OpenGL Programming Guide

Chapter 10The Framebuffer

Buffers and Their Uses

Color Buffers

The color buffers are usually the ones you draw to. They contain either color-index or RGB color data and may also contain alpha values. An OpenGL implementation that supports stereoscopic viewing has left and right color buffers for the left and right stereo images. If stereo isn't supported, only the left buffers are used. Similarly, double-buffered systems have front and back buffers, and a single-buffered system has the front buffers only. Every OpenGL implementation must provide a front-left color buffer.

Up to four optional, nondisplayable auxiliary color buffers can also be supported. OpenGL doesn't specify any particular uses for these buffers, so you can define and use them however you please. For example, you might use them for saving an image that you use repeatedly; rather than redrawing the image, you can just copy it into the usual color buffers. See the description of glCopyPixels() in "Reading, Writing, and Copying Pixel Data" for more information about how to do this.

You can use GL_STEREO or GL_DOUBLE_BUFFER with glGetBooleanv() to find out if your system supports stereo (that is, has left and right buffers) or double-buffering (has front and back buffers). To find out how many auxiliary buffers are present, use glGetIntegerv() with GL_AUX_BUFFERS.

Depth Buffer

The depth buffer stores a depth value for each pixel. As described in "Hidden-Surface Removal Survival Kit" depth is usually measured in terms of distance to the eye, so pixels with larger depth-buffer values are overwritten by pixels with smaller values. This is just a useful convention, however, and the depth buffer's behavior can be modified as described in "Depth Test." The depth buffer is sometimes called the z buffer (the z comes from the fact that x and y values measure horizontal and vertical displacement on the screen, and the z value measures distance perpendicular to the screen).

Stencil Buffer

One use for the stencil buffer is to restrict drawing to certain portions of the screen, just as a cardboard stencil can be used with a can of spray paint to make fairly precise painted images. For example, if you want to draw an image as it would appear through an odd-shaped windshield, you can store an image of the windshield's shape in the stencil buffer, and then draw the entire scene. The stencil buffer prevents anything that wouldn't be visible through the windshield from being drawn. Thus, if your application is a driving simulation, you can draw all the instruments and other items inside the automobile once, and as the car moves, only the outside scene need be updated.

Accumulation Buffer

The accumulation buffer holds RGBA color data just like the color buffers do in RGBA mode. (The results of using the accumulation buffer in color-index mode are undefined.) It's typically used for accumulating a series of images into a final, composite image. With this method, you can perform operations like scene antialiasing by supersampling an image and then averaging the samples to produce the values that are finally painted into the pixels of the color buffers. You don't draw directly into the accumulation buffer; accumulation operations are always performed in rectangular blocks, usually transfers of data to or from a color buffer.

OpenGL Programming Guide