Such computers had limited graphical abilities and usually a fixed number of colours or 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 (CLUT), 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 beam), 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 colour 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. ==Vertical raster bars==