Understanding how 3 x 8 = 24 bits of binary data may be used to define a single pixel is fundamentally important. However, technology moves on at a rapid pace and image capture and manipulation is already being handled with even greater precision. Many devices now work with 10, 12 , 14 or even 16 bits of data per pixel, giving rise to RGB images based upon a theoretical 30, 36, 42 or 48-bit colour. Fortunately confusion is reduced by regarding data using any number of bits greater than 8 as 16-bit data. The missing bits are just filled in as zeros, leaving standard 16-bit data with a somewhat lower precision determined by the originating device. Many image manipulation processes within the latest versions of Photoshop uses 16-bit data. This results in greater arithmetic precision and hence colour gradation, even though the source may be accurate to only 10, 12 or 14 bits.
Although this approach is not as good as using true 16-bit data an improvement arises because, for example, processing 12-bit binary values may lead to a result that does not exactly equal one of numbers on the 12-bit scale. It may fall between two adjacent values and hence be compromised when rounded to the nearest available level. This means that two values which should have been slightly different are forced to be the same, potentially damaging colour gradation. Other result values that were only slightly further apart may be rounded to different levels, further exaggerating the effect. The damage can be seen in histograms where, after processing, some levels or colour value become completely empty. These are revealed by the appearance of a comb-like structure in a previously solid mass of data. The gaps represent small jumps in colour gradation in an image. The same effect is present when processing 16-bit data, but with 216 = 65,536 levels available the damage is not apparent to the human eye.
Where 16-bit data is used, RGB images have 3 x 16 = 48 bits per pixel and CMYK images have 4 x 16 = 64 bits per pixel, so things start to get confusing.. Images constructed in this manner are therefore probably better described as 16-bits per channel, that is16 bits for each of the red, green and blue channel pixels in an RGB image, and 16 bits for each of the cyan, magenta, yellow and black channel pixels in a CMYK image.
It is essential to understand that, in broad terms, layers are the same as channels. RGB colour images essentially consist of three layers (normally known as channels) - one red, one green and one blue. These are laid on top of each other like coloured semi-transparent acetate sheets, and their combined effect is a natural coloured image such as the eye normally sees. However, it is also important to realise that layers of various descriptions can be combined, or flattened, in countless different ways. These combination techniques, or blending modes, determine to a large extent the appearance of the final image.