GLSLでマルチテクスチャ

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    GLuint texa = 0, texb = 0, texc = 0;
    glActiveTexture(GL_TEXTURE0);
    LoadGLTexture("texture/image0.png", texa, false, false);
    glBindTexture(GL_TEXTURE_2D, texa);
 
    glActiveTexture(GL_TEXTURE1);
    LoadGLTexture("texture/image1.png", texb, false, false);
    glBindTexture(GL_TEXTURE_2D, texb);
 
    glActiveTexture(GL_TEXTURE2);
    LoadGLTexture("texture/image2.png", texc, false, false);
    glBindTexture(GL_TEXTURE_2D, texc);

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        GLint num_tex_units;
    glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS, &num_tex_units);
    cout << "Max texture units = " << num_tex_units << endl;

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void main(void)
{
    gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex;        gl_TexCoord[0] = gl_MultiTexCoord0;        }

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uniform sampler2D tex0;
uniform sampler2D tex1;
uniform sampler2D tex2;
 
void main(void)
{
        vec3 color_tex0 = texture2D(tex0, gl_TexCoord[0].st).xyz;
    vec3 color_tex1 = texture2D(tex1, gl_TexCoord[0].st).xyz;
    vec3 color_tex2 = texture2D(tex2, gl_TexCoord[0].st).xyz;
 
    float gray = (color_tex0.r+color_tex0.g+color_tex0.b)/3.0;
 
    if(gray > 0.999){
        gl_FragColor.rgb = color_tex1+color_tex2;
    }
    else{
        gl_FragColor.rgb = color_tex0;
    }
    gl_FragColor.a = 1.0;
}