Digital storage and transmission promise noise-free images, but it is important to keep in mind that even digital is not perfect. Digital images have their own sources of noise: round-off error and quantization error. Whenever you do any sort of image arithmetic, such as contrast enhancement or compositing, you get roundoff error. In fact, since the arithmetic is often done in only X-bit accuracy, sometimes the round-off error can be substantial. You get quantization error, on the other hand, whenever you go from an analog signal to a digital signal or whenever you go from a high color-resolution signal (for example, 24 bits per pixel) to a low resolution signal (for example, 8 bits per pixel). The author considers the quantization error from analog to digital.
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