Rasters

"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.

Links

• Wikipedia link to Raster concepts

CPCWiki Links

• Gate Array
• CRTC
• ASIC

• Synchronising with the CRTC and display

• Programming:Simple Raster Example 1
• Programming:Simple Raster Example 2 (uses CPC plus features)
• Programming:Simple Raster Example 3 (uses CPC plus features)
• Programming:Simple Split Raster Example 1