Otsu's method

An example image thresholded using Otsu's algorithm

Original image

In computer vision and image processing, Otsu's method is used to automatically perform clustering-based image thresholding, or, the reduction of a graylevel image to a binary image. The algorithm assumes that the image contains two classes of pixels following bi-modal histogram (foreground pixels and background pixels), it then calculates the optimum threshold separating the two classes so that their combined spread (intra-class variance) is minimal. The extension of the original method to multi-level thresholding is referred to as the Multi Otsu method. Otsu's method is named after Nobuyuki Otsu (大津展之, Ōtsu Nobuyuki).

Method

In Otsu's method we exhaustively search for the threshold that minimizes the intra-class variance (the variance within the class), defined as a weighted sum of variances of the two classes:

$\sigma^2_w(t)=\omega_1(t)\sigma^2_1(t)+\omega_2(t)\sigma^2_2(t)$

Weights $\omega_i$ are the probabilities of the two classes separated by a threshold $t$ and $\sigma^2_ i$ variances of these classes.

Otsu shows that minimizing the intra-class variance is the same as maximizing inter-class variance:

$\sigma^2_b(t)=\sigma^2-\sigma^2_w(t)=\omega_1(t)\omega_2(t)\left[\mu_1(t)-\mu_2(t)\right]^2$

which is expressed in terms of class probabilities $\omega_i$ and class means $\mu_i$.

The class probability $\omega_1(t)$ is computed from the histogram as $t$:

$\omega_1(t)=\Sigma_0^t p(i)$

while the class mean $\mu_1(t)$ is:

$\mu_1(t)=\left[\Sigma_0^t p(i)\,x(i)\right]/\omega_1$

where $x(i)$ is the value at the center of the $i$th histogram bin. Similarly, you can compute $\omega_2(t)$ and $\mu_2$ on the right-hand side of the histogram for bins greater than $t$.

The class probabilities and class means can be computed iteratively. This idea yields an effective algorithm.

Algorithm

1. Compute histogram and probabilities of each intensity level
2. Set up initial $\omega_i(0)$ and $\mu_i(0)$
3. Step through all possible thresholds $t = 1 \ldots$ maximum intensity
   1. Update $\omega_i$ and $\mu_i$
   2. Compute $\sigma^2_b(t)$
4. Desired threshold corresponds to the maximum $\sigma^2_b(t)$
5. You can compute two maxima (and two corresponding thresholds). $\sigma^2_{b1}(t)$ is the greater max and $\sigma^2_{b2}(t)$ is the greater or equal maximum
6. Desired threshold = $\frac{\text{threshold}_1 + \text{threshold}_2 }{2}$

JavaScript implementation

NB: The input argument total is the number of pixels in the given image. The input argument histogram is a 256-element histogram of a grayscale image different gray-levels (typical for 8-bit images). This function outputs the threshold for the image.

function otsu(histogram, total) {
    var sum = 0;
    for (var i = 1; i < 256; ++i)
        sum += i * histogram[i];
    var sumB = 0;
    var wB = 0;
    var wF = 0;
    var mB;
    var mF;
    var max = 0.0;
    var between = 0.0;
    var threshold1 = 0.0;
    var threshold2 = 0.0;
    for (var i = 0; i < 256; ++i) {
        wB += histogram[i];
        if (wB == 0)
            continue;
        wF = total - wB;
        if (wF == 0)
            break;
        sumB += i * histogram[i];
        mB = sumB / wB;
        mF = (sum - sumB) / wF;
        between = wB * wF * Math.pow(mB - mF, 2);
        if ( between >= max ) {
            threshold1 = i;
            if ( between > max ) {
                threshold2 = i;
            }
            max = between;            
        }
    }
    return ( threshold1 + threshold2 ) / 2.0;
}

External links

• Lecture notes on thresholding - covers the Otsu method.
• A plugin for ImageJ using Otsu's method to do the threshold.
• A full explanation of Otsu's method with a worked example and Java implementation.
• Implementation of Otsu's method in ITK
• Otsu Thresholding in C# A straightforward C# implementation with explanation.
• Otsu's method using MATLAB