Color depth

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Color depth
8-bit color 15/16 bit: Highcolor 24/32 bit: Truecolor Web-safe color
Related
RGB color model Palette

Color depth is a computer graphics term describing 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 (bpp), particularly when specified along with the number of bits used. Higher color depth gives a broader range of distinct colors.

1 Indexed color
2 Direct color
3 Selection of color depth
4 See also
5 References
6 External links

[edit] Indexed color

A 2-bit indexed-color image. The color of each pixel is represented by a number; each number corresponds to a color in the palette.

With relatively low color depth, the stored value is typically an index into a color map or palette. The colors available in the palette itself may be fixed by the hardware or modifiable. Modifiable palettes are sometimes referred to as pseudocolor palettes.

1-bit color (2¹ = 2 colors) monochrome, often black and white
2-bit color (2² = 4 colors) CGA
4-bit color (2⁴ = 16 colors) as used by EGA and by the least common denominator VGA standard at higher resolution
5-bit color (2⁵ = 32 colors) Original Amiga chipset
6-bit color (2⁶ = 64 colors) Original Amiga chipset
8-bit color (2⁸ = 256 colors) Most early color Unix workstations, VGA at low resolution, Super VGA, AGA
12-bit color (2¹² = 4096 colors) some Silicon Graphics systems and Amiga systems in HAM mode.

Old graphics chips, particularly those used in home computers and video game consoles, often feature an additional level of palette mapping in order to increase the maximum number of simultaneously displayed colors. For example, in the ZX Spectrum, the picture is stored in a two-color format, but these two colors can be separately defined for each rectangular block of 8x8 pixels.

1 bit

4 bits

8 bits

Truecolor (24+ bits)

[edit] Direct color

As the number of bits increases, the number of possible colors becomes impractically large for a color map (a 20 bit depth would require more memory to store the colormap than is required to store the pixels themselves). So in higher color depths, the color value typically directly encodes relative brightnesses of red, green, and blue to specify a color in the RGB color model.

[edit] 8-bit direct color

A very limited but true direct color system, there are 3 bits (8 possible levels) for both the R and G components, and the two remaining bits in the byte pixel to the B component (four levels), enabling 256 (8 × 8 × 4) different colors. The normal human eye is less sensitive to the blue component than to the red or green, so it is assigned one bit less than the others. Used, at least, in the MSX2 system series of computers in the early 1990's.

Do not confuse with an indexed color depth of 8bpp (although it can be simulated in such systems selecting the adequate table).

[edit] 12-bit direct color

In 12-bit direct color, there are 4 bits (16 possible levels) for each of the R, G, and B components, enabling 4,096 (16 × 16 × 16) different colors. This color depth is sometimes used in devices with a color display, such as mobile telephones and other equipment.

[edit] HighColor

Highcolor or HiColor is considered sufficient to provide life-like colors, and is encoded using either 15 or 16 bits:

15-bit uses 5 bits to represent red, 5 for green, and 5 for blue. Since 2⁵ is 32 there are 32 levels of each color which can therefore be combined to give a total of 32,768 (32 × 32 × 32) mixed colors .

16-bit color uses 5 bits to represent red, 5 bits to represent blue, but (since the human eye is more sensitive to the color green) uses 6 bits to represent 64 levels of green. These can therefore be combined to give 65,536 (32 × 64 × 32) mixed colors. Sixteen-bit color is referred to as "thousands of colors" on Macintosh systems.

[edit] LCD Displays

Some modern LCD displays use dithered 18-bit color (64 × 64 × 64 = 262,144 combinations) to achieve faster transition times, without sacrificing truecolor display levels entirely.

[edit] Truecolor

Truecolor can frequently mimic many colors found in the real world, producing 16.7 million distinct colors. This approaches the level at which the human eye can distinguish colors for most photographic images, though image manipulation, some black-and-white images (which are restricted to 256 levels with Truecolor) or "pure" generated images may reveal the limitations.

24-bit Truecolor uses 8 bits to represent red, 8 bits to represent blue, and 8 bits to represent green. 2⁸ = 256 levels of each of these three colors can therefore be combined to give a total of 16,777,216 mixed colors (256 × 256 × 256). Twenty-four-bit color is referred to as "millions of colors" on Macintosh systems.

[edit] 32-bit color

"32-bit color" is a misnomer when regarding display color depth. A common misconception is that 32-bit color produces 4,294,967,296 distinct colors.

In reality, 32-bit color actually refers to 24-bit color (Truecolor) with an additional 8 bits, either as empty padding space or to represent an alpha channel. Considering red, green, and blue use the same amount of bits for their respective color (with the exception of 16-bit color), the total bits used will be a multiple of 3: like 15-bit color (5 bits each) and 24-bit color (8 bits each). The reason for using empty space is that all but the newest modern computers process data internally in units of 32 bits; as such, using this amount for each pixel can allow speed optimizations, but increasing the installed video memory.

[edit] Beyond truecolor

In the late 1990s, some high-end graphics hardware and scanners, such as from SGI, started to use more than 8 bits per channel, such as 12 or 16. This has never become common, as the gain in color resolution is almost invisible – 10 bits per channel seem to be enough to reach the absolute limits of human vision under almost all circumstances^{[citation needed]}.

However, professional-quality image manipulation software has started to employ 16 bits per color channel for internal storage, providing protection against accumulating rounding errors when multiple consecutive manipulations are performed on a picture.

For extended dynamic range imaging, including high dynamic range imaging (HDRI), floating point numbers are used to describe numbers in excess of 'full' white and black. This allows an image to describe accurately the intensity of the sun and deep shadows in the same colour space. Various models are used to describe these ranges, many employing 32 bit accuracy per channel. A new format is the ILM "half" using 16-bit floating point numbers, it appears this is a much better use of 16 bits than using 16-bit integers and is likely to replace it entirely as hardware becomes fast enough to support it.

[edit] Television Color

Most of today's TVs and computer screens form images by varying the intensity of just three primary colors: red, green, and blue. Bright yellow, for example, is composed of equal parts red and green, with no blue component. However, this is only an approximation, and is not as saturated as actual yellow light. For this reason, recent technologies such as, Texas Instruments's BrilliantColor augment the typical red, green, and blue channels with up to three others: cyan, magenta and yellow. Mitsubishi and Samsung, among others, use this technology in some TV sets. Assuming that 8 bits are used per color, such six-color images would have a color depth of 48 bits.

The ATI FireGL V7350 graphics card supports 40-bit and 64-bit color^[1].

[edit] Selection of color depth

In graphics intensive applications such as computer games, a tradeoff of performance and quality can be achieved by lowering the color depth of the display; graphics with lower color depths do not require as much frame buffer memory or display bandwidth, allowing them to be generated and displayed more quickly. Increasing color depth results in higher color quality at the expense of display speed and responsiveness.