Unexpected results for projection on to plane
- by ravenspoint
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! )
