View Full Version : Transperant Textures / Billboarding? (cont)

02-15-2000, 04:51 AM
By "manually create an alpha channel" I think he means this:
At the moment I guess you're loading your image into an RGB bitmap - one with 3 bytes per texel, 1 each for red, green, blue. Instead you need an RGBA bitmap, with an extra byte per texel for alpha making 4 bytes per texel total.

Load the color components as normal, then check to see whether they match your "invisible" colour. If they do, put 0 in the alpha byte. Otherwise, put 255 in the alpha byte. (I think Nate got these the wrong way round.)

Alpha represents the "opacity" of a texel; 255 is fully opaque, 0 is fully transparent. It's a lot like generating a blitter mask on the 'Miggy, except that it can do partial transparency rather than just on/off, and the alpha is stored together with the colour info, rather than separately. (i.e. texel data is chunky rather than planar - be thankful!)

Mike C.

Great I seem to have this working 95% now. The problem now is the underlying polygons color seems to show itself through the transperant parts of the texture map. Im not sure what to do here http://www.opengl.org/discussion_boards/ubb/smile.gif



02-15-2000, 05:59 AM
Sounds like you're texture mapping with GL_MODULATE mode for glTexEnv, which _is_ affected by the underlying polygon colour. I don't have the GL spec handy at the moment - I'll have a look tonight - but maybe try using GL_DECAL instead?

Note that people generally use GL_MODULATE with a white "undercoat" to achieve vertex lighting for textured polys. With GL_DECAL you won't get this, but for billboarded trees you shouldn't be using vertex lighting anyway.

02-16-2000, 04:59 AM
Okay, we've sorted this one out by email. Just in case anyone else was wondering, it needed a glTexEnv mode of GL_REPLACE.