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I need to set up a billboarding matrix but I don't have any LookAt- and EyePoint vector. My view matrix is set up like this:
D3DXQuaternionRotationYawPitchRoll( &qR, m_fYaw, m_fPitch, m_fRoll );
D3DXMatrixAffineTransformation( &m_matOrientation, 1.25f, NULL, &qR, &m_vecPosition );
D3DXMatrixInverse( &m_matView, NULL, &m_matOrientation );

Does anyone have an idea of how to calculate the angle between the camera and the object and set up the billboaring matrix correctly?

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A billboard needs to face the camera. Typically you Y-Axis align them, so the object spins on the Y-Axis. If you don't need that, then you can look at the code and change it accordingly.

Basically you get the difference between the Objects location and Cameras location on the X/Z axis only. This will create a forward vector. Then you normalize it to bind it to the -1/1 range, and get the cross product to get the right vector. Then the up vector is hard coded since its Y-Axis aligned.

D3DXVECTOR3 tempVec1, tempVec2;
D3DXVECTOR3 tempForward, tempUp, tempRight;

tempVec1.x = tempCamera(3,0);
tempVec1.y = 0;
tempVec1.z = tempCamera(3,2);

tempVec2.x = tempObject(3,0);
tempVec2.y = 0;
tempVec2.z = tempObject(3,2);

tempForward = tempVec2 - tempVec1;

D3DXVec3Normalize(&tempForward, &tempForward);

tempUp.x = 0;
tempUp.y = 1;
tempUp.z = 0;

D3DXVec3Cross(&tempRight, &tempUp, &tempForward);

tempObject(0,0) = tempRight.x;
tempObject(0,1) = tempRight.y;
tempObject(0,2) = tempRight.z;

tempObject(1,0) = tempUp.x;
tempObject(1,1) = tempUp.y;
tempObject(1,2) = tempUp.z;

tempObject(2,0) = tempForward.x;
tempObject(2,1) = tempForward.y;
tempObject(2,2) = tempForward.z;

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find the angle between cameralookat and the positive Z-axis (0,0,1). That is the yaw amount for billboarding.
Do the same for pitch, thou for games that are on a flat plane (most games) I find yaw is enough.

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If you keep track of your view matrix, you can also just take the transpose of that and multiply it by a translation matrix for the billboard object and set that to the world transform. Simple as 1, 2, 3. :)


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err... can't you just reset the rotation part of your matricies?

over in the openGL forum it goes a little like this..

void CBillboard::Render()
float mv[16];
// save the current modelview matrix

// get the current modelview matrix
glTranslatef(position.x, position.y, position.z);
glGetFloatv(GL_MODELVIEW_MATRIX , mv);

// undo all rotations and scaling
for( int i=0; i<3; i+=2 ) {
for( int j=0; j<3; j++ ) {
if ( i==j )
mv[i*4+j] = 1.0;
mv[i*4+j] = 0.0;

// set the modelview with no rotations and scaling

// restores the modelview matrix

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