Possible wrong Otsu (auto) threshold value for binarisation

Submitted by Vincent

Description

Created attachment 218327 Picture-test to compute Otsu Thresholding

Hello,

Gimp use the Otsu method to compute binarization threshold.

If you compute the auto-threshold for the attached picture with Gimp 2.8 (menu "Colors / Threshold - Auto" or "Couleurs / Seuil - Auto in french), Gimp find 185.

If you compute the auto-threshold for the attached picture with ImageJ (menu "Image / Adjust / Threshold - Otsu"), ImageJ find 195.

I think these two threshold (195 and 185) can be improved.

Gimp return the threshold "s1" for maximum (GREATER) histogram standard variation.
ImageJ return the threshold "s2" for maximum (GREATER OR EQUAL) histogram standard variation.

I think you must compute s1 and s2 and return s = ( s1 + s2 ) / 2

Otsu threshold must be ( 185 + 195 ) / 2 = 190 for the attached picture. Look at the histogram, it is the middle...

(In most cases, s1 = s2 = s...)

Best regards,

Vincent

Attachment 218327, "Picture-test to compute Otsu Thresholding":
Imagette_test_Otsu_8bits.tif

Version: 2.8.8