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8-Bit and 16-Bit Indexed Images

If the class of X is uint8 or uint16, its values are offset by 1 before being used as colormap indices. The value 0 points to the first row of the colormap, the value 1 points to the second row, and so on. The image command automatically supplies the proper offset, so the display method is the same whether X is double, uint8, or uint16.

The colormap index offset for uint8 and uint16 data is intended to support standard graphics file formats, which typically store image data in indexed form with a 256-entry colormap. The offset allows you to manipulate and display images of this form in MATLAB using the more memory-efficient uint8 and uint16 arrays.

Because of the offset, you must add 1 to convert a uint8 or uint16 indexed image to double. For example,

Conversely, subtract 1 to convert a double indexed image to uint8 or uint16.


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