OpenGL matrix vs DirectX matrix

Question

I'm working on an animation and model exporter from Maya to use within my game. Currently, models work perfectly, and animations have some issues with the joints. Since Maya works in an OpenGL environment and my game works with DirectX, to load the mesh in I negated the Z component of all vertex positions, normals, and inverted the UV's V coordinate.

Currently, joints loaded in are reversed along the Z axis. My question is, what do I need to do to get bone matrices working properly from OpenGL to DirectX? My original thought was to just transpose them, but that doesn't work. I've also tried negating the z-column but that causes some odd stretching.

I've only been trying to invert the root bone.

Notice the feet joints pointing backward. https://i.stack.imgur.com/6kNr2.png

Inverting the root joints Z-Column, weird stretching and it looks like he's slouched. https://i.stack.imgur.com/KysH8.png

Any thoughts on this?

Answer 1

You need to flip your matrices and all points or use a Right handed projection matrix for everything in your game.

I wrote my own exporter and noticed the same problem but like Left handed so I just multiplied all Maya matrices by the following

// A float version of a right to left handed converison matrix
float RightToLeftMatData[4][4] = {
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, -1, 0,
    0, 0, 0, 1,
};

//Not sure why but transforms must be built off of the dagPath
MStatus status;
MMatrix tMMat = dagPath.inclusiveMatrix(&status);
if (status != MS::kSuccess)
    Error("Failed to get inclusive matrix");
tMMat.get(tMat.m);

// Cobalt always uses left handed coordinates so convert the coordinate system
if (ConvertFromRightHandedCoordinate)
{
    tMat = mRightToLeft * tMat * mRightToLeft;
}

Also you'll need to modify every vertex's position, normal, binormal, tangent, and bitangent like so...

//Animation requires that we do not get vertex points pretransformed to world space
fnMesh.getPoints( vertexList, MSpace::kObject);
for ( uint32 i = 0; i < vertexList.length(); i++ )
{
    vertexList[i].cartesianize();
    MPoint point = vertexList[i];
    if (ConvertFromRightHandedCoordinate) vPoint.push_back(cVector3(point.x, point.y, -point.z));
    else vPoint.push_back(cVector3(point.x, point.y, point.z));
}

fnMesh.getNormals( normalList, MSpace::kObject);
for (uint32 i = 0; i < normalList.length(); i++)
{
    MPoint point = normalList[i];
    if (ConvertFromRightHandedCoordinate) vNormal.push_back(cVector3(point.x, point.y, -point.z));
    else vNormal.push_back(cVector3(point.x, point.y, point.z));
}

// Same thing for binormal and bitangent

I also flip the V coordinates for textures though you could always do this in a shader if you'd prefer. A lot of shader examples from NVidia actually have a flag to do that.

If you need some reference material feel free to take anything you want from my maya conversion. http://code.google.com/p/cobaltlibrary/source/browse/trunk/Tools/Source/MayaConvert/MayaFile.cpp

Answer 2

I don't think you need to flip the z-coordinate of your vertex, just transpose you matrix and use it in your DirectX program A command misunderstanding about this is LH or RH system, but it's really nothing about which coordinate system you are using, it's about row-major(DirectX) or column-major(OpenGL) matrix.