GL_BACK material properties.

OpenGL Development JOGL Development
#1

Hi,

Thanks for helping me figure out the previous question about lighting.

I’m still having trouble giving the back side of polygons a color different from the front side. The same sphere (with fixed lighting!) with half the triangles missing so you can see through can be found.

http://www.geocities.com/bura3no/index.htm

The relevant code is here:

  gl.glMaterialfv(GL.GL_FRONT, GL.GL_AMBIENT_AND_DIFFUSE, Color.GREEN.getComponents(null) );
  gl.glMaterialfv(GL.GL_BACK, GL.GL_AMBIENT_AND_DIFFUSE, Color.RED.getComponents(null) );

  gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_EMISSION, Color.BLACK.getComponents(null));

  gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_SPECULAR, Color.WHITE.getComponents(null) );
  gl.glPolygonMode(GL.GL_FRONT_AND_BACK, GL.GL_FILL);

  gl.glBegin(GL.GL_TRIANGLES);
  for (int t = 0; t < myMesh.pGetNumberOfElements()/2; t++) {
    Element elem = myMesh.pGetElement(t);
    for (int v = 0; v < elem.pGetNumberOfNodes(); v++) {
      VDouble x = myMesh.pGetX(elem.pGetNode(v));
      VDouble n = normals[elem.pGetNode(v)];
      gl.glNormal3d(n.pGet(0), n.pGet(1), n.pGet(2));
      gl.glVertex3d(x.pGet(0), x.pGet(1), x.pGet(2));
    }
  }
  gl.glEnd();

}

gl.glEndList();
#2

At a first glance that render loop looks fine, so make sure you’re not culling the back-facing polygons and enable two-sided lighting:

GL.glDisable(GL.GL_CULL_FACE) ;
GL.glLightModeli(GL.GL_LIGHT_MODEL_TWO_SIDE, GL.GL_TRUE) ;
#3

Thanks for response.

Your suggestion helped with lighting of the back side (something I did need), but not with the coloring. It’s still green rather than some other color.

#4

Well, the code you’ve posted looks fine, as long as Color.RED really contains a red colour. How are the lights set up?

Not sure what to suggest… anyone else got any ideas?

#5

Need more code.

#6

Hi,

I would very much like to post more code, but I’m not quite sure what part of my program to quote. If I have

  gl.glMaterialfv(GL.GL_FRONT, GL.GL_AMBIENT_AND_DIFFUSE, Color.GREEN.getComponents(null) );

and

  gl.glMaterialfv(GL.GL_BACK, GL.GL_AMBIENT_AND_DIFFUSE, Color.RED.getComponents(null) );

right next to each other and green works but red doesn’t, can you give me an idea of I might be doing wrong?

Thanks!

Pavel

#7

Try removing gl.glMaterialfv(GL.GL_FRONT, GL.GL_AMBIENT_AND_DIFFUSE, Color.GREEN.getComponents(null) ); and see what happens. Your code looks fine but obviously something is wrong.

#8

EVERYTHING goes black.

#9

Try reversing your colors, front red, back green. If everything turns red then we know it isn’t your color methods. What kind of graphics card do you have?

#10

Yes, it all turns red.

I have a Mobility Fire GL 9000.
Chip type: Mobility Radeon 9000 (LF).

And here’s an interesting thing: When I switch to

  gl.glPolygonMode(GL.GL_FRONT, GL.GL_LINE);
  gl.glPolygonMode(GL.GL_BACK, GL.GL_FILL);

the program crashes with the following error message:

http://www.geocities.com/bura3no/error.txt

#11

Try making a little c app that does the same type of lighting and see if you get the same results. If it is the same you might have found a driver bug.