Amiga Halfbrite mode

Extra Half-Brite (EHB) mode is a planar display mode of the Commodore Amiga computer. It uses six bitplanes (six bits/pixel). The first five bitplanes index 32 colors selected from a 12-bit color space (4096 possible colors). If the bit on the sixth bitplane is set the display hardware halves the brightness of the corresponding color component. This way 64 simultaneous colors are possible (32 arbitrary colors plus 32 half-bright components) while only using 32 color registers. The number of color registers is a hardware limitation of pre-AGA chipsets used in Amiga computers.

Some contemporary game titles and animations used EHB mode as a hardware-assisted means to display shadows or silhouettes. EHB was also often used as general-purpose 64 color mode with the aforementioned restrictions.^[1][2]

Some early versions of the first Amiga, the Amiga 1000, sold in the United States lack the EHB video mode, which is present in all later Amiga models.

EHB Sliced Mode

With EHB palette switching, it is possible to produce yet more colors in a single image; this can be achieved by splitting the image into multiple horizontal blocks (slices), between which the color registers are modified during the vertical scan. This is not an official graphics mode, but a software technique made possible by the hardware. For example, by switching the palette eight times during a vertical scan, it is possible to produce up to 512 on-screen colors. Unlike the 4096 color HAM mode, this technique places no restrictions on color combinations between horizontally adjacent pixels. For example, moving parts of the image (within slice) do not require complex operations.

Comparison to EGA

Although the IBM PC Enhanced Graphics Adapter (EGA) standard offers a fixed 64 color space, it only allows 16 simultaneous colors (16 out of 64), and then only in the relatively slow and barely accelerated high-res (640x350) mode. Hi-res also required a dedicated EGA monitor - the lower resolutions (320x200 + 640x200, same as CGA) use 16 fixed colours, identical to those of the CGA RGBI palette, for compatibility with older RGBI monitors.

EHB exceeds this by allowing 32 colors out of 4096 plus their half-bright counterparts (32 + 32 out of 4096), at 320x200 to 360x576, on a standard composite-video or video-frequency RGB monitor. The Amiga could easily display any 32 EGA colors, but not all 64 simultaneously (without in-frame palette changes). It could also display at 640x400 to 720x576 using any 16 colours out of 4096 on a composite monitor with the same Agnus/Copper graphics acceleration facilities as in low res.

See also

• Original Chip Set
• Hold-And-Modify

References

External links

• Animated demo using Halfbrite mode (requires Java)