Texture Combiners

Texture combiners became core in GL 1.3. You can combine by multiplying, replacing, adding, subtracting, DOT3 product.
GL 1.4 added to the core a crossbar feature. This means that you sample GL_TEXTUREX from any other texture image unit (TIU).

Example : multiply tex0 and tex1

 glActiveTexture(GL_TEXTURE0);
 glBindTexture(GL_TEXTURE_2D, textureID0);
 //Simply sample the texture
 glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
 //------------------------
 glActiveTexture(GL_TEXTURE1);
 glBindTexture(GL_TEXTURE_2D, textureID1);
 glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
 //Sample RGB, multiply by previous texunit result
 glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_PREVIOUS);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR);
 //Sample ALPHA, multiply by previous texunit result
 glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_MODULATE);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PREVIOUS);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_ALPHA, GL_TEXTURE);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_ALPHA);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_ALPHA);

The equivalent with GL 2.0 shaders

 //Fragment shader
 uniform sampler2D Texture0;
 uniform sampler2D Texture1;
 //------------------------
 varying vec2 TexCoord0;
 varying vec2 TexCoord1;    //Or just use TexCoord0
 //------------------------
 void main()
 {
    vec4 texel = texture2D(Texture0, TexCoord0);
    texel *= texture2D(Texture1, TexCoord1);
    gl_FragColor = texel;
 }

Example : Blend tex0 and tex1 based on a blending factor you supply

 glActiveTexture(GL_TEXTURE0);
 glBindTexture(GL_TEXTURE_2D, textureID0);
 glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
 //------------------------
 glActiveTexture(GL_TEXTURE1);
 glBindTexture(GL_TEXTURE_2D, textureID1);
 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_PREVIOUS);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB, GL_CONSTANT);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB, GL_SRC_COLOR);
 //------------------------
 glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_INTERPOLATE);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PREVIOUS);
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_ALPHA, GL_TEXTURE);
 //GL_CONSTANT refers to the call we make with glTexEnvfv(GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, mycolor)
 glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_ALPHA, GL_CONSTANT);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_ALPHA);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_ALPHA);
 glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_ALPHA, GL_SRC_ALPHA);
 //------------------------
 float mycolor[4];
 mycolor[0]=mycolor[1]=mycolor[2]=0.0;    //RGB doesn't matter since we are not using it
 mycolor[3]=0.75;                         //Set the blend factor with this
 glTexEnvfv(GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, mycolor);

The equivalent with GL 2.0 shaders

 //Fragment shader
 uniform sampler2D Texture0;
 uniform sampler2D Texture1;
 uniform float BlendFactor;
 //------------------------
 varying vec2 TexCoord0;
 varying vec2 TexCoord1;    //Or just use TexCoord0
 //------------------------
 void main()
 {
    vec4 texel0 = texture2D(Texture0, TexCoord0);
    vec4 texel1 = texture2D(Texture1, TexCoord1);
    gl_FragColor = mix(texel0, texel1, BlendFactor);
 }