Unexpected results for projection on to plane [closed]

Question

I want to use this projection matrix:

GLfloat shadow[] = { 
    -1,0,0,0,
    1,0,-1,1,
    0,0,-1,0,
    0,0,0,-1 };

It should cast object shadows onto the y = 0 plane from a point light at 1,1,-1.

I create a rectangle in the x = 0.5 plane

    glBegin( GL_QUADS );
    glVertex3f( 0.5,0.2,-0.5);
    glVertex3f( 0.5,0.2,-1.5);
    glVertex3f( 0.5,0.5,-1.5);
    glVertex3f( 0.5,0.5,-0.5);
    glEnd();

Now if I manually multiply these vertices with the matrix, I get.

    glBegin( GL_QUADS );
    glVertex3f( 0.375,0,-0.375);
    glVertex3f( 0.375,0,-1.625);
    glVertex3f( 0,0,-2);
    glVertex3f( 0,0,0);
    glEnd();

Which produces a reasonable display ( camera at 0,5,0 looking down y axis )

So rather than do the calculation manually, I should be able to use the opengl model transormation. I write this code:

    glMatrixMode (GL_MODELVIEW);
      GLfloat shadow[] = { 
    -1,0,0,0,
    1,0,-1,1,
    0,0,-1,0,
    0,0,0,-1 };
    glLoadMatrixf( shadow );
    glBegin( GL_QUADS );
    glVertex3f( 0.5,0.2,-0.5);
    glVertex3f( 0.5,0.2,-1.5);
    glVertex3f( 0.5,0.5,-1.5);
    glVertex3f( 0.5,0.5,-0.5);
    glEnd();

But this produces a blank screen!

What am I doing wrong?

Is there some debug mode where I can print out the transformed vertices, so I can see where they are ending up?

Note: People have suggested that using glMultMatrixf() might make a difference. It doesn't. Replacing

glLoadMatrixf( shadow );

with

     glLoadIdentity();
     glMultMatrixf( shadow );

gives the identical result ( of course! )

Answer 1

glLoadMatrix replaces the current matrix, so it is not equivalent to your manual multiplication if the current modelview matrix was not the identity. If that may be the problem, try glMultMatrix instead (and a surrounding push/pop if needed).