Vertical Raster Bars
A similar effect can be generated vertically, although it often does not extend into the border area. To generate vertical bars, the same line of video memory is repeatedly output every scanline. At the top of the frame, the video memory is typically blank, and every horizontal blanking interval it is updated with a new "bar" in a slightly different position, creating a "stepped" effect.
Vertical raster bars are often called Kefrens bars, after the Amiga demo group that popularized them. However, the effect was implemented earlier by the Alcatraz demo group.
Read more about this topic: Raster Bar
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