Highcolour
Encyclopedia
High color graphics is a method of storing image information in a computer
's memory such that each pixel
is represented by two byte
s. Usually the color
is represented by all 16 bit
s, but some devices also support 15-bit high color.
More recently, high color has been used by Microsoft to distinguish display systems that can make use of more than 8-bits per color channel (10:10:10:2 or 16:16:16:16 rendering formats) from traditional 8-bit per color channel formats. This is a distinct usage from the 15-bit (5:5:5) or 16-bit (5:6:5) formats traditionally associated with the phrase high color.
, green
, and blue
components of the final color, like this:
Sample
layout of a 15 bit color data in a 16 bit pixel (in RGBAX notation)
Sample
layout of a 15 bit color data and Alpha channel in a 16 bit pixel (in RGBAX notation)
Each of the RGB components has 5 bits associated, giving 25 = 32 intensities of each component. 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 8-bit index into a 256-color palette (with bits 8 through 14 remaining unused.) 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.
Sample
layout of real 16 bit color data in a 16 bit pixel (in RGBAX notation)
This can lead to small discrepancies in encoding, e.g. when one wishes to encode the 24-bit color RGB (40, 40, 40) with 16 bits (a problem common to subsampling). 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 color 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 true color, i.e., 24 or 32 bits) where dark shades of red, green and blue are shown using 128 levels of intensities for each component (7 bits). Readers with normal vision should see the individual 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 color depth on the red or blue channel, usually in applications where that color is more prevalent (photographing of skin tones or skies, for example).
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
's memory such that each pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
is represented by two byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s. Usually the color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
is represented by all 16 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s, but some devices also support 15-bit high color.
More recently, high color has been used by Microsoft to distinguish display systems that can make use of more than 8-bits per color channel (10:10:10:2 or 16:16:16:16 rendering formats) from traditional 8-bit per color channel formats. This is a distinct usage from the 15-bit (5:5:5) or 16-bit (5:6:5) formats traditionally associated with the phrase high color.
15-bit high color
In 15 bit high color, one of the bits of the two bytes is ignored or set aside for an alpha channel, and the remaining 15 bits are split between the redRed
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
, green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
, and blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
components of the final color, like this:
Sample
Sample (graphics)
In computer graphics, a sample is an intersection of channel and a pixel.The diagram below depicts a 24-bit pixel, consisting of 3 samples for Red, Green, and Blue....
layout of a 15 bit color data in a 16 bit pixel (in RGBAX notation)
Sample
Sample (graphics)
In computer graphics, a sample is an intersection of channel and a pixel.The diagram below depicts a 24-bit pixel, consisting of 3 samples for Red, Green, and Blue....
layout of a 15 bit color data and Alpha channel in a 16 bit pixel (in RGBAX notation)
Each of the RGB components has 5 bits associated, giving 25 = 32 intensities of each component. This allows 32,768 possible colours for each pixel.
The popular Cirrus Logic
Cirrus Logic
Cirrus Logic is a fabless semiconductor supplier specializing in analog, mixed-signal, and audio DSP integrated circuits . They are presently headquartered in Austin, Texas. Their audio processors and audio converters are found in many professional audio and consumer entertainment products,...
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 8-bit index into a 256-color palette (with bits 8 through 14 remaining unused.) This would have enabled display of (comparatively) high-quality color images side by side with palette-animated
Palette-shifting
Color cycling, also known as palette shifting, is a technique used in computer graphics in which colors are changed in order to give the impression of animation...
screen elements, but in practice, this feature was hardly used by any software.
16-bit high color
When all 16 bits are used, one of the components (usually green, see below) gets an extra bit, allowing 64 levels of intensity for that component, and a total of 65,536 available colors.Sample
Sample (graphics)
In computer graphics, a sample is an intersection of channel and a pixel.The diagram below depicts a 24-bit pixel, consisting of 3 samples for Red, Green, and Blue....
layout of real 16 bit color data in a 16 bit pixel (in RGBAX notation)
This can lead to small discrepancies in encoding, e.g. when one wishes to encode the 24-bit color RGB (40, 40, 40) with 16 bits (a problem common to subsampling). Forty in binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
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 color 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 true color, i.e., 24 or 32 bits) where dark shades of red, green and blue are shown using 128 levels of intensities for each component (7 bits). Readers with normal vision should see the individual 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 color depth on the red or blue channel, usually in applications where that color is more prevalent (photographing of skin tones or skies, for example).
Other notes
There is generally no need for a color look up table (CLUT, or palette) when in high color mode, because there are enough available colors per pixel to represent graphics and photos reasonably satisfactorily, although the lack of precision has an impact on image fidelity. As a result, some image formats (e.g., TIFF) can save paletted 16-bit images with an embedded CLUT.See also
- True color
- Deep color
- Color depthColor depthIn computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
- Planar
- Packed pixel
- List of monochrome and RGB palettes — 15-bit RGB and 16-bit RGB sections.