Why does ISWT2 (R2017a) fail for coefficients in single precision?

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Shogo Muramatsu 2017-5-14

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评论： Walter Roberson 2017-5-17

ISWT2 (R2017a) seems not to work properly for coefficients in single precision. The code below fails only for the single precision case.

nLevels = 1;
%%For image in double
X = im2double(imread('cameraman.tif'));
C = swt2(X,nLevels,'haar');
Y = iswt2(C,'haar');
psnr(Y,X)
%%For image in single
X = im2single(imread('cameraman.tif'));
C = swt2(X,nLevels,'haar');
Y = iswt2(C,'haar'); % <- error
psnr(Y,X)

Why does this phenomenon happen?

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Answer 1

Walter Roberson 2017-5-16

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在 MATLAB Online 中打开

It is a bug in the code. If you look near line 110 of iswt2 you will see

    % Ambiguity:
    % (level=3 and indexed BW image) or (level=1 and truecolor image)
    % To suppress this Ambiguity, the function SWT2 in case of a true 
    % color image and a level 1 analysis, produce single for approximation
    % coefficients !!

So the swt2 code is encoding information about BW vs truecolor in the datatype (!) and then iswt2 flubs the reconstruction.

The work-around is to use

[A,H,V,D] = swt2(X,nLevels,'haar');
Y = iswt2(A, H, V, D);

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Shogo Muramatsu 2017-5-16

I understood the cause clearly. Thank you very much!

Walter Roberson 2017-5-17

The problem occurs for any grayscale data that is not of type double

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Answer 2

Ankit Bhatnagar 2017-5-16

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Hi,

The ISWT2 works only with double precision as it uses functionality which expects the value to be double. Kindly use the double precision for this function.