Hello. I've got a little problem concerning blending. The thing is that I have coded a little landscape engine, and now I'm adding trees to it. The trees are simple texture which are alligned like a cross, to reproduce the effect off a real tree. Currently I can add about 40000 trees without having frame rate problems, but I just can't seem to get the bitmaps to be transparent. If I use blending, I have to disable depth testing, right? And if I do that, the trees that are hidden will be drawn, and whatever blend function i use (i use a mask map and a texture map) the trees glow, or are semi-transparent. What I really wonder is, is there any function which just skips to draw the black pixels on my bitmap, so that I can skip the masking map, and still have depth testing, or do I need to depth sort every tree, which will reduce my framerate quite much. I would be really grateful if someone could help me with this stupid prolem. [edited by - Erim on December 3, 2002 8:38:39 AM]

There is an intermediate solution that works without blending, it is called alpha testing.
First of all, you need an alpha channel for your picture. Since you already have a RGB channel, you can easily compute yourself the channel using the following formula :
Alpha = (Red + Green + Blue) / 3

Then activate alpha testing by calling :
glEnable(GL_ALPHA_TEST); // Enable alpha testing.
glAlphaFunc(GL_NOTEQUAL, 1.0f); // Discard pixels when their alpha component is null

Pros and cons :
- You don''t need to sort trees by depth, and you don''t need tricks such as enabling depth reading while disabling depth writing,
- You can''t blend trees. Like the GIF picture format, your texture will be rendered in two states : show the pixel or don''t show the pixel. There is no level between them (there is no 50%).

If you wanter finer detail, you should try with glAlphaFunc(GL_GREATER, 0.5f) or you can replace 0.5 with any value between 0 and 1.

Hope this helps.

Hmm, thank you both, I think the method vincoof posted works best, but where do I put the alpha information in the texture?
I''m using my own bmp loader, but I am thinking of using tga instead, so that I can use alpha too, but how do I tell OpenGL to use the alpha channel?

You simply tell him while you call glTexImage2D. Could you please post the line of code where you call glTexImage2D ?

Well, this is how it looks, I''m really grateful you''re helping me!

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, tex->width, tex->height, 0, GL_RGB, GL_UNSIGNED_BYTE, tex->data);

ok, you have to set an alpha channel, and then call this line :
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, tex->width, tex->height, 0, GL_RGBA , GL_UNSIGNED_BYTE, tex->data);
(Note the GL_RGBA instead of GL_RGB)

If the pixels into tex->data are RGB, you can easily setup the fourth channel with the following code snippet :


You can optimize it easily, but I''ve tried to be clear instead.
If you prefer C++, you may call new/delete instead of malloc/free :




And please note that you may NOT replace the alpha computation with this line :


because the r+g+b sum may exceed the capacity of an unsigned byte (8 bits) thus would yield to errors.

Whee, thanks!
It works really good now, I wrote a TGA loader so that OpenGL loads the alpha from the TGA.
Thank you very much for your help!