Back in 1985 when the Amiga was first released, the HAM (Hold And Modify) screenmode was revolutionary. It allowed 4096 colours to be displayed simultaneously, meaning near true-colour images could be displayed while only using the amount of memory that a normal 64-colour screen would use.
HAM does have its disadvantages, though – while a regular bitplane image can be faded out very easily by simply fading the palette, this trick doesn’t work for HAM images.
Fading out a HAM image can be done, however, using the Blitter.
This AMOSPro program uses some “evil” trickery to fade a HAM image to black, using the Screen Copy command with blitter modes, peeks and pokes to shuffle bitplanes around between screens, and what amounts to a ShadeBOB routine over the entire screen.
Step by step, the theory of operation is something like this:
- Logical OR the two control planes together to make a mask plane. This mask plane is “0” wherever pixels use the base palette, and “1” wherever a direct R, G or B level is being specified.
- Use the mask plane to perform binary subtraction on the four data planes, using another spare plane to hold carry data.
- Check for overflow, and zero the image wherever overflow has occurred.
This is done simultaneously with fading out the palette, and the end result is quite a nice fade-out of a HAM image.