Why does the Image Processing Toolbox (R2007a) ENTROPY command return a value of zero when the image is cast as a double?

Currently, the documentation for the ENTROPY function states that it should work with images of data type double. However, if an image 'I' is explicitly cast as a double by:

   I=double(I);

ENTROPY returns a value of zero due to scaling issues. The code I used is as follows:

   I = rgb2gray(ColorImage); % use any image

ent1 = entropy(I);

I = double(I);

ent2 = entropy(I);

Answer 1

[MathWorks Support Team]

Some of the Image Processing Toolbox functions have to assume a particular dynamic range. By convention, this range is [0,1] for data of type 'double', but [0,255] for data of type 'uint8'. Therefore, the IM2DOUBLE function should be used as it handles this scaling automatically. For example, your code thus becomes:

I = rgb2gray(ColorImage);           % use any true color image

ent1 = entropy(I);

I = im2double(I);

ent2 = entropy(I);

Both 'ent1' and 'ent2' should be equal.