It's magic. R2017a gives a different result from R2016a/b with the following simple code...

R2017a gives a different result from R2016a/b with the following code:

I = magic(16); H = magic(4); imfilter(I,H,'circular')

Has the implementation of imfilter been changed?

It is a huge problem for me.

[Shogo Muramatsu]

As well, R2016b gives the same answer to the following commands:

I = magic(16); H = magic(4); ansimf = imfilter(I,H,'conv','circular')
I = fft2(magic(16),16,16); H = fft2(magic(4),16,16); ansfft = circshift(ifft2(I.*H,16,16),-[2 2])

while R2017a gives different results.

[Shogo Muramatsu]

An official response was obtained by a bug report. This is a malfunction of the imfilter function of MATLAB R2017a. They told me that this bug will be improved in the next release version (probably MATLAB R2017b).

[Adam]

I'm not sure if a bug being improved is a good thing or a bad thing!

[Shogo Muramatsu]

This bug is a fatal problem especially for researchers and developers concerning signal processing, and should be fixed immediately.

For a double precision input image, the circular extension may fail when using an even size kernel. Please also see the answer made by myself.

Circular convolution is a special operator because

• It is diagonalized by the discrete Fourier transform(DFT), i.e., FFT.
• The adjoint operator is also circular convolution with flipped kernel, i.e., circular correlation.

This bug is an obstacle to using these properties on MATLAB.

Answer 1

[Walter Roberson]

Yes, the implementation did change; I do not see any release notes saying so, though.

I see in R2016b that non-separable filters have two branches: filterSingle2DWithConv() for single(), and filterPartOrWhole() for everything else. filterPartOrWhole() calls mex routines to do the work. You are using double(), so you would get the "everything else" mex routines.

I see in R2017a that non-separable filters have three branches: filterDouble2DWithConv() for double() for 2D arrays; filterDouble2DWithConv() (same routine) for double() with 3D arrays; and filterPartOrWhole() otherwise. (Notice filterSingle2DWithConv is gone and handled by filterPartOrWhole() now.) You are using double(), so you get the new filterDouble2DWithConv() routine. It calls upon conv2() to do the work.

I think you would be justified in filing a bug report on this.

[Shogo Muramatsu]

Thank you very much for your prompt and detailed comments.

I've checked the result from imfilter in R2017 by the counterpart frequency domain element-wise product result. They are conflict with each other.

I'll send them a bug report.

Thanks again.

[Image Analyst]

Which release do you think is the most "correct"?

[Shogo Muramatsu]

R2016a/b give correct results for the above commands.

[Shogo Muramatsu]

I've confirmed that the single precision routine gives the correct result by the following code:

% Input 2D array
I = magic(16);

% Filter kernel
H = magic(4);

% Circular convolution with double precision
ansimfd = imfilter(double(I),H,'conv','circular');

% Circular convolution with single precision
ansimfs = imfilter(single(I),H,'conv','circular');

% Frequency domain filtering
Iw = fft2(I,16,16);
Hw = fft2(H,16,16);
ansfft2 = circshift(ifft2(Iw.*Hw,16,16),-[2 2]);

% Evaluation of double precision filtering
norm(ansimfd-ansfft2)

% Evaluation of single precision filtering
norm(ansimfs-ansfft2)

Casting the input array to single precision would be a way to avoid the malfunction for the double precision data.

Answer 2

[Shogo Muramatsu]

From line 450 to line 451 in imfilter.m, we can see the following codes:

 a = padarray_algo(a, prePadSize, method, padVal, 'pre');
 a = padarray_algo(a, padSize, method, padVal, 'post');

These two lines seem to periodically expand the image boundary. In this second line, it seems that the periodicity is collapsing as a result of postpadding by duplicating the result of prepadding.

Answer 3

[Shogo Muramatsu]

Visit https://mathworks.com/support/bugreports/ and search #BugID: 1554862.

Answer 4

[Prerana Paravastu]

Which is the recent version?