I'm rendering two crossed quads (one polygon intersects another at the middle point) and applying a grass texture on them to simulate a field of grass in real time. like this image: Im using glEnable(GL_BLEND) and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) for the blend func. But as they are at the same level on Z buffer, I think opengl cant know who's first and the second one doesnt get the trasparency correctly. Im using a TGA 32-bit with alpha channel as textures. Anyone knows a solution for this? Thnxs in advance, Bruce Sinner [Edited by - brucesinner on September 1, 2004 10:47:54 AM]

try this:

glDepthMask(GL_FALSE);
glDisable(GL_CULL_FACE);
glBlendFunc(GL_ONE_MINUS_DST_COLOR,GL_ONE);
glEnable(GL_BLEND);

Draw();

glEnable(GL_CULL_FACE);
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);

use alphatesting, and no semi transparent stuff, it's a good way of getting it right.
or you could stop using crossed quads, it doesn't work well like that.

Or render 4 quads sorted back to front.

Thnxs b3s3rk but didnt work well too :-(

i thought on using 4 quads but i think it could be done with two since i read bout this on chapter 7 of gpu gems where the author teachs how to move grass with vertex shaders and he says that one way of doing a big field of grass is making three crossed quads, forming a 'star'. But he dont explain the ogl stuff, only the shader part. But if he can do it, i can too! (perhaps i can) :-)

Im putting a link to my project, it is based on nehe_glut basecode i made, it only needs glut. The main stuff is in glutDisplay function. If anyone is willing to take a look i will appreciate too much the help.

Theres 4 grass textures, only change the CreateTexture call with grass_1 ... grass_4.tga.

Today I managed to put it working but with Alpha_Test and Alpha_func set to GREATER than 0.5. But it leads me to some nasty visual artefacts on the texture.

Project Link

Thnxs for the help folks,
Bruce Sinner

Quote: Original post by b3rs3rk
try this:

glDepthMask(GL_FALSE);
glDisable(GL_CULL_FACE);
glBlendFunc(GL_ONE_MINUS_DST_COLOR,GL_ONE);
glEnable(GL_BLEND);

Draw();

glEnable(GL_CULL_FACE);
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);

try this here i think that worked fine in on of my former engines

in the redbook third edition on page 226 they do it the same way just for a tree texture

glDepthMask(GL_FALSE);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
Draw();
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);

Quote: Original post by Basiror

Quote: Original post by b3rs3rk
try this:

glDepthMask(GL_FALSE);
glDisable(GL_CULL_FACE);
glBlendFunc(GL_ONE_MINUS_DST_COLOR,GL_ONE);
glEnable(GL_BLEND);

Draw();

glEnable(GL_CULL_FACE);
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);

try this here i think that worked fine in on of my former engines

in the redbook third edition on page 226 they do it the same way just for a tree texture

glDepthMask(GL_FALSE);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
Draw();
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);

Not good too. It does the transparency well but the second poly goes over the first... thanks anyway.

I am not sure, but aren't you supposed to turn the depth mask on, glDepthMask(GL_TRUE);, before you render the transparent polygons, and then turn it off ?

In other words transparent polygons should test the depth buffer but never write into it.

So i guess what you should try is

glDepthMask(GL_TRUE); <----------- first turn it on
glDisable(GL_CULL_FACE);
glBlendFunc(GL_ONE_MINUS_DST_COLOR,GL_ONE);
glEnable(GL_BLEND);

Draw();

glEnable(GL_CULL_FACE);
glDisable(GL_BLEND);
glDepthMask(GL_FALSE); <------- then turn it off