In this chapter, we will focus on techniques that work directly with the pixels in a framebuffer. These techniques typically involve multiple passes. An initial pass produces the pixel data and subsequent passes apply effects or further processes those pixels. To implement this, we often make use of the ability provided in OpenGL for rendering directly to a texture or set of textures (refer to the Rendering to a texture recipe in Chapter 5, Using Textures).

The ability to render to a texture, combined with the power of the fragment shader, opens up a huge range of possibilities. We can implement image processing techniques such as brightness, contrast, saturation, and sharpness by applying an additional process in the fragment shader prior to output. We can apply convolution filters such as edge detection, smoothing (blur), or sharpening. We'll take a closer look at convolution filters in the recipe on edge detection.

A related set of techniques involves rendering additional information to textures beyond the traditional color information and then, in a subsequent pass, further processing that information to produce the final rendered image. These techniques fall under the general category that is often called deferred shading.

In this chapter, we'll look at some examples of each of the preceding techniques. We'll start off with examples of convolution filters for edge detection, blur, and bloom. Then, we'll move on to the important topics of gamma correction and multisample anti-aliasing. Finally, we'll finish with a full example of deferred shading.

Most of the recipes in this chapter involve multiple passes. In order to apply a filter that operates on the pixels of the final rendered image, we start by rendering the scene to an intermediate buffer (a texture). Then, in a final pass, we render the texture to the screen by drawing a single fullscreen quad, applying the filter in the process. You'll see several variations on this theme in the following recipes.