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

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Shogo Muramatsu
Shogo Muramatsu 2017-5-14
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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采纳的回答

Walter Roberson
Walter Roberson 2017-5-16
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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更多回答（1 个）

Ankit Bhatnagar
Ankit Bhatnagar 2017-5-16
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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