I have tested Avinoam's latest patch (file #39195) and it looks all fine to me:

• it resolves all mentioned issues of this bug report
  • it solves the original issue (major/minoraxislength with multiple regions), see comment #0
  • it removes the division by zero in the calculation of the ellipse orientation property, see comment #1
• it gives Matlab compatible results on all mentioned examples in this bug report
• it adds new tests for those improvements
  • those new tests areMatlab compatible
  • the tests all pass

If noone else finds a problem wit this patch, then I would like to see this patch commited to the image repository.

Thanks for supplying this patch, Avinoam.

There is a small typo on your last patch, Avinoam. (It also makes one of your newly added tests fail): The number "90" from the last verbatim-box in comment #17 is mistakenly written as a "0".

I will try to run all your new tests in Matlab during the next days. If they all pass, I would be happy to see this patch applied (with the number 90 corrected).

Yes, I think the last "patch" in comment #17 would be fine.

Could you add some relevant tests from the discussed example images?

With Matlab I get for the test script from comment #15:

orientation_A = 0
orientation_B = 90

This is different compared to the results with the patch in comment #14.

@Avinoam: Could you change your if-statement to also make those results ML compatible?

Current implementation of orientation

    orientation(idx) = -(180/pi) .* atan (major_vec(2) ./ major_vec(1));

gives:

orientation =

  -35.96725
   82.06034
  -56.90598
  -86.07882
  -62.03271
   42.99091
   69.42293
   37.99984
   84.71827
  -90.00000
   31.92248
   -0.00000
  -31.02163
   14.03068
   34.96791
   64.73045
  -13.02547
   23.02190
   12.62266
  -77.01055
  -20.73767

Changing to:

    if (major_vec(1) == 0 || major_vec(2) == 0)
      orientation(idx) = 0;
    else
      orientation(idx) = -(180/pi) .* atan (major_vec(1) ./ major_vec(2));
    end

 
gives a Matlab compatible results. What do you think?

Thanks @Hartmut, and sorry, I did not write the correct lines:

for the commands

props1 = regionprops(L,'Orientation');
orientation = [props1.Orientation]'

I get with Matlab 2010a

orientation =

  -54.0328
    7.9397
  -33.0940
   -3.9212
  -27.9673
   47.0091
   20.5771
   52.0002
    5.2817
         0
   58.0775
         0
  -58.9784
   75.9693
   55.0321
   25.2696
  -76.9745
   66.9781
   77.3773
  -12.9895
  -69.2623

Do you get the same results?

Thanks.

To check Matlab compatability, can some check the result of the
script from comment #0 with the line

Orientation = [props.MinorAxisLength]'

Thanks.

I can confirm that Avinoam's first patch (file #39060) fixes the original issue. And all tests of regionprops.m now pass on linux and on Windows (w32 tested).

@Avinoam: Do you understand why Windows gives different results (some eps only) compared to Linux for those tests you have changed?

Sorry, the argumentation in my last post was wrong. A single pixel gets 1/12 in size, this leads to the guarantee that the majoraxislength and the minoraxislength will never become zero, I think. But this doesn't prove yet that the major_vec(1) will never be 0 in our code. Sorry.

The following image really gives me a value of major_vec(1) = 0, nevertheless the current regionprops.m DOES return the right value for the orientation:

BW = logical([0 0 0 0; 0 1 1 0; 0 0 0 0]);
L = bwlabel(BW);
props = regionprops(L,'Orientation');
props.Orientation

This is due to the result of "atan(1/0)" which does give a warning (no error!), but nevertheless returns the "correct" value of pi/2 (that results in the correct orientation value of -90 degrees).

I have also tried the simple change of atan to atan2, and I do get a failing test of the Orientation feature. So a quick substitute of atan into atan2 will not do.

To conclude: I have no objection about switching from atan to atan2, but the resulting orientation values SHOULD stay the same, please. (So this would mean some additional adjustments to this code line, and double checking that the orientation results stay the same.)

I will make two csets with tests.

Note that the tan/atan2 should be discussed, because it will
not be back compatible, since atan range is [-90:90] and atan2
range is [-180:180]

Adding package maintainer.

This problem happens with the current version 2.6.1 of the image package. It still worked fine with the older version of regionprops.m that is in the release image-2.4.1.

The property "majoraxislenght" and "minoraxislength" seem to be corrupted somewhere on their way through regionprops.m. If I observe this right, the results are fine when I only request those properties from a single region. But once I request those properties simultanously from several regions at once, I get corrupted results.

Here is a small script to reproduce the behavior. It uses the attached png-file "rechtecke.png":

clear, close all

I = imread('rechtecke.png');
schwelle = graythresh(I);
BW = im2bw(I, schwelle);
L = bwlabel(BW);
L_rgb = label2rgb(L, 'hsv');
imshow(L_rgb);
title('labelled regions');

props = regionprops(L,'MajorAxisLength','MinorAxisLength');
major=[props.MajorAxisLength]';
minor=[props.MinorAxisLength]';
figure, plot(major, minor, 'x');
xlabel('major axis length');
ylabel('minor axis length');

The second plot obviously does not show the correct major and minor axis length with image-2.6.1. If you use image-2.4.1 instead you can see the correct and expected result. I will also attach those two screenshots.