Framebuffer
Encyclopedia
The information in the memory buffer typically consists of color values for every pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
(point that can be displayed) on the screen. Color values are commonly stored in 1-bit binary
Binary image
A binary image is a digital image that has only two possible values for each pixel. Typically the two colors used for a binary image are black and white though any two colors can be used. The color used for the object in the image is the foreground color while the rest of the image is the...
(monochrome), 4-bit palettized
Palette (computing)
In computer graphics, a palette is either a given, finite set of colors for the management of digital images , or a small on-screen graphical element for choosing from a limited set of choices, not necessarily colors .Depending on the context In computer graphics, a palette is either a given,...
, 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
Display resolution
The display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by all different factors in cathode ray tube , flat panel or projection...
of the output signal, and on the color depth
Color depth
In computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
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
Vertex (geometry)
In geometry, a vertex is a special kind of point that describes the corners or intersections of geometric shapes.-Of an angle:...
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
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
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 (pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s).
History
Computer researchers had long discussed the theoretical advantages of a framebuffer, but were unable to produce a machine with sufficient memory at an economically practicable cost. In 1969, Joan Miller of Bell LabsBell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
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
SuperPaint
SuperPaint was a pioneering graphics program and framebuffer computer system developed by Richard Shoup at Xerox PARC. The system was first conceptualized in late 1972 and produced its first stable image in April 1973...
system at Xerox PARC
Xerox PARC
PARC , formerly Xerox PARC, is a research and co-development company in Palo Alto, California, with a distinguished reputation for its contributions to information technology and hardware systems....
. 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.
Shoup was also able to use the SuperPaint framebuffer to create an early digital video-capture system. By synchronizing the output signal to the input signal, Shoup was able to overwrite each pixel of data as it shifted in. Shoup also experimented with modifying the output signal using color tables. These color tables allowed the SuperPaint system to produce a wide variety of colors outside the range of the limited 8-bit data it contained. This scheme would later become commonplace in computer framebuffers.
In 1974 Evans & Sutherland
Evans & Sutherland
Evans & Sutherland is a computer firm involved in the computer graphics field. Their products are used primarily by the military and large industrial firms for training and simulation, and in digital projection environments like planetariums.-History:...
released the first commercial framebuffer, costing about $15,000. It was capable of producing resolutions of up to 512 by 512 pixels in 8-bit grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
color, and became a boon for graphics researchers who did not have the resources to build their own framebuffer. The New York Institute of Technology
New York Institute of Technology
New York Institute of Technology is a private, non-sectarian, co-educational research university in New York City. NYIT has five schools and two colleges, all with a strong emphasis on technology and applied scientific research...
would later create the first 24-bit color system using three of the Evans & Sutherland framebuffers. Each framebuffer was connected to an RGB color output (one for red, one for green and one for blue), with a minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
controlling the three devices as one.
The rapid improvement of integrated-circuit technology made it possible for many of the home computers of the late 1970s (such as the Apple II
Apple II
The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
) to contain low-color framebuffers. While initially derided for poor performance in comparison to the more sophisticated graphics devices used in computers like the Atari 400, framebuffers eventually became the standard for all personal computers. Today, nearly all computers with graphical capabilities utilize a framebuffer for generating the video signal.
Framebuffers also became popular in high-end workstations throughout the 1980s. SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...
, Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
, HP
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
, DEC
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
and IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
all released framebuffers for their computers. These framebuffers were usually of a much higher quality than could be found in most home computers, and were regularly used in television, printing, computer modeling and 3D graphics.
Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
computers, due to their special design attention to graphics performance, created in the 1980s a vast market of framebuffer based graphics cards. Noteworthy to mention was the graphics card in Amiga A2500 Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, which was in 1991 the first computer to implement an X11 server program as a server for hosting graphical environments and the Open Look
OPEN LOOK
OPEN LOOK is a graphical user interface specification for UNIX workstations. It was originally defined in the late 1980s by Sun Microsystems and AT&T.-History:...
GUI
Gui
Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...
graphical interface in high resolution (1024x1024 or 1024x768 at 256 colors). The graphics card for A2500 Unix was called the A2410 (Lowell TIGA Graphics Card) and was an 8-bit graphics board based on the Texas Instruments
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
TMS34010
TMS34010
The TMS34010 was the first programmable graphics processor integrated circuit . First silicon was working at Texas Instruments in Houston in December 1985, and first shipment was to IBM's workstation facility in Kingston, New York, in January 1986...
clocked at 50 MHz. It was a complete intelligent graphics coprocessor. The A2410 graphics card for Amiga was co-developed with Lowell University. Other noteworthy Amiga framebuffer based cards were: the Impact Vision IV24 graphics card from GVP
Great Valley Products
Great Valley Products is a former third-party Amiga hardware supplier.The company was mostly known for CPU-Accelerators and SCSI-Hostadapters for the Commodore Amiga 500 and the Amiga 2000 computer series. The company liquidated itself in July 1995...
