What is a digital image

A digital image is fundamentally an array of numbers that represent levels of brightness (Figure 13.1).

13.1.1 Bit depth

Depending on the bit depth of the image, the numbers may range from 0 to 255 (8 bits), 0 to 65 535 (16 bits), or some other range.

The eye cannot distinguish even 256 levels, so 8-bit graphics are sufficient for finished pictures. The reason for wanting more levels during manipulation is that we may not be using the full range at all stages of processing. For instance, a badly underexposed 16-bit image might use only levels 0 to 1000, which are still enough distinct levels to provide smooth tones. An 8-bit image underexposed to the same degree would be unusable.

For greatest versatility, some software supports floating-point data, so that levels can be scaled with no loss of precision; in a floating-point system, you can divide 65 535 by 100 and get 655.35. You can also use large numbers without going out of range; if you do something that produces a value greater than 65 535, it will not be clipped to maximum white.

Note that Photoshop always reports brightness levels on a scale of 0 to 255, regardless of the actual bit depth of the image. This is to help artists match colors.

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Figure 13.1. A digital image is an array of numbers that represent levels of brightness. (From

Astrophotography for the Amateur.)

13.1.2 Color encoding

A color image normally has three numbers for each pixel, giving the brightness in red, green, and blue (RGB color). The alternative is CMYK color, which describes color in terms of cyan, magenta, yellow, and black printing inks; the two are interconvertible. Further alternatives are the Lab and L*a*b* systems, which use three coordinates based on the two-dimensional CIE chromaticity chart plus luminosity.

Astronomical images are sometimes constructed by combining the luminosity (L) from one image with the color (RGB) from another; this technique is called LRGB. It is not normally done with DSLRs, whose output is already in full color.

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