Hold-And-Modify Mode
The original HAM mode allows all 4096 colors to be displayed using just six bitplanes. This is achieved by modifying the color of the previous playfield pixel encountered on the scanline, or in the case of the first playfield pixel of each scanline the color of the first entry in the color palette. The color of each successive pixel is defined as either a selection from the color palette, or a modification of the color of the previous pixel. In the latter case, only one of the color components may be modified per pixel. Two of the color components are 'held', and one component is 'modified', hence the name Hold-And-Modify.
HAM can be considered a lossy compression technique. Under HAM6 mode the playfield is encoded in half the memory normally required for a 12-bit color space. The greater overall color fidelity is achieved at the expense of horizontal chroma resolution. The image is 'decompressed' by display hardware in realtime as the graphics buffer is being displayed.
HAM was only originally created as an experimental mode. The chipset was originally designed to interpret bitplane data as the hue and luminance portions of an NTSC signal. To quote Jay Miner:
| “ | Hold and Modify came from a trip to see flight simulators in action and I had a kind of idea about a primitive type of virtual reality. NTSC on the chip meant you could hold the hue and change the luminance by only altering four bits. When we changed to RGB I said that wasn't needed any more as it wasn't useful and I asked the chip layout guy to take it off. He came back and said that this would either leave a big hole in the middle of the chip or take a three-month redesign and we couldn't do that. I didn't think anyone would use it. I was wrong again as that has really given the Amiga its edge in terms of the color palette. | ” |
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