MATLAB: Does the Image Processing Toolbox (R2007a) ENTROPY command return a value of zero when the image is cast as a double

doubleentropyim2doubleImage Acquisition ToolboxImage Processing Toolbox

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);

Best Answer

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.
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