Applying a 2D texture

In GLSL, applying a texture to a surface involves accessing texture memory to retrieve a color associated with a texture coordinate, and then applying that color to the output fragment. The application of the color to the output fragment could involve mixing the color with the color produced by a shading model, simply applying the color directly, using the color in the reflection model, or some other mixing process. In GLSL, textures are accessed via sampler variables. A sampler variable is a handle to a texture unit. It is typically declared as a uniform variable within the shader and initialized within the main OpenGL application to point to the appropriate texture unit. 

In this recipe, we'll look at a simple example involving the application of a 2D texture to a surface, as shown in the following image. We'll use the texture color as...