Hold And Modify Graphics
| Field | Computer Graphics |
| Went Obsolete | 1992 |
| Made Obsolete By | Affordability of 18 and 24 bit graphics |
| Knowledge Assumed | Amiga computer programming |
| When useful | from 1985 to about 1992 |
Hold And Modify was a graphics mode on the Amiga computer systems which was totally unique. It was added almost as an afterthought and wasn't considered to be a very practical mode, but with clever programming turned out to be pretty usable.
The concept was simple. At the time, having 6 bits of color information per pixel on a computer screen on low end computers was pretty advanced. This would allow up to 64 colors. On the Amiga, those 64 colours could be chosen from a palette of 4096 (also a pretty good range at the time). Actually, you couldn't even truly to do this, because there were only 32 color registers, so the Amiga employed another mode called Extra Half Bright which made the additional 32 colors simply half as bright as the first 32.
Hold And Modify was a trick where the first 4 bits would select from a bank of 16 color registers, again each of those was a 12 bit color (4 bits for each of red, green and blue, for 4096 total colours). The other 2 bits would determine an alternate interpretation, however. If they were set to 0, the first four bits would act like normal. However, if they were set to 1, 2 or 3 this would cause the first four bits to take the color of the previous pixel (its full 12 bit value) and modify the red, green or blue component.
This would allow you to make small color changes and thus enable you to potentialy display every single color in the 4096 color palette. As it turns out, for photographs and other graphics, small color changes are frequently all you need (an in fact modern compression systems like JPEG rely on this fact), so a properly rendered HAM image can look spectacular.
Though it required quite a bit of overhead to calculate, it became possible to create paint programs that would let you draw in real time in HAM mode. Of course it wasn't pixel perfect, but some skilled people made it work pretty well.
Later generations of chips enabled an 8 bit HAM mode that allowed for 18 bit color (262144 colors).
As computers gained more memory and speed it was possible to directly have 18 bit and then 24 bit or higher colour thus eliminating the need for such a mode, and even though it was a form of compression as well, things like JPEG went much further.
