Raster Bar - Horizontal Raster Bars

Horizontal Raster Bars

Such computers had limited graphical abilities and usually a fixed number of colours/inks (e.g. a maximum of 16 on the Amstrad CPC) that could be displayed at any one time, which were often assigned from a colour look-up table, which maps each displayable colour to one of a larger selection of possible colours (palette) of which the hardware was capable (e.g. 27 on the CPC). Raster bars and similar effects (e.g. having a HUD that uses a different set of colours than does the playing area) are achieved by changing the entries in the CLUT at specific times while the screen is being drawn (originally by the electron gun), in order to display a different set of colours in the subsequent portion of the screen. The most basic raster bar simply affects a single scanline by changing the value in the CLUT for the ink covering that line just before the electron gun draws it, and then changes it back to the previous colour once the line is finished. By using multiple colours in succession and carefully gradating the changes, an effect of metallic-looking horizontal bars can be achieved.

Many graphics chips can trigger an interrupt, specifically called a raster interrupt, when the horizontal blanking interval or the vertical sync begins; thus, an interrupt handler can precisely time and perform the task of updating CLUT entries for raster-bars and other colour-changing effects.

This effect may have been considered impressive to those who were unfamiliar with how it worked, because the computer appeared to be displaying more colours than are normally possible - potentally in the 'off-limits' border area, too - 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.

The Commodore Amiga had a graphics coprocessor 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.