Rasterbar redirects here. For the computing library libtorrent from Rasterbar see libtorrent (Rasterbar)

Libtorrent (Rasterbar)

libtorrent is an open source implementation of the BitTorrent protocol. It is written in and has its main library interface in C++. Its most notable features are support for Mainline DHT, IPv6 , HTTP seeds and µTorrent's peer exchange....

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The raster bar (also referred to as rasterbar or copperbar) is an effect

Demo effect

Demo effects are computer-based real-time visual effects found in demos created by the demoscene.The main purpose of demo effects in demos is to show off the skills of the programmer...

 used in demo

Demo (computer programming)

A demo is a non-interactive multimedia presentation made within the computer subculture known as the demoscene. Demogroups create demos to demonstrate their abilities in programming, music, drawing, and 3D modeling...

s that displays animated horizontal bars of colour that extend into the overscan

Overscan

Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...

 area of the display. The effect was very common in demos on the Commodore 64

Commodore 64

The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

, Commodore Amiga, 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...

, ZX Spectrum

ZX Spectrum

The ZX Spectrum is an 8-bit personal home computer released in the United Kingdom in 1982 by Sinclair Research Ltd...

 and Amstrad CPC

Amstrad CPC

The Amstrad CPC is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom,...

.
The computers of the 8 and 16 bit era typically did not or could not display video memory across the entire screen, leaving a border around the regular display area. The graphics chip commonly used a fixed entry in the colour look-up table

CLUT

A colour look-up table is a mechanism used to transform a range of input colours into another range of colours. It can be a hardware device built into an imaging system or a software function built into an image processing application...

 (CLUT) to colour this border area. The basic principle of the raster bar effect is that this CLUT entry is updated with a new colour during the horizontal blanking interval, when the TV's electron beam is returning to the left to begin drawing a new scanline, thus changing the border colour for just that scanline. By carefully gradiating the colour changes, an effect of metallic-looking horizontal bars can be achieved.

Many graphics chips can trigger an interrupt

Interrupt

In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

 when the horizontal blanking interval begins, and thus an interrupt handler

Interrupt handler

An interrupt handler, also known as an interrupt service routine , is a callback subroutine in microcontroller firmware, operating system or device driver whose execution is triggered by the reception of an interrupt...

 can perform the task of updating the CLUT entry.

This effect may have been considered impressive to those who were unfamiliar with how it worked, because the computer appeared to be displaying multiple colours in the "off-limits" border area, and because the traditional way of creating and animating such horizontal lines (by colouring pixels individually) would require intensive CPU performance that most CPUs of the time could not attain.

It also has to be said that changing the CLUT was not the only way that graphics could be displayed in the border area of the screen. The Commodore 64

Commodore 64

The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

 had a trick in which the programmer could fool the graphics chip to display sprites in the border, and the 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...

 could use other tricks to extend the area available for displaying graphics to the borders. In both of these cases, however, the tricks involved triggering features in the graphics chips that were not actually implemented by design.

The Commodore Amiga had a graphics coprocessor

Coprocessor

A coprocessor is a computer processor used to supplement the functions of the primary processor . Operations performed by the coprocessor may be floating point arithmetic, graphics, signal processing, string processing, or encryption. By offloading processor-intensive tasks from the main processor,...

 referred to as the Copper that could be programmed to perform the effect, hence the term "Copperbars". In each frame of the demo, a new Copper list (Copper program) would be generated in order to animate the bars.

Vertical raster bars

A similar effect can be generated vertically, although it often does not extend into the overscan border areas. To generate vertical bars, the same line of video memory is repeatedly output every scanline. At the top of the frame, the video memory is typically blank, and every horizontal blanking interval it is updated with a new "bar" in a slightly different position, creating a "stepped" effect.

Vertical raster bars are often called Kefrens bars, after the Amiga demo group that popularized them. However, the effect was implemented earlier by the Alcatraz demo group.