Color depth

Color depth
1-bit monochrome 8-bit grayscale 8-bit color 15/16-bit color (High color) 24-bit color (True color) 30/36/48-bit color (Deep color)
Related
Indexed color Palette RGB color model Web-safe color
This box: view · talk · edit

In 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 (bpp), particularly when specified along with the number of bits used. Higher color depth gives a broader range of distinct colors.

Color depth is only one aspect of color representation (formally, the gamut: which colors can be expressed), expressing how finely levels of color can be expressed (formally, gamut depth); the other aspect is how broad a range of colors can be expressed. The RGB color model, as used below, cannot express many colors, notably saturated colors such as yellow. Thus, the issue of color representation is not simply "sufficient color depth" but also "broad enough gamut".

Indexed color

Main article: Indexed color

With relatively low color depth, the stored value is typically a number representing the index into a color map or palette. The colors available in the palette itself may be fixed by the hardware or modifiable within the limits of the hardware (for instance, both color Macintosh systems and VGA-equipped IBM-PCs typically ran at 8-bit due to limited VRAM, but while the best VGA systems only offered an 18-bit (262,144 color) palette from which colors could be chosen, all color Macintosh video hardware offered a 24-bit (16 million color) palette). Modifiable palettes are sometimes referred to as pseudocolor palettes.

1 bit (2 colors)

• 1-bit color (2¹ = 2 colors) monochrome, often black and white, compact Macintoshes, Atari ST.
• 2-bit color (2² = 4 colors) CGA, gray-scale early NeXTstation, color Macintoshes, Atari ST.

2 bits (4 colors)

• 3-bit color (2³ = 8 colors) many early home computers with TV displays

4 bits (16 colors)

• 4-bit color (2⁴ = 16 colors) as used by EGA and by the least common denominator VGA standard at higher resolution, color Macintoshes, Atari ST.
• 5-bit color (2⁵ = 32 colors) Original Amiga chipset
• 6-bit color (2⁶ = 64 colors) Original Amiga chipset

8 bits (256 colors)

• 8-bit color (2⁸ = 256 colors) most early color Unix workstations, VGA at low resolution, Super VGA, color Macintoshes, Atari TT, AGA, Falcon030.
• 12-bit color (2¹² = 4096 colors) some Silicon Graphics systems, Neo Geo, Color NeXTstation systems, and Amiga systems in HAM mode.
• 16-bit color (2¹⁶ = 65536 colors) some color Macintoshes.

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.

Direct color

As the number of bits increases, the number of possible colors becomes impractically large for a color map. 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.

A typical computer monitor and video card may offer 8 bits of resolution (256 output levels) per R/G/B color channel, for an overall 24-bit color space (or 32-bit space, with alpha transparency bits, which have no bearing on the output resolution), though earlier standards offered 6 bits per channel (64 levels) or less; the DVD standard defines up to 10 bits of resolution (1024 levels) for each of the Y/U/V video encoding channels (luminance plus two chrominance channels).

8-bit direct color

A very limited but true direct color system, there are 3 bits (8 possible levels) for each of 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,^{[citation needed]} so it is assigned one bit less than the others. Used, amongst others, in the MSX2 system series of computers in the early to mid 1990s.

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

16-bit direct color

In 16-bit direct color, there can be 4 bits (16 possible levels) for each of the R, G, and B components, plus optionally 4 bits for alpha (transparency), enabling 4,096 (16 × 16 × 16) different colors with 16 levels of transparency. Or in some systems there can be 5 bits per color component and 1 bit of alpha (32768 colors, just fully transparent or not); or there can be 5 bits for red, 6 bits for green, and 5 bits for blue, for 65536 colors with no transparency.^[1] These color depths are sometimes used in small devices with a color display, such as mobile telephones.

Variants with 5 or more bits per color component are sometimes called high color,^[2] which is sometimes considered sufficient to display photographic images.^[3]

LCD displays

Almost all cheap LCD displays (such as typical twisted nematic types) use dithered 18-bit color (64 × 64 × 64 = 262,144 combinations) to achieve faster transition times, but they must use either dithering or Frame Rate Control to fake 24-bit-per-pixel truecolor, or throw away 6 bits of color information away entirely. The best LCD displays can display 24-bit or greater color depth.