, an interesting integrated video suite, capable of mixing 24-bit framebuffer, with Genlock
Genlock
Genlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...
, Chromakey, TV signal pass-thru and TV in a window capabilities; the DCTV a graphics card and video capture system; the Firecracker 32-bit graphics card; the Harlequin card, the Colorburst; the HAM-E external framebuffer. The Graffiti external graphics card is still available on the market.
Most Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...
(Mega STE model), and Atari TT framebuffers were created for the VME rear connector slot of Atari machines dedicated to video expansion cards: Leonardo 24-bit VME graphics adapter, CrazyDots II 24-bit VME graphics card, Spektrum TC graphics card, NOVA ET4000 VME SVGA graphics card (capable of resolutions up to 1024x768 at 256 colors or 800x600 at 32768 colors), whose design came from the ISA
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
/PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
world (it was effectively an ATI Mach32 S: with 1 MB of video RAM).
Display modes
Refresh rate
The refresh rate is the number of times in a second that a display hardware draws the data...
timings.
In the world of Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
machines and operating systems, such conveniences were usually eschewed in favor of directly manipulating the hardware settings. This manipulation was far more flexible in that any resolution, color depth and refresh rate was attainable – limited only by the memory available to the framebuffer.
An unfortunate side-effect of this method was that the display device
Display device
A display device is an output device for presentation of information in visual or tactile form...
could be driven beyond its capabilities. In some cases this resulted in hardware damage to the display. More commonly, it simply produced garbled and unusable output. Modern CRT monitors fix this problem through the introduction of "smart" protection circuitry. When the display mode is changed, the monitor attempts to obtain a signal lock on the new refresh frequency. If the monitor is unable to obtain a signal lock, or if the signal is outside the range of its design limitations, the monitor will ignore the framebuffer signal and possibly present the user with an error message.
LCD monitors tend to contain similar protection circuitry, but for different reasons. Since the LCD must digitally sample the display signal (thereby emulating an electron beam), any signal that is out of range cannot be physically displayed on the monitor.
Color palette
Framebuffers have traditionally supported a wide variety of color modes. Due to the expense of memory, most early framebuffers used 1-bit (2 color), 2-bit (4 color), 4-bit (16 color) or 8-bit (256 color) color depths. The problem with such small color depths is that a full range of colors cannot be produced. The solution to this problem was to add a lookup tableLookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
to the framebuffers. Each "color" stored in framebuffer memory would act as a color index; this scheme was sometimes called "indexed color".
The lookup table served as a palette that contained data to define a limited number (such as 256) of different colors. However, each of those [256] colors, itself, was defined by more than 8 bits, such as 24 bits, eight of them for each of the three primary colors. With 24 bits available, colors can be defined far more subtly and exactly, as well as offering the full range gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...
which the display can show. While having a limited total number of colors in an image is somewhat restrictive, nevertheless they can be well chosen, and this scheme is markedly superior to 8-bit color.
The data from the framebuffer in this scheme determined which of the [256] colors in the palette was for the current pixel, and the data stored in the lookup table (sometimes called the "LUT") went to three digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
s to create the video signal for the display.
The framebuffer's output data, instead of providing relatively-crude primary-color data, served as an index – a number – to choose one entry in the lookup table. In other words, the index determined which color, and the data from the lookup table determined precisely what color to use for the current pixel.
Memory access
While framebuffers are commonly accessed via a memory mappingMemory-mapped I/O
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
directly to the CPU memory space, this is not the only method by which they may be accessed. Framebuffers have varied widely in the methods used to access memory. Some of the most common are:
- Mapping the entire framebuffer to a given memory range.
- Port commands to set each pixel, range of pixels or palette entry.
- Mapping a memory range smaller than the framebuffer memory, then bank switchingBank switchingBank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer...
as necessary.
The framebuffer organization may be chunky (packed pixel) or planar.
Virtual framebuffers
Many systems attempt to emulate the function of a framebuffer device, often for reasons of compatibility. The two most common "virtual" framebuffers are the Linux framebufferLinux framebuffer
The Linux framebuffer is a graphic hardware-independent abstraction layer to show graphics on a computer monitor, typically on the console...
device (fbdev) and the X Virtual Framebuffer (Xvfb
Xvfb
In the X Window System, Xvfb or X virtual framebuffer is an X11 server that performs all graphical operations in memory, not showing any screen output. From the point of view of the client, it acts exactly like any other server, serving requests and sending events and errors as appropriate....
