The preceding code that loads the two textures into the OpenGL program is very similar to the code from the previous recipe, Applying a 2D texture. The main difference is that we load each texture into a different texture unit. When loading the brick texture, we set the OpenGL state such that the active texture unit is unit zero:
glActiveTexture(GL_TEXTURE0);
And when loading the second texture, we set the OpenGL state to texture unit one:
glActiveTexture(GL_TEXTURE1);
In the fragment shader, we specify the texture binding for each sampler variable using the layout qualifier corresponding to the appropriate texture unit. We access the two textures using the corresponding uniform variables, and store the results in brickTexColor ...