Truecolor

24 bits (16,777,216 colors, "truecolor")

Truecolor is a method of representing and storing graphical image information (especially in computer processing) in an RGB color space such that a very large number of colors, shades, and hues can be displayed in an image, such as in high quality photographic images or complex graphics. Usually, truecolor is defined to mean at least 256 shades of red, green, and blue, for a total of at least 16,777,216 color variations. The human eye is popularly believed to be capable of discriminating among as many as ten million colors.^[4] Truecolor can also refer to an RGB display mode that does not need a color look-up table (CLUT).^[5]

For each pixel, generally one byte is used for each channel while the fourth byte (if present) is being used either as an alpha channel data or simply ignored. Byte order is usually either RGB or BGR. Some systems exist with more than 8 bits per channel, and these are often also referred to as truecolor (for example a 48-bit truecolor scanner).

Even with truecolor, monochromatic images, which are restricted to 256 levels, owing to their single channel, can sometimes still reveal visible banding artifacts.

Truecolor, like other RGB color models, cannot express colors outside of the gamut of its RGB color space (generally sRGB).

On Macintosh systems, 24-bit color is referred to as "millions of colors."

Beyond truecolor

Video cards with 10 bits per color, or 30-bit color, started coming into the market in the late 1990s. An early example was the Radius ThunderPower card for the Macintosh, which included extensions for QuickDraw and Adobe Photoshop plugins to support editing 30-bit images.^[6]

Deep color is a term used to describe a gamut comprising a billion or more colors.^[7] The xvYCC, sRGB, and YCbCr color spaces can be used with deep color systems.^[8]

Systems using more than 24 bits in a 32-bit pixel for actual color data exist, but most of them opt for a 30-bit implementation with two bits of padding so that they can have an even 10 bits of color for each channel, similar to many HiColor systems.

While some high-end graphics workstation systems and the accessories marketed toward use with such systems, as from SGI, have always used more than 8 bits per channel, such as 12 or 16 (36-bit or 48-bit color), such color depths have only worked their way into the general market more recently.

As bit depths climb above 8 bits per channel, some systems use the extra bits to store more intensity range than can be displayed all at once, as in 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 color space for less distortion after intensive editing. Various models 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.^{[according to whom?]}

Many modern desktop systems (Mac OS X, GNOME, KDE, Windows XP/Vista/7, etc.) offer a 32-bit color option (given a suitably modern video card), but in that context, 32-bit color refers to 24-bit TrueColor with 8 bits for an alpha channel. When switching to an 8/16/24-bit color option in those systems, generally transparency/translucency effects are disabled, and the only reduction in color depth is seen when going to 8/16-bit color.

Industry support

The HDMI 1.3 specification defines bit depths of 30 bits (1.073 billion colors), 36 bits (68.71 billion colors), and 48 bits (281.5 trillion colors).^[8] In that regard, the NVIDIA Quadro graphics cards support 30-bit deep color^[9] as do some models of the Radeon 5800 series such as the HD 5970.^[10][11] The ATI FireGL V7350 graphics card supports 40-bit and 64-bit color.^[12]

The DisplayPort specification also supports color depths greater than 24 bpp.

At WinHEC 2008, Microsoft announced that color depths of 30 bits and 48 bits would be supported in Windows 7, along with the wide color gamut scRGB (which can be converted to xvYCC output).^[13][14]

Television color

Virtually all television displays and computer displays form images by varying the strength (technically, tristimulus values) of just three primary colors: red, green, and blue. Bright yellow, for example, is formed by roughly equal red and green contributions, with little or no blue contribution. Recent technologies such as Texas Instruments's BrilliantColor augment the typical red, green, and blue channels with up to three other primaries: cyan, magenta and yellow.^[15] Mitsubishi and Samsung, among others, use this technology in some TV sets. However, the associated signal processing typically fails to mimic additive mixing; hence, the displayed colors are distorted (for example, compared to sRGB source values). The Sharp Aquos line of televisions has introduced Quattron technology, which augments the usual RGB pixel components with a yellow subpixel. Again, signal processing fails in general to follow the laws of additive mixing, and colors are distorted.

