Image Entropy
Description
Image entropy is a quantity which is used to describe the `business' of an
image, i.e. the amount of information which must be coded for by a compression
algorithm. Low entropy images, such as those containing a lot of black sky,
have very little contrast and large runs of pixels with the same or similar
DN values. An image that is perfectly flat will have an entropy of
zero. Consequently, they can be compressed to a relatively small size.
On the other hand, high entropy images such as an image of heavily cratered
areas on the moon have a great deal of contrast from one pixel to the next
and consequently cannot be compressed as much as low entropy images.
Calculating Image Entropy
Image entropy as used in my compression tests is calculated with the same
formula used by the Galileo Imaging Team:
In the above expression, P i is the probability that the
difference between 2 adjacent pixels is equal to i, and Log 2
is the base 2 logarithm. Here
is an IDL program I wrote to calculate the entropy of an image using this
expression.
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