# OpenGL How do I swap Y and Z axis with D3DXMATRIX?

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I seem to be having problem exporting camera with 3ds max.
The object is sitting horizontally in max, now the exported object is standing upright in dx9
I believe the y-z axis is wrong. How do I flip it around? I mean what matrix can I multiply with this wrong matrix?
[code]
///
float mat[4][4];
Matrix3 invTM;
int persp;
float hither;
float yon;
D3DXMATRIX m_d3dWorldXform; // your app world transform matrix
D3DXMATRIX m_d3dViewXform; // your app view transform matrix
D3DXMATRIX m_d3dProjXform; // your app projection transform matrix
Interface *ip2 = GetCOREInterface();
ViewExp * pView = ip2->GetActiveViewport(); // Get the viewport in question
GraphicsWindow *gw = pView->getGW(); // Get the GraphicsWindow context

gw->getCameraMatrix( mat, &invTM, &persp, &hither, &yon); // getting these values to work with ... see above for their types
float oneOverDepth = 1.0f / (yon - hither);

// Set the Direct3D Camera View Position and Camera Projection Transforms.
//
// The first matrix is the full projection transformation matrix that
// converts World Coordinates into NPC. This means that the matrix is the
// product of the Camera View Position transformation matrix and the Camera
// Projection matrix. The second matrix is the inverse of the Camera View
// Position transformation matrix so if we multiply this second matrix by
// the first, we get the Camera Projection matrix. If we take the inverse
// of the second matrix, we get the Camera View Position matrix.
//
// The Camera View Position transformation converts World coordinates into
// Camera View Position coordinates where the camera is located at the
// origin. We have been given the inverse of the Camera View Position
// matrix so the first step is to take the inverse of this transform to
// obtain the Camera View Position matrix.
// General conversion from 3ds max coords to Direct3D coords:
//
// 3ds max: (Up, Front, Right) == (+Z, +Y, +X)
//
// Direct3D: (Up, Front, Right) == (+Y, +Z, +X)
//
// Conversion from 3ds max to Direct3D coords:
//
// 3ds max * conversion matrix = Direct3D
//
// [ x y z w ] * | +1 0 0 0 | = [ X Y Z W ]
// | 0 0 +1 0 |
// | 0 +1 0 0 |
// | 0 0 0 +1 |
//
// The View transform below accomplishes this. The standard View transform
// received makes the rotation about the X axis because the assumption was
// to transform to RH coords with the XY plane being the vertical plane
// instead of the XZ plane. The negation of the the Z column does the RH
// to LH flip. Thus, the View transform makes the transition from RH 3ds
// max coords to LH Direct3D coords.
Matrix3 camTM = Inverse(invTM);

// We now have an affine matrix (4x3) with no perspective column (it is
// understood to be (0, 0, 0, 1)). We add the fourth column and flip the
// Z-axis because Direct3D uses a left-handed coordinate system and MAX
// uses a right-handed coordinate system.
// Copy the affine view matrix data
int ki, kj;
for (ki = 0; ki < 4; ki++) {
for (kj = 0; kj < 3; kj++) {
m_d3dViewXform.m[ki][kj] = pcvm[ki][kj];
}
}
// Assign the fourth column (perspective terms)
m_d3dViewXform.m[0][3] = m_d3dViewXform.m[1][3] = m_d3dViewXform.m[2][3] = 0.0f;
m_d3dViewXform.m[3][3] = 1.0f;

// Scale the Z-axis (third column) by -1 to flip to left-handed Direct3D
// coordinate system
for (ki = 0; ki < 4; ki++) {
m_d3dViewXform.m[ki][2] *= -1.0f;
}

// Calculate the Direct3D Camera Projection transformation matrix.
//
// First, multiply the MAX full projection matrix by the inverse of the MAX
// Camera View Position matrix to obtain the MAX Camera Projection matrix.
//
// This gives us a correct Direct3D Camera Projection matrix except for the
//
for (ki = 0; ki < 4; ki++) {
float val = (float)(ki==3);
for (kj = 0; kj < 4; kj++) {
m_d3dProjXform.m[ki][kj] = pa[ki][0] * mat[0][kj] +
pa[ki][1] * mat[1][kj] +
pa[ki][2] * mat[2][kj] +
val * mat[3][kj];
}
}

// Now calculate the lower right quadrant of the Camera Projection matrix
// using the facts that MAX uses an NPC Z-axis range of +1 to -1 whereas
// Direct3D uses an NPC Z-axis range of zero to +1.
//
// For ease of reference, the general forms of the Direct3D Projection
// matrix for perspective and orthographic projections are given below.
//
// Please note that the matrices are specified in row-major order. This
// means that the translate terms are located in the fourth row and the
// projection terms in the fourth column. This is consistent with the way
// MAX, Direct3D, and OpenGL all handle matrices. Even though the OpenGL
// documentation is in column-major form, the OpenGL code is designed to
// handle matrix operations in row-major form.

