Sounds like you want to enable alpha blending.
 
You should do:
 
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
 
glEnable(GL_BLEND);
 
/*
 * Render your transparent geom here.
 */
 
glDisable(GL_BLEND);
 
Note, that the alpha component of the diffuse material colour you set(or your per vertex colours), will have to be set to the appropriate transparency value (0.0f - 1.0f, with  0 being completely transparent).
 
Also, note that if you are rendering multiple transparent meshes, that you should probably queue up your transparent geom, and render it after all normal geometry. I'm a bit fuzzy on this stuff, but I think you'll also have to disable Z writes to get the transparency looking good.
 
BTW, you could also choose to do some simple sorting of your transparent geom, if you want it to appear correctly(some sort of bucket sort usually does the trick).
 
-----Original Message-----
From: Iker@gamepro.es [mailto:Iker@gamepro.es]
Sent: 18 July 2001 16:38
To: gdalgorithms-list@lists.sourceforge.net
Subject: [Algorithms] Making an object transparent in openGL

Hi there!

I would like to make a 3d rendered object transparent acording to a global factor. Imagine a cube. It is completely black while rendering. Now I set my material/shader (call it as you whish) to a transparency factor of 0.5 anf then the cube "bends" with the background image. What openGL calls do I have to do to make this happen? In a software renderer this sounds pretty simple, but I dunno how to accomplish this prob in openGL...

Thanx... maybe is a very simple question...



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