Fred's ImageMagick Scripts



    Licensing:

    Copyright © Fred Weinhaus

    My scripts are available free of charge for non-commercial use, ONLY.

    For use of my scripts in commercial (for-profit) environments or non-free applications, please contact me (Fred Weinhaus) for licensing arrangements. My email address is fmw at alink dot net.

    If you: 1) redistribute, 2) incorporate any of these scripts into other free applications or 3) reprogram them in another scripting language, then you must contact me for permission, especially if the result might be used in a commercial or for-profit environment.

    Usage, whether stated or not in the script, is restricted to the above licensing arrangements. It is also subject, in a subordinate manner, to the ImageMagick license, which can be found at: http://www.imagemagick.org/script/license.php

SAHOOTHRESH


Automatically thresholds an image to binary (b/w) format using Sahoo's entropy technique.

Download Script

last modified: September 26, 2017



USAGE: sahoothresh [-p power] [-g graph] infile outfile
USAGE: sahoothresh [-help]

-p ..... power .......... power or exponent to use with method=4
......................... float; power>0; default=2
-g ..... graph .......... graph specifies whether to generate a
......................... histogram graph image displaying the
......................... location and value of the threshold;
......................... choices are: view or save;
......................... default is no graph

PURPOSE: To automatically thresholds an image to binary (b/w) format using Sahoo's entropy technique.

DESCRIPTION: SAHOOTHRESH automatically thresholds an image to binary (b/w) format. It assume the histogram is bimodal, i.e. is the composite of two bell-shaped distributions representing the foreground and background classes. The Sahoo appoach computes computes one measure of Entropy for each of the foreground (above threshold data) and background (at and below threshold value) classes. The optimal threshold is the one that maximizes the Sum of the Foreground and Background Entropies.

ARGUMENTS:

-p power ... POWER is the exponent used in method 4. The value for power may be a float greater than zero. The default is 2.

-g graph ... GRAPH specifies whether to generate a graph (image) of the histogram, displaying the location and value of the threshold. The choices are: view, save and none. If graph=view is selected, the graph will be created and displayed automatically, but not saved. If graph=save is selected, then the graph will be created and saved to a file using the infile name, with "_histog_sahoo.gif" appended, but the graph will not be displayed automatically. If -g option is not specified, then no graph will be created.

NOTE: It is highly recommended that the output not be specified as a JPG image as that will cause compression and potentially a non-binary (i.e. a graylevel) result. GIF is the recommended output format.

REFERENCES: see the following:
http://climate.gsfc.nasa.gov/publications/fulltext/RSEpaper.pdf
http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf

CAVEAT: No guarantee that this script will work on all platforms, nor that trapping of inconsistent parameters is complete and foolproof. Use At Your Own Risk.


EXAMPLES


Sahoo Thresholding Of Various Images At Default Power = 2 (default)

Pictures Were Obtained from:
blood.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
fingerprint.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
flower.jpg from http://www.jhlabs.com/ip/blurring.html
house.jpg from http://en.wikipedia.org/wiki/Otsu's_method
kanji.jpg from http://www.measurement.sk/2004/S1/Yong.pdf
parts.gif from http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fip-Segmenta.html
rice.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
tank.jpg from http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=ADA464347&Location=U2&doc=GetTRDoc.pdf
textsample.jpg from http://signal.ece.utexas.edu/seminars/dsp_seminars/01fall/211_seeger_mf.pdf
lena2g_edge1.jpg was created using the IM function -edge 1

Original Image

Thresholded Image

Histogram



Sahoo Thresholding Of Various Images At Default Power = 0.1

Pictures Were Obtained from:
blood.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
fingerprint.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
flower.jpg from http://www.jhlabs.com/ip/blurring.html
house.jpg from http://en.wikipedia.org/wiki/Otsu's_method
kanji.jpg from http://www.measurement.sk/2004/S1/Yong.pdf
parts.gif from http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fip-Segmenta.html
rice.jpg from http://www.istanbul.edu.tr/eng/ee/jeee/main/pages/issues/is62/62008.pdf
tank.jpg from http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=ADA464347&Location=U2&doc=GetTRDoc.pdf
textsample.jpg from http://signal.ece.utexas.edu/seminars/dsp_seminars/01fall/211_seeger_mf.pdf
lena2g_edge1.jpg was created using the IM function -edge 1

Original Image

Thresholded Image

Histogram



See A Comparison Of Each Image Against Each Thresholding Technique



What the script does is as follows:

  • Converts the image to grayscale
  • Generatess the histogram
  • Compute the normalized histogram counts by dividing each bin count
    by the total pixels in the image
  • Generates cumulative arrays for each bin from the normalized count
    and from the product of normalized counts times bin graylevel
    starting from each end of the histogram. These are called histogram
    moments zero and one
  • Computes the cumulative histogram entropy arrays from these moment arrays
    using the specified power
  • For each possible bin, finds the total entropy from the
    two cumulative entropy arrays and chooses the threshold with the
    largest total entropy
  • Thresholds the image at the graylevel corresponding to the selected bin