Rasterbar Demo Effect

The rasterbar demo effect is an effect used in demos to produce colour effects that are not normally possible on the computer. The effect was very common in demos on the Commodore 64, Commodore Amiga and Atari ST.

The computers of the 8 and 16 bit era usually used indexed colour as opposed to true colour. In indexed colour, each pixel in memory does not contain the precise colour of the pixel but rather an index into a colour map. This colour map then contains the RGB values that should be used. The advantage of this approach is that the image on the screen takes up significantly less memory, which means that the graphics chip can be made cheaper (it can run much slower since it has to read less memory every frame).

The graphics chips of the day also displayed a border around the viewable area. This was done since the most common type of display was a standard tv. The border was usually quite large, and the only property of the border that could be altered by the programmer was its colour.

The graphics chips worked by reading the content of the graphics memory (which was simply a part of main memory), cross-referenced the colour index with the colour map which was stored inside the graphics chip and sent the resulting colour values to the display.

The graphics chip could send an interrupt to the CPU when it started painting the screen, and usually also when it started to display the next line. The purpose of these interrupts were to synchronise the back buffer switching to prevent graphics artifacts when displaying animated graphics. The raster bar effect was achieved by changing the border (and background, which on some architectures were the same) colour right before the graphics chip started to display a new line of graphics. The end result was that the programmer had full control of the colour of every line on the background and the screen. Displaying any forms of patters on the border was impossible or at least very difficult in any other way, but it was also used to present pattern on the screen area as well. The reason for this was that this was a much faster way of achieving the desired visual effect than to modify the actual pixel values themselves. In fact, most computers at the time did not have sufficient processing power to actually update all the pixels on the screen during the time it took for the graphics chip to display a single frame, which is needed in order for the animation effects to look smooth.

It also has to be said that the rasterbar effect was not the only way that graphics could be displayed in the border area of the screen. The Commodore 64 had a trick in which the programmer could fool the graphics chip to display sprites in the border, and the Atari ST 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.