->GetTransformLinkMatrix(transformLinkMatrix); // The transformation of the cluster(joint) at binding time from joint space to world space globalBindposeInverseMatrix = transformLinkMatrix.Inverse() * transformMatrix * geometryTransform; // Update the information in mSkeleton mSkeleton.mJoints[currJointIndex].mGlobalBindposeInverse = globalBindposeInverseMatrix;
So let's start from this GetTransformMatrix. The TransformMatrix is actually a legacy thing. It is the Global Transform of the entire mesh at binding time and all the clusters have exactly the same TransformMatrix. This matrix would not be needed if your artists have good habits and before they rig the model, they "Freeze Transformations" on all channels of the model. If your artists do "Freeze Transformations", then this matrix would just be an identity matrix.
Now we go on to GetTransformLinkMatrix. This is the very essence of the animation exporting code. This is the transformation of the cluster (joint) at binding time from joint space to world space in Maya.
So now we are all set and we can get our inverse of global bind pose of each joint. What we want eventually is the InverseOfGlobalBindPoseMatrix in VertexAtTimeT = TransformationOfPostAtTimeT * InverseOfGlobalBindPoseMatrix * VertexAtBindingTime
To get this, we do this: transformLinkMatrix.Inverse() * transformMatrix * geometryTransform
Now we are 2 steps away from animation. We need to get the SkinningWeight-JointIndex pair for each vertex and we still need to get the transformations at different times in the animation
Let's deal with SkinningWeight-JointIndex pair first.
In our game engine, we have this relationship: Vertex -> 4 SkinningWeight-JointIndex pairs. However, in FBX SDK the relationship is inverted. Each cluster has a list of all the control points (vertices) it affects and how much it affects. The code below gets the relationship in the format we favor but please recall that when I process control points, I stored all the control points into a map based on their indices. This is where we can profit. With this map, here we can lookup and update the control point a cluster affects in O(1).
// Associate each joint with the control points it affects
unsigned int numOfIndices = currCluster->GetControlPointIndicesCount();
for (unsigned int i = 0; i < numOfIndices; ++i)
{
BlendingIndexWeightPair currBlendingIndexWeightPair;
currBlendingIndexWeightPair.mBlendingIndex = currJointIndex;
currBlendingIndexWeightPair.mBlendingWeight = currCluster->GetControlPointWeights()[i];
mControlPoints[currCluster->GetControlPointIndices()[i]]->mBlendingInfo.push_back(currBlendingIndexWeightPair);
}
Now we only need the last piece in the puzzle: the Transformations at time t in the animation. Note that this part is something I did not do well, my way is not very optimized since I get every keyframe. What should ideally be done is to get the keys and interpolate between them, but I guess this is a trade-off between space and speed. Also, I did not get down to my feet and study the animation hierarchy of FBX. There is actually an animation curve stored inside FBX file and with some work, you can access it and get lean and clean what you need.
// Get animation information
// Now only supports one take
FbxAnimStack* currAnimStack = mFBXScene->GetSrcObject<FbxAnimStack>(0);
FbxString animStackName = currAnimStack->GetName();
mAnimationName = animStackName.Buffer();
FbxTakeInfo* takeInfo = mFBXScene->GetTakeInfo(animStackName);
FbxTime start = takeInfo->mLocalTimeSpan.GetStart();
FbxTime end = takeInfo->mLocalTimeSpan.GetStop();
mAnimationLength = end.GetFrameCount(FbxTime::eFrames24) - start.GetFrameCount(FbxTime::eFrames24) + 1;
Keyframe** currAnim = &mSkeleton.mJoints[currJointIndex].mAnimation;
for (FbxLongLong i = start.GetFrameCount(FbxTime::eFrames24); i <= end.GetFrameCount(FbxTime::eFrames24); ++i)
{
FbxTime currTime;
currTime.SetFrame(i, FbxTime::eFrames24);
*currAnim = new Keyframe();
(*currAnim)->mFrameNum = i;
FbxAMatrix currentTransformOffset = inNode->EvaluateGlobalTransform(currTime) * geometryTransform;
(*currAnim)->mGlobalTransform = currentTransformOffset.Inverse() * currCluster->GetLink()->EvaluateGlobalTransform(currTime);
currAnim = &((*currAnim)->mNext);
}
This part is pretty straightforward - the only thing to be noted is that Maya currently does not support multi-take animations (Perhaps MotionBuilder does). I will decide if I write about exporting materials based on how many people read this article, but it is pretty easy and can be learnt through the "ImportScene" example
DirectX and OpenGL Conversions
FBX uses the same coordinate system as OpenGL does, which is (X-Right, Y-Up, Z-Out). However, what if we want to use our animations in DirectX?
Basically:
Position, Normal, Binormal, Tangent -> we need to negate the Z component of the Vector
UV -> we need to make V = 1.0f - V
Vertex order of a triangle -> change from Vertex0, Vertex1, Vertex2 to Vertex0, Vertex2, Vertex1 (Basically invert the culling order)
Matrices:
- Get translation component of the matrix, negate its Z component
- Get rotation component of the matrix, negate its X and Y component
- I think if you are using XMMath library, you don't need to take the transpose. But don't quote me on that.
Corrections, Improvements, Advice
Well, I am actually quite a noob on FBXSDK and game programming in general. If you see any mistakes, or you see any space for improvements, please comment on this article and help me get better. I know there are a lot of pros on this forum, they just don't have enough time to write an article like this to teach people step by step.
Conclusion
Well, FBXSDK can be pretty nasty to work with. However, once you know what data in FBX means, it is actually very easy to use. I think my article is enough to get people started on using FBXSDK. Please leave a comment if you have any questions.
Article Update Log
2.19.2014 First Version Submitted