- Amiga Halfbrite mode
Extra Half-Brite (EHB) mode is a
screenmode of the CommodoreAmiga computer. It uses 6 bitplanes (6 bits/pixel), where the first 5 bitplanes index a color from the color palette (consisting of 32 colors). If the bit on the 6th plane is set the color brightness is halved for each color component. This way 64 simultaneous colors are possible while only using 32 color palette registers.The 32 colors per palette is a hardware limitation for the pre-AGA chipsets of the Amiga computer.
Some early versions of the
Amiga 1000 sold in theUnited States lacked the EHB video mode.Note that although the PC
EGA 64 color adapter also offered a 64 color space, it only allowed to pick 16 colors out of it. Colors that can be produced by EGA and EHB aren't directly comparable. EGA allowed values 0,1,2, and 3 for red, green and blue, and their combinations, making a linear 64 color system (but with 16 colors displayed at once), while with EHB you could pick any 32 colors out of 4096 and have half-bright transformations. EGA colors can be closely emulated in EHB but the process isn't linear. Even if EGA could display all 64 colors at once, EHB could produce much better image quality.With palette switching it was possible to produce yet more colors in a single image, this was achieved by splitting the image to multiple horizontal blocks, between which the palette data was modified on the fly when displaying the image. This was not an official graphics mode, but a software controlled trick that the hardware allowed to perform. For example, by switching the palette 8 times during a vertical scan, it was theoretically possible to produce what appeared like up to 512 colors in an image. Also it had no restrictions on color combinations between adjacent pixels like the HAM mode, and for example, moving parts of the image (within their own palette block) didn't require complex operations.
ee also
*
Amiga Hold-and-Modify External links
* [http://www.randelshofer.ch/animations/anims/electronic_arts/HBriteHill.html Animated demo using Halfbrite mode] (requires Java)
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