Framebuffer

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A framebuffer is a video output device that drives a video display from a memory buffer containing a complete frame of data.

The information in the memory buffer typically consists of color values for every pixel (point that can be displayed) on the screen. Color values are commonly stored in 1-bit binary (monochrome), 4-bit palettized, 8-bit palettized, 16-bit highcolor and 24-bit truecolor formats. An additional alpha channel is sometimes used to retain information about pixel transparency. The total amount of the memory required to drive the framebuffer depends on the resolution of the output signal, and on the color depth and palette size.

Framebuffers differ significantly from the vector displays that were common prior to the advent of the framebuffer. With a vector display, only the vertices of the graphics primitives are stored. The electron beam of the output display is then commanded to move from vertex to vertex, tracing an analog line across the area between these points. With a framebuffer, the electron beam (if the display technology uses one) is commanded to trace a left-to-right, top-to-bottom path across the entire screen, the way a television renders a broadcast signal. At the same time, the color information for each point on the screen is pulled from the framebuffer, creating a set of discrete picture elements (pixels).

The term "framebuffer" has also entered into colloquial usage to refer to a backing store of graphical information. The key feature that differentiates a framebuffer from memory used to store graphics – the output device – is lost in this usage.

Contents

History

Computer researchers had long discussed the theoretical advantages of a framebuffer, but were unable to produce a machine with sufficient memory. In 1969, Joan Miller of Bell Labs experimented with the first known instance of a framebuffer. The device displayed an image with a color depth of three bits. However, it was not until the 1970s that advances in integrated-circuit memory made it practical to create the first framebuffer capable of holding a standard video image.

In 1972, Richard Shoup developed the SuperPaint system at Xerox PARC.[1] This system had 311,040 bytes of memory and was capable of storing 640 by 480 pixels of data with 8 bits of color depth. The memory was scattered across 16 circuit boards, each loaded with multiple 2-kilobit shift register chips. While workable, this design required that the total framebuffer be implemented as a 307,200 byte shift register that shifted in synchronization with the television output signal. The primary drawback to this scheme was that memory was not random access. Rather, a given position could be accessed only when the desired scan-line and pixel time rolled around. This gave the system a maximum latency of 33 ms for writing to the framebuffer.

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