duckisking

04-07-2012, 02:31 PM

Hi everyone, I've got a question as to the workings of glMultMatrix(). Basically, my program is a car driving around on a track. This is some rough pseudocode of my drawCar() method.

double x,y,z = findCarPosition(time); //coordinates of car

double dx,dy,dz = findCarDirection(time); //forward unit vector

glPushMatrix();

glTranslated(x,y,z);

//(0,1,0) is global up vector

double sx,sy,sz = cross_product(0,1,0 , dx,dy,dz); // left vector

double ux,uy,uz = cross_product(dx,dy,dz , sx,sy,sz); //local up vector

glRotated(-90,ux,uy,uz);

double m[16] = {dx,dy,dz,0 , ux,uy,uz,0, sx,sy,sz,0, 0,0,0,1};

glMultMatrixd(m);

glBegin(GL_QUADS);

glColor3d(0.0,0.0,1.0);

//draw car

glEnd();

glPopMatrix();

I read somewhere the m[16] would rotate an object to the correct way. When I tried this it was off my 90 degrees, hence the other glRotated.

Anyways, the problem...The car starts out as the light blue I've set it as. But as it drives around the track, it varies in color from dark blue to light blue. If I comment out the glMultMatrixd() line, the car will obviously not be rotated correctly, but the color stays constant.

I thought glMultMatrix will only affect 1 matrix, but it seems to be affecting both my color and modelview? Will this always happen with glMultMatrix, and do I need to find another way to rotate the car?

Thanks for any help or advice you can offer.

-duckisking

double x,y,z = findCarPosition(time); //coordinates of car

double dx,dy,dz = findCarDirection(time); //forward unit vector

glPushMatrix();

glTranslated(x,y,z);

//(0,1,0) is global up vector

double sx,sy,sz = cross_product(0,1,0 , dx,dy,dz); // left vector

double ux,uy,uz = cross_product(dx,dy,dz , sx,sy,sz); //local up vector

glRotated(-90,ux,uy,uz);

double m[16] = {dx,dy,dz,0 , ux,uy,uz,0, sx,sy,sz,0, 0,0,0,1};

glMultMatrixd(m);

glBegin(GL_QUADS);

glColor3d(0.0,0.0,1.0);

//draw car

glEnd();

glPopMatrix();

I read somewhere the m[16] would rotate an object to the correct way. When I tried this it was off my 90 degrees, hence the other glRotated.

Anyways, the problem...The car starts out as the light blue I've set it as. But as it drives around the track, it varies in color from dark blue to light blue. If I comment out the glMultMatrixd() line, the car will obviously not be rotated correctly, but the color stays constant.

I thought glMultMatrix will only affect 1 matrix, but it seems to be affecting both my color and modelview? Will this always happen with glMultMatrix, and do I need to find another way to rotate the car?

Thanks for any help or advice you can offer.

-duckisking