Implementing the depth-of-field effect

The classic depth-of-field (DOF) means that when camera lens is focused on an object, only the object and its nearby area will appear sharp in the final image, but subjects outside the area will appear blurred. It is a famous and common post-processing effect used in most 'next generation' computer games, especially the first-person ones, to enhance the reality.

We will use more than one pass to implement a basic DOF effect.