Texture alpha between multitexture passes

OpenGL OpenGL: Advanced Coding
1

Let’s say I had a metal wall texture with some “holes” in it. I use a 32-bit TGA to provide an alpha channel. Underneath the metal is a cubic map, so when you move by the wall, you only see some parts of the wall with the reflective cubic map. It’s a really nice effect.

I can easily do this in two passes. Is there a way to do it using one-pass multitexturing? I want a full range of alpha values, so it’s a blend, not a mask.

2

Sure,

First stage : Bind diffuse with alpha holes

RGB : Diffuse texture
Alpha: Alpha holes

Second stage: Bind cube map

RGB : Lerp between the previous color and the cube map by the previous alpha
Alpha: (probably) Use primary alpha

3

Ummm…I have no idea what that means.

4

This is the advanced forum??

Ok just do this:

//Satge 0
glActiveTextureARB(GL_TEXTURE0)
glEnable(GL_TEXTURE_2D);

glBindTexture(GL_TEXTURE_2D, /* funky texture with alpha holes */ );

//Modulate the texture by the vertex color
glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_COMBINE)
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_RGB,GL_MODULATE)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_RGB,GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_RGB,GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_RGB,GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_RGB,GL_SRC_COLOR)

//Modulate the vertex alpha by the texture alpha
// (May want to just use texture alpha -depends on your case)
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_ALPHA,GL_MODULATE)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_ALPHA,GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_ALPHA,GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_ALPHA,GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_ALPHA,GL_SRC_ALPHA)

//Stage 1
glActiveTextureARB(GL_TEXTURE1)

glEnable(GL_TEXTURE_CUBE_MAP);

//Enable reflection texturegen here

glBindTexture(GL_TEXTURE_CUBE_MAP, /* funky reflective cube map */ );

//Lerp between the two textures by the previous textures’ (and vertex) alpha
glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_COMBINE)
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_RGB,GL_INTERPOLATE)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_RGB,GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_RGB,GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_RGB,GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_RGB,GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE2_RGB,GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND2_RGB,GL_SRC_ALPHA)

//Just use the primary alpha
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_ALPHA,GL_REPLACE)
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_ALPHA,GL_PRIMARY_COLOR)
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_ALPHA,GL_SRC_ALPHA)

5

Sweet, thanks.