Why does ISWT2 (R2017a) fail for coefficients in single precision?
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Shogo Muramatsu
le 14 Mai 2017
Commenté : Walter Roberson
le 17 Mai 2017
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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Walter Roberson
le 16 Mai 2017
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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Ankit Bhatnagar
le 16 Mai 2017
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.
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