RGB - RGB Devices - RGB and Displays - Video Framebuffer

Video Framebuffer

A framebuffer is a digital device for computers which stores data in the so-called video memory (comprising an array of Video RAM or similar chips). This data goes either to three digital-to-analog converters (DACs) (for analog monitors), one per primary color, or directly to digital monitors. Driven by software, the CPU (or other specialized chips) write the appropriate bytes into the video memory to define the image. Modern systems encode pixel color values by devoting eight bits to each of the R, G, and B components. RGB information can be either carried directly by the pixel bits themselves, or provided by a separate color look-up table (CLUT) if indexed color graphic modes are used.

A CLUT is a specialized RAM that stores R, G, and B values that define specific colors. Each color has its own address (index)—consider it as a descriptive reference number that provides that specific color when the image needs it. The content of the CLUT is much like a palette of colors. Image data that uses indexed color specifies addresses within the CLUT to provide the required R, G, and B values for each specific pixel, one pixel at a time. Of course, before displaying, the CLUT has to be loaded with R, G, and B values that define the palette of colors required for each image to be rendered.

This indirect scheme restricts the number of available colors in an image (typically 256), although each color in the table has typically 8 bits for each of the R, G, and B primaries. This means that any given color can be one of approx. 16.7 million possible colors. However, the advantage is that an indexed-color image file can be significantly smaller than it would be with 8 bits per pixel for each primary. Modern storage, however, is far less costly, greatly reducing the need to minimize image file size.

By using an appropriate combination of red, green, and blue intensities, many colors can be displayed. Current typical display adapters use up to 24-bits of information for each pixel: 8-bit per component multiplied by three components (see the Digital representations section below). With this system, 16,777,216 (2563 or 224) discrete combinations of R, G and B values are allowed, providing millions of different (though not necessarily distinguishable) hue, saturation, and lightness shades.

For images with a modest range of brightnesses from the darkest to the lightest, eight bits per primary color provides good-quality images, but extreme images require more bits per primary color as well as advanced display technology. For more information see High Dynamic Range (HDR) imaging.