====== FAQ: What is the algorithm used in automatic thresholding? ======

**What is the algorithm used by ImageJ to find the ideal threshold value?**

The automatic thresholding function used by **Image>Adjust>Threshold** and **Process>Binary>Threshold** is an iterative procedure based on the //isodata// algorithm [T.W. Ridler, S. Calvard, Picture thresholding using an iterative selection method, IEEE Trans. System, Man and Cybernetics, SMC-8 (1978) 630-632.]

The online [[http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fip.html|Image Processing Fundamentals]] tutorial [[http://www.ph.tn.tudelft.nl/Courses/FIP/noframes/fip-Segmenta.html#Heading118|discusses]] the isodata algorithm among several others.\\  Briefly, the procedure divides the image into //objects// and //background// by taking an initial threshold, then the averages of the pixels **at or below the threshold** and pixels **above** are computed. The averages of those two values are computed, the threshold is incremented and the process is repeated until the threshold is larger than the composite average. That is,

  threshold = (average background + average objects)/2

The code in ImageJ that implements this function is the **getAutoThreshold()** method in the **ImageProcessor** class.

Note that there are many more methods for setting an automatic threshold (such as Otsu's method, entropy method, triangle method, k-means clustering, etc).