Sliced HAM Mode (SHAM)
Sliced HAM mode, also known as dynamic HAM mode uses the Amiga's standard HAM6 color mode, 6-bitplanes (4 bitplanes for the base colour palette of 16 colors and 2 bitplanes for modifying the base colors). It requires the use of the 'copper' coprocessor to increase the number of colors available on screen to the full 4096 palette by reprogramming the color registers every scanline. Slicing was common when using copper graphics in demos, image manipulation, and certain games. It is also possible to reprogram color registers at arbitrary points along individual scanlines, meaning that individual scanlines can be sliced in the same way.
SHAM expands the colour palette of the overall screen, each scan line can have a unique 16 colour base palette from 4096 possible colors. The advantage of SHAM mode is in having a more precise representation of the image's intended colors, since it would not be limited by the palette choices made for the previous scanline, thus helping to reduce HAM6 colour fringing that occurs more often when an image uses just a single 16 colour base palette for all scan lines.
Dynamic HiRes uses a similar palette changing technique. Where SHAM is limited to low resolution HAM modes, Dynamic HiRes uses the 4bitplane (16 colour) high resolution modes. Each scan line can have a unique 16 colour palette. Unlike the low resolution HAM6 modes, the hires screen modes of the original chipset have no ability to modify the base colors to get more than 16 colors per scan line.
The SHAM idea was deprecated when HAM8 was introduced, since even an unsliced HAM8 image has far more color resolution than a sliced HAM6 image. However, SHAM remains the best available HAM mode on those Amigas with the original chipset.
Modern graphics hardware does not have an Amiga-style copper, so in order to correctly display sliced HAM images, the Amiga hardware must be emulated. The programming required to do this can be computationally expensive, since to emulate a screen that is arbitrarily sliced, the only way to make certain that the emulation is correct is to emulate every bus cycle. However, if the goal is merely to display a SHAM image on a non-Amiga platform, the required color values may be pre-calculated based on the palette entries that are programmed via the copper-list, regardless of whether the palette is modified in the middle of a scanline.
Slicing an image no longer has any practical value on modern graphics hardware, since the color fidelity and bandwidth of modern hardware do not impose the limitations that slicing is designed to circumvent.
Read more about this topic: Hold-And-Modify, Hold-And-Modify Mode
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