questions of transform matrix in VR

问题

I have a problem about matrix transformation in OpenVR api.

m_compositor->WaitGetPoses(m_rTrackedDevicePose, vr::k_unMaxTrackedDeviceCount, nullptr, 0);

in the demo which the openvr gives:

const Matrix4 & matDeviceToTracking = m_rmat4DevicePose[ unTrackedDevice ];
        Matrix4 matMVP = GetCurrentViewProjectionMatrix( nEye ) * matDeviceToTracking;
        glUniformMatrix4fv( m_nRenderModelMatrixLocation, 1, GL_FALSE, matMVP.get() );

where GetCurrentViewProjectionMatrix is calculated with

Matrix4 CMainApplication::GetCurrentViewProjectionMatrix( vr::Hmd_Eye nEye )
{
    Matrix4 matMVP;
    if( nEye == vr::Eye_Left )
    {
        matMVP = m_mat4ProjectionLeft * m_mat4eyePosLeft * m_mat4HMDPose;
    }
    else if( nEye == vr::Eye_Right )
    {
        matMVP = m_mat4ProjectionRight * m_mat4eyePosRight *  m_mat4HMDPose;
    }

    return matMVP;
}

the question is:

1, which space is matDeviceToTracking transformed from to which space?

2, If I have modelview matrix already, and already can rotate with hmd, how can I render the rendermodel correctly? I tried using projection*modelview*m_rmat4DevicePose[ unTrackedDevice ] but there is no effect.

回答1:

1.

In the sample code, the matDeviceToTracking is a reference to m_rmat4DevicePose[unTrackedDevice], which is copied from TrackedDevicePose_t::mDeviceToAbsoluteTracking. This is a model matrix mapping from the model space to the world space.

There is one pitfall, though. If you included the UpdateHMDMatrixPose() function from the sample, this function inverts m_rmat4DevicePose[vr::k_unTrackedDeviceIndex_Hmd] while updating the value of m_mat4HMDPose, leaving m_rmat4DevicePose[0] mapping from the world space to the model/HMD view space, exactly the other way around to the other matrices in the array.

2.

If you already have the model-view matrix, then you only need to multiply the projection matrix by it to obtain the MVP matrix. For rendering into the HMD, use m_mat4ProjectionLeft * m_mat4eyePosLeft * modelview and m_mat4ProjectionRight * m_mat4eyePosRight * modelview for left and right eye, respectively. For rendering on a monitor, you can generate your own frustum and multiply it by your model-view matrix. The following website is a good reference on how to create a projection matrix: http://www.songho.ca/opengl/gl_projectionmatrix.html

来源：https://stackoverflow.com/questions/38910609/questions-of-transform-matrix-in-vr