Hi!
I have a trouble with my cartoon-style shading algorithm…I just draw the object as it is and then I add a black contour line with the following steps:
draw_the_object();
glCullFace(GL_FRONT);
glPolygonMode(GL_LINE);
glLineWidth(4.0);
draw_the_object();
…the problem is: IF the object is transparent I will see all the the wireframe lines of the back polygons, and not only the contour lines…it is obviuos, but how can I avoid this in an efficient way?
stencil!
draw your transparent object with stencil writes enabled - no testing, just replacing when depthtest ist passed, and then draw your outlines with stencil test enabled.
for more details about stencil, just take a look at the tutorials & faqs on www.opengl.org.
(btw. you can accomplish the same effect with alphatesting)
Thanx! 
That’s the right solution!
Just to know…I haven’t understood how you would solve this problem with ALPHA_TEST…immagine you have to render in cartoon-style a transparent cube:
1- fist, you draw the transparent cube.
2- you enable front-face-culling
3- you set glPolygonMode(GL_BACK, GL_LINE)
4- …and now ??? …
if you enable alhpa-testing you will discard everything!..
He didn’t mention Alpha Testing, he did mention Stencil Testing.
Though, I would rather recommend simple Depth Testing if you can.
But if your transparent object does not write the depth buffer, well… use the stencil buffer 
i meant alphaBLENDING(you can use the alphabuffer as a mask as well). but this would only work with opaque objects. so it’s not a solution for your problem…