Frame buffers and color mapsfigure

IV.1 FRAME BUFFERS AND COLOR MAPS

Figure 2. The highlighted pixel in the frame buffer indicates that color number 54 should be displayed at the corresponding screen location. The color-map entry for index 54 contains the color (32, 20, 50), which is then displayed on the screen.

GRAPHICS GEMS I Edited by ANDREW S. GLASSNER 216

Often there is a color map associated with these frame buffers as well. In this arrangement, there are actually three individual color maps, one each for the red, green, and blue components of the image. When a pixel is to be displayed, its three color components are retrieved from the frame buffer. The red value is used as an index into the red color map to determine a new intensity for the red component at that point; similar look-ups are performed for green and blue (see Fig. 3). This allows one to apply overall intensity changes to the image without directly affecting the pixel values in the image. A common use of these color maps is to implement gamma-correction, which compensates for the nonlinearity of the display monitor.

Each layer of information in a frame buffer is called a plane (or channel). Thus, the first form of frame buffers discussed above has but a single plane (which is 8 bits deep), whereas frame buffers that store the red, green, and blue components at each pixel are three planes deep. A frame buffer may be constructed with an arbitrary number of planes at each pixel. Typically these other planes hold information that varies from pixel to pixel across the image, such as transparency (often called alpha),