if (persp) {

// Perspective projection. The general form of the Direct3D Camera
// Projection matrix is:
//
// | 2n/(r-l) 0 0 0 |
// | 0 2n/(t-b) 0 0 |
// | (r+l)/(r-l) (t+b)/(t-b) f/(f-n) 1 |
// | 0 0 -fn/(f-n) 0 |
//
// Construct the lower right four terms correctly for Direct3D.
//
m_d3dProjXform.m[2][2] = yon*oneOverDepth;
m_d3dProjXform.m[2][3] = 1.0f;
m_d3dProjXform.m[3][2] = -(yon*hither*oneOverDepth);
m_d3dProjXform.m[3][3] = 0.0f;

} else {

// Orthographic projection. The general form of the Direct3D Camera
// Projection matrix is:
//
// | 2/(r-l) 0 0 0 |
// | 0 2/(t-b) 0 0 |
// | 0 0 1/(f-n) 0 |
// | (r+l)/(r-l) (t+b)/(t-b) -n/(f-n) 1 |
//
// Construct the lower right four terms correctly for Direct3D.
//
m_d3dProjXform.m[2][2] = oneOverDepth;
m_d3dProjXform.m[2][3] = 0.0f;
m_d3dProjXform.m[3][2] = -(hither*oneOverDepth);
m_d3dProjXform.m[3][3] = 1.0f;
}
[/code]

Thanks
Jack Edited by lucky6969b

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Depending on whether you use a left- or a right-handed coordinate system, you'd need to do something like this:

first pitch the model backwards: D3DXMatrixRotationYawPitchRoll(&matrixPitch, 0.f, PI * .5f, 0.f);
then flip it along the z-axis when needed: D3DXMatrixScaling(&matrixScale, 1.f, 1.f, -1.f);
then concatenate all of them together: matrixPitch * matrixScale * matrixWorld * matrixView * matrixProjection

Of course it would be better to correct a mesh' orientation directly after loading it, not only until you render it.

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You can always do it while exporting, I think it's easier. When You export .FBX model you can go to Advanced Options -> Axis Conversion, in .OBJ you can chcek (or uncheck)convert yz-axis, most of model exporters got this option.

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Hi eppo, thanks for your help. I am actually following the typical article found on the net. I think you have read it too.
Basically, it should be correct except I didn't export the world transformation of the object. So would that be the root cause of the problem?
Thanks
Jack

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max
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http://img254.imageshack.us/img254/3271/maxz.jpg

dx9
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http://img687.imageshack.us/img687/6381/56040738.png Edited by lucky6969b

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How do you feel that it's the view transformation problem or the projection transformation problem?
If the definition of projection matrix is to transforming the view frustum to cuboid shape, it won't do any harm to the camera, will it?
So it must be the view transformation problem.

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How do I dismantle the up-right-view vectors of the transformation?

[code]
// Scale the Z-axis (third column) by -1 to flip to left-handed Direct3D
// coordinate system
for (ki = 0; ki < 4; ki++) {
m_d3dViewXform.m[ki][2] *= -1.0f;
}
[/code]

Can this simple operation change it from right-handed to left-handed?
Thanks
Jack Edited by lucky6969b

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I probably should have been clearer on this: the problem isn't really in the object's world-xfrm, it's in the mesh vertices themselves. You either correct this by applying a pre-transformation when rendering or by simply swapping vertex components during loading; D3DXVECTOR3(vec3DS.x, vec3DS.z, (-)vec3DS.y).

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I copied this camera exporting code from the web and using the pandasoft exporter. Are they not compatible?
Thanks
Jack

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Thanks all of you, I unchecked the "left-coordinate system" checkbox in the pandasoft exporter and it solved the problem.
Much appreciated.

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Hello eppo,

                       CameraObject *cam = (CameraObject*) node->EvalWorldState(ip->GetTime()).obj;

Matrix3 atm = node->GetObjectTM(ip->GetTime(), 0);// 4x3 matrix

int ki, kj;
for (ki = 0; ki < 4; ki++) {
for (kj = 0; kj < 3; kj++) {
d3dViewXform.m[ki][kj] = pcvm[ki][kj];
}
}
// Assign the fourth column (perspective terms)

d3dViewXform.m[0][3] = d3dViewXform.m[1][3] = d3dViewXform.m[2][3] = 0.0f;
d3dViewXform.m[3][3] = 1.0f;

D3DXMATRIX matrixPitch, matrixScale;

D3DXMatrixIdentity(&matrixPitch);
D3DXMatrixIdentity(&matrixScale);

D3DXMatrixRotationYawPitchRoll(&matrixPitch, 0.f, PI * .5f, 0.f);
D3DXMatrixScaling(&matrixScale, 1.f, 1.f, -1.f);

d3dViewXform = matrixPitch * matrixScale * d3dViewXform;


The camera still got distorted. I evaluated this thread again because I want +y to go upward and +z go outward.

Thanks

Jack

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