Image Thresholding

Abstract

The subject of this chapter is image thresholding in which we transform an input image, A, into a binary image B, where the pixel gray-levels in B are restricted to {0,1}. If a _m is the gray level of the mth pixel in A, then the corresponding value in B is

$$ b_{m}=\left\{ \begin{array}{ll} 1 & \mbox{if $a_{m} \geq t_{m}$} \;, \\ 0 & \mbox{otherwise} \;, \end{array} \right. $$

where t _m is the threshold value for the mth pixel. The thresholds t _m , m ∈ {1,2,...,M}, may all be equal to a global threshold t _G or they may vary locally (i. e. from pixel to pixel). In this chapter we shall concentrate on unsupervised thresholding methods. These are thresholding algorithms in which we only use information contained in the current input image to calculate t _m and t _G .