Hu moments comparison

Question

I tried to compare two images and use the Hu moment to compare the outline extracted from these images: https://docs.google.com/file/d/0ByS6Z5WRz-h2WHEzNnJucDlRR2s/edit and https://docs.google.com/file / d / 0ByS6Z5WRz-h2VnZyVWRRWEFva0k / edit The second image is equal to the first, it just rotated, and I expected the result to be the same as that of Humoments. They are a little different.

The sign of humutes on the right (first image):

[[ 6.82589151e-01] [ 2.06816713e-01] [ 1.09088295e-01] [ 5.30020870e-03] [ -5.85888607e-05] [ -6.85171823e-04] [ -1.13181280e-04]] 

The sign of humutes on the right (second image):

 [[ 6.71793060e-01] [ 1.97521128e-01] [ 9.15619847e-02] [ 9.60179567e-03] [ -2.44655863e-04] [ -2.68791106e-03] [ -1.45592441e-04]] 

In this video: http://www.youtube.com/watch?v=O-hCEXi3ymU in the fourth round I watched how he got exactly the same. Where am I wrong?

Here is my code:

 nomeimg = "Sassatelli 1984 ruotato.jpg" #nomeimg = "Sassatelli 1984 n. 165 mod1.jpg" img = cv2.imread(nomeimg) gray = cv2.imread(nomeimg,0) ret,thresh = cv2.threshold(gray,127,255,cv2.THRESH_BINARY_INV) element = cv2.getStructuringElement(cv2.MORPH_CROSS,(4,4)) imgbnbin = thresh imgbnbin = cv2.dilate(imgbnbin, element) #find contour contours,hierarchy=cv2.findContours(imgbnbin,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE) #Elimination small contours Areacontours = list() area = cv2.contourArea(contours[i]) if (area > 90 ): Areacontours.append(contours[i]) contours = Areacontours print('found objects') print(len(contours)) #contorus[3] for sing in first image #contours[0] for sign in second image print("humoments") mom = cv2.moments(contours[0]) Humoments = cv2.HuMoments(mom) print(Humoments) 

Answer 1

I think your numbers are probably good, the differences between them are moderately small. As the guy says in the video you are linking to (about 3 minutes):

To get some meaningful answers, we take the log transformation

therefore, if we do -np.sign(a)*np.log10(np.abs(a)) on the data that you publish above, we get:

First image:

 [[ 0.16584062] [ 0.68441437] [ 0.96222185] [ 2.27570703] [-4.23218495] [-3.16420051] [-3.9462254 ]] 

Second image:

 [[ 0.17276449] [ 0.70438644] [ 1.0382848 ] [ 2.01764754] [-3.61144437] [-2.57058511] [-3.83686117]] 

That they are not identical is to be expected. You start with rasterized images, which you then process quite a bit to get some of the outlines you go through.

From opencv docs :

In the case of bitmap images, the calculated Hu invariants for the original and converted images are slightly different.