Raster bar

It can be programmed to change the display colors per scan line without requiring the CPU, except to update the position of the bars once per frame.

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.

To generate vertical bars, the same line of video memory is repeatedly output every scanline.