Question

all, I am trying to multiply a matrix to a vector in OpenGL, but it turn out that the rendering result I got by calling my own multiplication function(OpenGLUtility::VectorMultiMatrix4d()) is different by calling the opengl function glMultMatrixf(). The two lines drawn on the screen is not same.

I put my code by calling two different ways as bellow.
The function OpenGLUtility::VectorMultiMatrix4d() is simple, just colume-major multiplication.

Can anyone give me some advices on this? Thanks a lot.

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Code1:

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
double inverse[16];
OpenGLUtility::InverseMatrix(m_mat, inverse);


double tOrigin[4] = { 
    g_bottom_plane.m_Origin[0], 
    g_bottom_plane.m_Origin[1],
    g_bottom_plane.m_Origin[2],
    1.0 };

OpenGLUtility::VectorMultiMatrix4d(tOrigin,inversed);

double tNormal[4] = {
    g_bottom_plane.m_Normal[0],
    g_bottom_plane.m_Normal[1],
    g_bottom_plane.m_Normal[2],
    0.0 };


OpenGLUtility::VectorMultiMatrix4d(tNormal,inversed);

glBegin(GL_LINES);
glColor3f(0.0,1.0, 0.0);
glVertex4f(tOrigin[0], tOrigin[1], tOrigin[2], tOrigin[3]);
glVertex4f( tNormal[0]*tOrigin[3] + tOrigin[0], 
            tNormal[1]*tOrigin[3] + tOrigin[1], 
            tNormal[2]*tOrigin[3] + tOrigin[2], 
            tOrigin[3] );
glEnd();

//

void OpenGLUtility::VectorMultiMatrix4f(float* pVector, float* pMat)
{
    pVector[0] = pMat[0]*pVector[0] + pMat[4]*pVector[1] + pMat[ 8]*pVector[2] + pMat[12]*pVector[3] ;
    pVector[1] = pMat[1]*pVector[0] + pMat[5]*pVector[1] + pMat[ 9]*pVector[2] + pMat[13]*pVector[3] ;
    pVector[2] = pMat[2]*pVector[0] + pMat[6]*pVector[1] + pMat[10]*pVector[2] + pMat[14]*pVector[3] ;
    pVector[3] = pMat[3]*pVector[0] + pMat[7]*pVector[1] + pMat[11]*pVector[2] + pMat[15]*pVector[3] ;
}

Code 2

float inverse[16];

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glMultMatrixf(m_mat);

OpenGLUtility::InverseMatrix(m_mat, inverse);
glMultMatrixf(inverse);

double tOrigin[4] = { 
    g_bottom_plane.m_Origin[0], 
    g_bottom_plane.m_Origin[1], 
    g_bottom_plane.m_Origin[2], 
    1.0 };

double tNormal[4] = { 
    g_bottom_plane.m_Normal[0], 
    g_bottom_plane.m_Normal[1], 
    g_bottom_plane.m_Normal[2],
    0.0 };


glBegin(GL_LINES);
glColor3f(0.0,1.0, 0.0);
glVertex4f( tOrigin[0], tOrigin[1], tOrigin[2], tOrigin[3] );
glVertex4f( tNormal[0]*tOrigin[3] + tOrigin[0], 
            tNormal[1]*tOrigin[3] + tOrigin[1], 
            tNormal[2]*tOrigin[3] + tOrigin[2], 
            tOrigin[3] );
glEnd();

glMultMatrixf(m_mat);

Answer 1

The problem with your multiplication is that you modify your pVector before you are done with the multiplication. You need to store it in a temporary or different variable!

Change your function to:

void OpenGLUtility::VectorMultiMatrix4f(float* pVector, float* pMat)
{
    float x = pMat[0]*pVector[0] + pMat[4]*pVector[1] + pMat[ 8]*pVector[2] + pMat[12]*pVector[3] ;
    float y = pMat[1]*pVector[0] + pMat[5]*pVector[1] + pMat[ 9]*pVector[2] + pMat[13]*pVector[3] ;
    float z = pMat[2]*pVector[0] + pMat[6]*pVector[1] + pMat[10]*pVector[2] + pMat[14]*pVector[3] ;
    float w = pMat[3]*pVector[0] + pMat[7]*pVector[1] + pMat[11]*pVector[2] + pMat[15]*pVector[3] ;

    pVector[0] = x;
    pVector[1] = y;
    pVector[2] = z;
    pVector[3] = w;
}