HIGHCOLOUR

(Redirected from Highcolor)

'Highcolour' graphics (variously spelled 'Hicolour', 'Highcolor', and 'Hicolor', and known as 'Thousands of colors' on a Macintosh) is a method of storing image information in a computer's memory such that each pixel is represented by two bytes. Usually the colour is represented by all 16 bits, but some video chipsets also support 15 bit highcolour.

Contents

15-bit highcolour

16-bit highcolour

Other notes

See also

15-bit highcolour

In 15 bit highcolour, one of the bits of the two bytes is ignored, and the remaining 15 bits are split between the red, green, and blue components of the final colour, like this:

Bit	15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00
Data	x	R	R	R	R	R	G	G	G	G	G	B	B	B	B	B

Each of the RGB components has 5 bits associated, giving 2^5 = 32 intensities of each hue. This allows 32,768 possible colours for each pixel.
The popular Cirrus Logic graphics chips of the early 1990s made use of the spare high-order bit for their so-called "mixed" video modes: with bit 15 clear, bits 0 through 14 would be treated as an RGB value as described above, while with bit 15 set, bit 0 through 7 would be interpreted as an index into a 256-color palette. This would have enabled display of (comparatively) high-quality color images side by side with palette-animated screen elements, but in practice, this feature was hardly used by any software.

16-bit highcolour

When all 16 bits are used, one of the hues (usually green, see below) gets an extra bit, allowing 64 levels of intensity for that hue, and a total of 65,536 available colours. This can lead to small discrepancies in encoding. Suppose we wish to encode the 24-bit colour RGB (40, 40, 40) with 16 bits. Forty in binary is 00101000. The red and blue channels will take the five most significant bits, and will have a value of 00101, or 5 on a scale from 0 to 31 (16.1%). The green channel, with six bits of precision, will have a binary value of 001010, or 10 on a scale from 0 to 63 (15.9%). Because of this, the colour RGB (40, 40, 40) will have a slight green tinge when displayed in 16 bits. Note that 40 on a scale from 0 to 255 is 15.7%.
Green is usually chosen for the extra bit in 16 bits because the human eye has its highest sensitivity for green shades. For a demonstration, look closely at the following picture (note: this will work only on monitors displaying truecolour, i.e., 24 or 32 bits) where dark shades of red, green and blue are shown using 128 levels of intensities for each hue (7 bits). Readers with normal vision should see the shades of green relatively easily, while the shades of red should be difficult to see, and the shades of blue are likely indistinguishable. More rarely, some systems support having the extra bit of colour depth on the red or blue channel, usually in applications where that colour is more prevalent (photographing of skin tones or skies, for example).

Other notes

Unlike planar or chunky (packed pixel) graphics, there is no need for a colour look up table (CLUT, or palette) when in Highcolour mode, because there are enough available colours per pixel to represent graphics and photos reasonably satisfactorily. Some image formats (TIF) can save paletted 16-bit images with embedded CLUT.

See also

★ Planar

★ Packed pixel

★ Truecolor

★ Color depth