For any Processing PImage, compute the histogram, the normalized histogram, and the overall Shannon entropy.

CVExtras.pde

class CVExtras {
  PImage pImage;
  int[] hist = new int[256];        // Raw Pixel Counts  
  double[] histN = new double[256]; // Pixel Percentages (Normalized)
  double entropy; 

  CVExtras(PImage img) {
    for (int i = 0; i < 256; i++) {
      this.hist[i] = 0;
      this.histN[i] = 0.0;
    }
    computeHistogram(img);
    computeEntropy(this.histN);
  }  

  void computeHistogram(PImage img) {
    int sum = 0;
    for (int i = 0; i < img.width; i++) {
      for (int j = 0; j < img.height; j++) {
        int bright = int(brightness(img.get(i, j)));
        this.hist[bright]++; 
        sum += bright;
      }
    }
    for (int i = 0; i < this.hist.length; i++) {
      this.histN[i] = ((double) this.hist[i]) / sum;
    }
  }

  void computeEntropy(double[] histN) {
    double e = 0;
    for (int i = 0; i < histN.length; i++) {
      double frequency = histN[i];
      if (frequency != 0) {
        e -= frequency * (Math.log(frequency) / Math.log(2));
      }
    }   
    this.entropy = Double.isNaN(e) ? 0.0 : e;
  }
}