"Raster" is a demo term which refers to the act of modifying the colours while the picture is being drawn by the display (TV or monitor). This is done to display more than the usual Video modes number of colours.
The display draws one line every 64microseconds, with the whole image drawn at a frequency of 50Hz. The CPU (Z80) runs at about 4 MHz. Most instructions are multiples of a microsecond and this makes timing for making rasters easy.
The traditional raster is made by changing the colour for one pen once each line creating a horizontal bar of colour.
But changing the colours can be used to have different areas of the screen in different colours, e.g. for a status panel and a game area.
It is also a good technic to display very coloured layers.
Exemples of Raster effect in Games
- Some GX4000 games are CPC games which include Rasters effect to add colours.
- Crazy Car 2 : extra colours layer in the sky.
- Fire and Forget 2 : Idem.
- Switchblade : also a good Red layer.
- Striker In The Crypts Of Trogan : one of the rare games in Mode 1 with "massive" use of in-game rasters (not only in the HUD actually).
This games looks a lot like Switchblade indeed...It may be a mod of the Switchblade engine. Yet there is a pseudo Colour attribute clash : when monster pass from one raster to another, they change colour.