Science Fair Project Encyclopedia
Highcolour (or Hicolour, Highcolor, Hicolor, Thousands on a Macintosh) graphics 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.
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 32 intensities of each hue. This allows 32768 possible colours for each pixel.
When all 16 bits are used, one of the hues (usually green, more on next paragraph) gets an extra bit, allowing a 64 levels of intensity for that hue, and a total of 65536 available colours. There is a problem with doing this because the green channel can have a different value than the other two. Suppose we wish to encode the 24-bit color rgb(40,40,40) with 16 bits. 40 in binary is 00101000. The red and blue channels will have a value of 0010, or 2 out of 32. The green channel will have a value of 00101, or 5 out of 64. Because of this, the color rgb(40,40,40) will have a slight green tinge when displayed in 16 bits.
The reason why green is usually chosen for the extra bit in 16 bits is because human eye has its highest sensitivity for green shades. You can convince yourself of that by looking at the following picture (warning: this will work only if your display is truecolour, e.g. 24 or 32 bits) where we show off dark shades of red, green, blue using 128 levels of intensities for each hue (7 bits); if you're not visually impaired, bring your eyes closer to the screen for a moment: you can easily see the lines of green, while you have difficulties to see the lines of red, and you almost can't distinguish the lines of blue. More rarely, some systems support having the extra bit of colour depth on the red or blue channel, usually in applications where that color is more prevalent (photographing of skin tones or skies, for example).
Unlike Planar or Chunky 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.
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