). The X Virtual Framebuffer was added to the X Window System distribution to provide a method for running X without a graphical framebuffer. While the original reasons for this are lost to history, it is often used on modern systems to support programs such as the Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
JVM that do not allow dynamic graphics to be generated in a headless
Headless system
A headless system is a computer system or device that has been configured to operate without a monitor , keyboard and mouse...
environment.
The Linux framebuffer device was developed to abstract the physical method for accessing the underlying framebuffer into a guaranteed memory map that is easy for programs to access. This increases portability, as programs are not required to deal with systems that have disjointed memory maps or require bank switching.
Page flipping
Since framebuffers are often designed to handle more than one resolution, they often contain more memory than is necessary to display a single frame at lower resolutions. Since this memory can be considerable in size, a trick was developed to allow for new frames to be written to video memory without disturbing the frame that is currently being displayed.The concept works by telling the framebuffer to use a specific chunk of its memory to display the current frame. While that memory is being displayed, a completely separate part of memory is filled with data for the next frame. Once the secondary buffer is filled (often referred to as the "back buffer"), the framebuffer is instructed to look at the secondary buffer instead. The primary buffer (often referred to as the "front buffer") becomes the secondary buffer, and the secondary buffer becomes the primary. This switch is usually done during the vertical blanking interval
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
to prevent the screen from "tearing" (i.e., half the old frame is shown, and half the new frame is shown).
Most modern framebuffers are manufactured with enough memory to perform this trick even at high resolutions. As a result, it has become a standard technique used by PC game programmer
Game programmer
A game programmer is a software engineer, programmer, or computer scientist who primarily develops codebase for video games or related software, such as game development tools. Game programming has many specialized disciplines all of which fall under the umbrella term of "game programmer"...
s.
Graphics accelerators
As the demand for better graphics increased, hardware manufacturers created a way to decrease the amount of CPU time required to fill the framebuffer. This is commonly called a "graphics accelerator" in the Unix world.Common graphics drawing commands (many of them geometric) are sent to the graphics accelerator in their raw form. The accelerator then rasterizes
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
the results of the command to the framebuffer. This method can save from thousands to millions of CPU cycles per command, as the CPU is freed to do other work.
While early accelerators focused on improving the performance of 2D GUI
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
systems, most modern accelerators focus on producing 3D imagery in real time. A common design is to send commands to the graphics accelerator using a library such as OpenGL
OpenGL
OpenGL is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL...
. The OpenGL driver then translates those commands to instructions for the accelerator's graphics processing unit
Graphics processing unit
A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
(GPU). The GPU uses those microinstructions
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
to compute the rasterized results. Those results are bit blit
Bit blit
Bit BLIT is a computer graphics operation in which several bitmaps are combined into one using a raster operator....
ted to the framebuffer. The framebuffer's signal is then produced in combination with built-in video overlay devices (usually used to produce the mouse cursor without modifying the framebuffer's data) and any analog special effects that are produced by modifying the output signal. An example of such analog modification was the anti-aliasing
Anti-aliasing
In digital signal processing, spatial anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution...
technique used by the 3dfx
3dfx
3dfx Interactive was a company that specialized in the manufacturing of 3D graphics processing units and, later, graphics cards. It was a pioneer in the field for several years in the late 1990s until 2000 when it underwent one of the most high-profile demises in the history of the PC industry...
Voodoo cards. These cards add a slight blur to output signal that makes aliasing of the rasterized graphics much less obvious.
Popular manufacturers of 3D graphics accelerators are Nvidia
GeForce
GeForce is a brand of graphics processing units designed by Nvidia. , there have been eleven iterations of the design. The first GeForce products were discrete GPUs designed for use on add-on graphics boards, intended for the high-margin PC gaming market...
and AMD
Radeon
Radeon is a brand of graphics processing units and random access memory produced by Advanced Micro Devices , first launched in 2000 by ATI Technologies, which was acquired by AMD in 2006. Radeon is the successor to the Rage line. There are four different groups, which can be differentiated by...
.
External links
- Interview with NYIT researcher discussing the 24-bit system
- Jim Kajiya – Designer of the first commercial framebuffer
- History of Sun Microsystems' Framebuffers
- DirectFB – An abstraction layer on top of the Linux Framebuffer device
- pxCore - A portable framebuffer abstraction layer for Windows, Windows Mobile, Linux and OSX.