Analog TVs use continuous signals which have no fixed number of different colors, although subject to noise introduced in transmission.

See also

• Bit plane
• RGB color model
• List of colors
• List of colors (compact)
• List of color palettes
• Mach banding

References

1. ^ Edward M. Schwalb (2003). iTV handbook: technologies and standards. Prentice Hall PTR. p. 138. ISBN 9780131003125. http://books.google.com/books?id=DM_EV4yJKYUC&pg=PA138. 
2. ^ Ben Waggoner (2002). Compression for great digital video: power tips, techniques, and common sense. Focal Press. p. 34. ISBN 9781578201112. http://books.google.com/books?id=4yXVZYMd-q4C&pg=PA34. 
3. ^ David A. Karp (1998). Windows 98 annoyances. O'Reilly Media, Inc. p. 156. ISBN 9781565924178. http://books.google.com/books?id=h4YYhcb0MsMC&pg=PA156. 
4. ^ D. B. Judd and G. Wyszecki (1975). Color in Business, Science and Industry. Wiley Series in Pure and Applied Optics (third ed.). New York: Wiley-Interscience. pp.  388. ISBN 0471452122. 
5. ^ Charles A. Poynton (2003). Digital Video and HDTV. Morgan Kaufmann. p. 36. ISBN 1558607927. http://books.google.com/?id=ra1lcAwgvq4C&pg=RA1-PA36&dq=truecolor#PRA1-PA36,M1. 
6. ^ "Radius Ships ThunderPower 30/1920 Graphics Card Capable of Super Resolution 1920 × 1080 and Billions of Colors". Business Wire. 1996-08-05. http://findarticles.com/p/articles/mi_m0EIN/is_1996_August_5/ai_18554540. 
7. ^ Keith Jack (2007). Video demystified: a handbook for the digital engineer (5th ed.). Newnes. p. 168. ISBN 9780750683951. http://books.google.com/books?id=BtCOgrpzbz4C&pg=PA168. 
8. ^ ^{a b} "HDMI Specification 1.3a Section 6.7.2". HDMI Licensing, LLC.. 2006-11-10. http://www.hdmi.org/learningcenter/kb.aspx?c=3. Retrieved 2009-04-09. 
9. ^ "30-Bit Color Technology for NVIDIA Quadro". NVIDIA. 2009-06-29. http://www.nvidia.com/docs/IO/40049/TB-04701-001_v02_new.pdf. Retrieved 2010-03-31. 
10. ^ "ATI Radeon HD 5970 Graphics Feature Summary". AMD. http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/hd-5970/Pages/ati-radeon-hd-5970-specifications.aspx. Retrieved 2010-03-31. 
11. ^ "AMD’s 10-bit Video Output Technology". AMD. http://ati.amd.com/products/pdf/10-Bit.pdf. Retrieved 2010-03-31. 
12. ^ Smith, Tony (20 March 2006). "ATI unwraps first 1GB graphics card". http://reghardware.co.uk/2006/03/20/ati_firegl_v7350/. Retrieved 2006-10-03. 
13. ^ "WinHEC 2008 GRA-583: Display Technologies". Microsoft. 2008-11-06. http://download.microsoft.com/download/5/E/6/5E66B27B-988B-4F50-AF3A-C2FF1E62180F/GRA-T583_WH08.pptx. Retrieved 2008-12-04. 
14. ^ "Windows 7 High Color Support". Softpedia. 2008-11-26. http://news.softpedia.com/news/Windows-7-High-Color-Support-98741.shtml. Retrieved 2008-12-05. 
15. ^ Hutchison, David C. (5 April 2006). "Wider color gamuts on DLP display systems through BrilliantColor technology". Digital TV DesignLine. http://www.digitaltvdesignline.com/showArticle.jhtml?printableArticle=true&articleId=184425677. Retrieved 2007-08-16. 

External links

• Preparing images for mobile devices: Reducing color depth related artifacts on mobile devices
• Banding in low colour resolution workflows: Comparison and recommendations