Pixel color depth or bits per pixel in digital photos
Digital photos are built of many pixels. Each pixel has a unique value which represents its color. When you are looking at a digital photo your eyes and brain merge these pixels into one continuous digital photo. Each pixel has a color value that is one out of a finite number of possible colors – this number is known as color depth.
Each pixel has a color value that is one out of a palette of unique colors. The number of such unique potential colors is known as color depth. Color depth is also known as bit depth or bits per pixel since a certain number of bits are used to represent a color and there is a direct correlation between the number of such bits and the number of possible unique colors. For example if a pixel color is represented by one bit - one bit per pixel or a bit depth of 1 - the pixel can have only two unique values or two unique colors – usually these colors will be black or white.
Color depth is important in two domains: the graphical input or source and the output device on which this source is displayed. Each digital photo source or other graphics sources are displayed on output devices such as computer screens and printed paper. Each source has a color depth. For example a digital photo can have a color depth of 16 bits. The source color depth depends on how it was created for example the color depth of the camera sensor used to shoot a digital photo. This color depth is independent of the output device used to display the digital photo. Each output device has a maximum color depth that it supports and can also be set to lower color depth (usually to save resources such as memory). If an output device has a higher color depth than the source the output device will not be fully utilized. If an output device has a lower color depth than the source the output device will display a lower quality version of the source.
Many times you will hear color depth expressed as a number of bits (bit depth or bits per pixel). Here is a table of common bits per pixel values and the number of colors they represent:
1 bit: only two colors are supported. Usually these are black and white but it can be any pair of colors. It is used for black and white sources and in rare cases of black and white screens.
2 bits: 4 colors are supported. Hardly used.
4 bits: 16 colors are supported. Hardly used.
8 bits: 256 colors are supported. Used for graphics and simple icons. Digital photos displayed using 256 colors are of poor quality.
12 bits: 4096 colors are supported. It is hardly used with computer screen but sometimes this color depth is used by mobile devices such as PDAs and phones. The reason is that 12 bits color depth is the limit for high quality digital photos display. Less than 12 bits screens distort the digital photo colors too much. The lower the color depth the less memory and resources are needed and such devices are resources limited.
16 bits: 65536 colors are supported. Provides high quality digital color photos display. This color depth is used by many computer screens and portable devices. 16 bits color depth is sufficient to display digital photo colors that are very close to real life.
24 bits: 16777216 (approximately 16 million) colors are supported. This is also known as “true color”. The reason for that nick name is that 24 bits color depth is considered more than the number of unique colors our eyes and brain can see. So using 24 bits color depth provides the ability to display digital photos in true real life colors.
32 bits: in contrast to what some people believe 32 bits color depth does not support 4294967296 (approximately 4 billion) colors. In fact 32 bits color depth supports 16777216 colors which is the same number as 24 bits color depth. The reason for 32 bit color depth existence is mainly for speed performance optimization. Since most computers use buses in multiplications of 32 bits they are more efficient using 32 bits chunks of data. 24 bits out of the 32 are used to describe the pixel color. The extra 8 bits are either left blank or are used for some other purpose such as indicating transparency or some other effect.
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