Amiga Halfbrite mode

The Extra Half Bright mode (short EHB) is a graphics mode the Amiga computer from Commodore, with the exception of the first Amiga 1000th It provides in addition to the normal 32 simultaneously displayable colors of the Amiga another 32 available which half the brightness of the first 32 possess. This allows 64 colors with only 32 color registers are displayed.

Amiga color palette

The available colors of the Amiga are stored in a special memory chip. The storage capacity of the chip is sufficient for 32 entries with 3 × 4 bits per color. Each color is defined by a red, green and blue components (see the RGB color space ), in which four bits are available for each color component. Each color has a stored index, i.e., a logical address by means of the color in the memory can be addressed. The adjacent table illustrates sample the logical schema of the so-called color palette.

Pixel representation

To display a pixel on the screen in color, the program needs the address of the desired color specify, for example the value " 31" for " green". In binary representation are responsible for addressing 32 values ​​5 bits required ( 25 = 32).

The extra Half-brightness mode, a bit is no longer used to address the color palette for a total of 6 bits. Thus, the addressing of 64 colors ( 26 = 64) would be possible in theory. However, because the color palette contains only 32 colors, even addresses can be controlled from 0 to 31. The sixth, most significant ( ie leftmost ) bit is therefore not used for the pointer to the color palette, but also acts as a so -called flag. If this bit is not set - so has the value 0 - such are the remaining five bits return to the normal address in the color palette. According to the above example ( 0) 11111 would therefore green for the color to the address 11111. If the sixth bit is set ( ie has the value 1 ), thus forming the remaining five bits return the address for the color palette. However, here the bit values ​​of the individual color components are shifted by one position to the right ( by the hardware in the Amiga chipset, corresponds to the decimal division by two), which changes the color shown. Since the resulting color has about half the brightness of the base color, this is called Extra - Half-brightness mode (of English helped half brightness and brightness). According to the example, the address value (1) would refer to the address 11111 11111 So the color green, the individual color components would be moved one position to the right, creating a new color.