Bit depth explained.. | Tutorial Freak – Online Tutorials

Bit depth explained..



Bit depth quantifies how many unique colours are available in an image’s colour palette in terms of the number of 0′s and 1′s, or “bits,” which are used to specify each colour. This does not mean that the image necessarily uses all of these colours, but that it can instead specify colours with that level of precision. For a grayscale image, the bit depth quantifies how many unique shades are available. Images with higher bit depths can encode more shades or colours since there are more combinations of 0′s and 1′s available.


Every colour pixel in a digital image is created through some combination of the three primary colours: red, green, and blue. Each primary colour is often referred to as a “colour channel” and can have any range of intensity values specified by its bit depth. The bit depth for each primary colour is termed the “bits per channel.” The “bits per pixel” (bpp) refers to the sum of the bits in all three colour channels and represents the total colors available at each pixel. Confusion arises frequently with colour images because it may be unclear whether a posted number refers to the bits per pixel or bits per channel. Using “bpp” as a suffix helps distinguish these two terms.


Most colour images from digital cameras have 8-bits per channel and so they can use a total of eight 0′s and 1′s. This allows for 28 or 256 different combinations—translating into 256 different intensity values for each primary color. When all three primary colours are combined at each pixel, this allows for as many as 16,777,216 different colours, or “true colour.” This is referred to as 24 bits per pixel since each pixel is composed of three 8-bit colour channels. The number of colours available for any X-bit image is just 2X if X refers to the bits per pixel and 23X if X refers to the bits per channel.


The human eye can only discern about 10 million different colours, so saving an image in any more than 24 bpp is excessive if the only intended purpose is for viewing. On the other hand, images with more than 24 bpp are still quite useful since they hold up better under post-processing .
Colour gradations in images with less than 8-bits per colour channel can be clearly seen in the image histogram.
The available bit depth settings depend on the file type. Standard JPEG and TIFF files can only use 8-bits and 16-bits per channel, respectively.


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