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The fact that the data used in remote sensing are digital makes
them most suitable for operations between channels. The principle
of such operations is to perform more or less complex mathematical
operations on each pixel that call upon the numerical values seen
for the pixel in the various spectral bands.
For example, we can calculate the sum of the spectral values of
a three-component image by doing the computation for each pixel,
then storing the results in a digital image having the same number
of pixels as the original image. In some cases, the result of these
operations may be negative or exceed 255, which is the highest value
that an image-processing system can handle. To get around these
problems we can use multiplicative coefficients and/or add a constant.
For example, if components A and B each vary between 0 and 255,
then C = (A-B) x 0.5 + 127 will definitely be a compromise between
0 and 255.