alpha blending

Hello all.

I am trying to blend two textures. One is a full RGB texture, with all 1.0’s for the Alpha, and the second texture is no RGB (set to 0.0’s), and an alpha pattern. I want to blend them so the final image is the RGB of the first texture, and the aplpha of the second texture.
tex1=(r,g,b,1) tex2=(0,0,0,a) final=(r,g,b,a)
I’m using the multitexture hardware extensions.
Here’s how I load the textures:

glBindTexture(GL_TEXTURE_2D, *tex);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR_MIPMAP_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP);
gluBuild2DMipmaps(GL_TEXTURE_2D,GL_RGBA8,cinfo.output_width,cinfo.output_height,GL_RGB,GL_UNSIGNED_BYTE,ourImageArray);
glActiveTextureARB(GL_TEXTURE1_ARB);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, *tex);

//and load the alpha image
glBindTexture(GL_TEXTURE_2D, *g); glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR_MIPMAP_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR_MIPMAP_NEAREST);
gluBuild2DMipmaps(GL_TEXTURE_2D,GL_RGBA,TextureImage[0]->sizeX,TextureImage[0]->sizeY,GL_RGBA,GL_UNSIGNED_BYTE,beta); glActiveTextureARB(GL_TEXTURE2_ARB);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, *g);

Here’s the what I do to draw them:

glEnable(GL_BLEND);
glAlphaFunc(GL_GREATER, 0.01f);
glEnable(GL_ALPHA_TEST);

glActiveTextureARB(GL_TEXTURE1_ARB);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D,*tex);

glActiveTextureARB(GL_TEXTURE2_ARB);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, *g);
…
glMultiTexCoord2fARB(GL_TEXTURE1_ARB,…);
glMultiTexCoord2fARB(GL_TEXTURE2_ARB,…);
glVertex3f(…);

The code above draws the texture with full transparencey where alpha < 0.01f. But I need to blend the alpha of the second texture with the first texture, so I get a smooth blend. Right now there is no blending.
I’ve been reading about blending, and I believe that I need a glBendFunc(…) call in there. I do not know where to put it, or what parameters to call.

Any ideas would be appreciated.
Thanks.

Firs of all, you don’t start counting texture units with TEXTURE1, you begin with TEXTURE0. So for two texture units, you should use TEXTRURE0 and TEXTURE1.

tex1=(r,g,b,1) tex2=(0,0,0,a) final=(r,g,b,a)

What kind of texture is tex2? Is it a single channel texture, uploaded as alpha only? Or is it a four channel image, with RGB explicitly set to 0? If it’s a pure alpha texture, RGB will have the value 1, not 0. This means to get the desired result, you can just modulate the two textures.

tex2 2 is a four channel image with RGB set to 0. I’ll try to make it a single channel and modulate the textures then.
Thanks for the advice.

Ahh… I figured it out. I needed to glDiable(GL_DEPTH_TEST) for the textures to blend correctly.