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Having thrashed the topic of masking within an inch of its life, it is time to return to the comfortable, less thoroughly thrashed realm of channels, but is it so very comfortable? So far we have we have been looking at images that contain 8 bits of data per pixel per channel. A single bit can convey one of two states, ON or OFF. So 8 bits permits up to 28 variations which adds up to 256 luminance levels on a channel-by-channel basis. Factoring the three channels required to produce an RGB image and you have 256^3 or 16.8 million possible colors. The name given to the number of bits assigned to a pixel is bit depth, so in addition to the height and width of an image, Photoshop sees a third dimension of bit depth associated with each and every pixel as I have illustrated here.
Given that no single computer screen can display 16.8 million pixels at a time, you might reasonably assume that 16.8 million unique colors are more than enough and quite frankly, it usually is. Every image we have been working with so far has been an 8 bit per channel image, but such images have a potential Achilles' Heel; they can fall apart under heavy editing. When you apply a color adjustment, for example, two pixels that were different colors maybe merged into one color which can result in clipping and banding.
Meanwhile, most reasonably capable scanners and digital cameras are capable of capturing higher bit depth images. A mid-range camera that captures say, 10 bits of data per channel, translates to 230 or 1.1 billion colors. Should you really be throwing all those extra colors away? That's why Photoshop includes a 16 bit per channel space which accommodates upward of a trillion color variations. I don't know of too many images that take full advantage of that space, but it gives you more than enough room to edit as you will see.
Then there is High Dynamic Range or HDR. Strictly speaking, HDR accommodates 32 bits of data per channel which means several octillion colors which are more possible colors than there are stars in the entire universe; but really, it means floating-point calculations which permits luminance values to move beyond white or black without clipping. It is pretty fantastic stuff as you are about to see.
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