Getting more colours with Bit Depth

Another area where there is a lot of misconception about colour is the topic of colour bit depth. Really there should be, because it is a simple the numbers of binary numbers (0 or 1) have to describe each primary colour (red, green, blue) in the pixel (colour channel) the more unique colours will be available (and the closer the steps between colours will be. It does not necessarily mean that a wider range of colours will be possible (ie colour gamut). This effect is most easily seen in the image histograms.

As an example and 8-bit coding system, of SRGB, gives each pixel up the 8 bits  or 2⁸ = 256 combination. By convention zero (0) is no colour (black for that channel) and 255 is the maximum intensity of colour in that channel. When all the three primary colours are combined there are 2^8*3  = 16,777,216 different colours definable for any given pixel. This is often described as “true colour”. This is often called the “bits per pixel” (bpp)to describe the sum of all three colour channels and that represents a pixel.

An interesting fact is most human eyes can only perceive around 10 million discreet colours, so displaying any image in more than 24 bpp will go unnoticed.

Most modern camera, will capture images in 8 bit SRGB (24 bpp) and standard .jpeg has this bit depth. Some higher end cameras now offer other colour spaces (Adobe RGB or ProPhoto) and greater bit depth. Remember you probably will not be able to see the difference in terms of enriched colour or image quality. The extra bit depth however can be very handy for post processing and particularly when “stretching” the tonal range of an underexposed RAW file (which can often lead to colour banding in the shadows). Much photo editing and computer graphics software can handle 16bit colour and  .tiff formats can be saved up to this bit depth.

The Cambridge in Colour site has a simple tutorial of Bit Depth, including a great visualization of the effect of bit depth. The African Shutha project has a wonder summary of the topic my Graeme Cookson.