Raster Effects
Per-scanline graphics tricks
Raster effects change graphics parameters during screen drawing, creating colour bars, split screens, and effects beyond normal hardware limits.
Overview
Raster effects exploit timing: the screen draws line by line, and code can change parameters mid-draw. Change the background colour at line 100, and the screen has two coloured regions. Chain multiple changes for rainbow effects. Reposition sprites after they’re drawn to multiplex them. The technique enabled visuals that specification sheets said were impossible.
Fast facts
- Mechanism: modify registers during screen draw.
- Timing: synchronise with raster beam position.
- Effects: colour bars, split screens, sprite multiplexing.
- Platforms: C64, Amiga, NES, and others.
- Precision: cycle-accurate timing often required.
Common effects
What raster programming enabled:
- Colour bars: different colours per region.
- Split screen: status bar with different mode.
- Sprite multiplexing: more sprites than hardware limit.
- Scroll splits: different scroll positions.
Implementation
Creating raster effects:
- Raster interrupt: trigger code at specific line.
- Timing: precise cycle counting.
- Register changes: update video chip mid-frame.
- Stable raster: eliminate timing jitter.