Background Subtraction by Golan Levin.

Detect the presence of people and objects in the frame using a simple background-subtraction technique. To initialize the background, press a key.

import processing.video.*;

int numPixels;
int[] backgroundPixels;
Capture video;

void setup() {
  // Change size to 320 x 240 if too slow at 640 x 480
  size(640, 480); 
  
  video = new Capture(this, width, height, 24);
  numPixels = video.width * video.height;
  // Create array to store the background image
  backgroundPixels = new int[numPixels];
  // Make the pixels[] array available for direct manipulation
  loadPixels();
}

void draw() {
  if (video.available()) {
    video.read(); // Read a new video frame
    video.loadPixels(); // Make the pixels of video available
    // Difference between the current frame and the stored background
    int presenceSum = 0;
    for (int i = 0; i < numPixels; i++) { // For each pixel in the video frame...
      // Fetch the current color in that location, and also the color
      // of the background in that spot
      color currColor = video.pixels[i];
      color bkgdColor = backgroundPixels[i];
      // Extract the red, green, and blue components of the current pixelÕs color
      int currR = (currColor >> 16) & 0xFF;
      int currG = (currColor >> 8) & 0xFF;
      int currB = currColor & 0xFF;
      // Extract the red, green, and blue components of the background pixelÕs color
      int bkgdR = (bkgdColor >> 16) & 0xFF;
      int bkgdG = (bkgdColor >> 8) & 0xFF;
      int bkgdB = bkgdColor & 0xFF;
      // Compute the difference of the red, green, and blue values
      int diffR = abs(currR - bkgdR);
      int diffG = abs(currG - bkgdG);
      int diffB = abs(currB - bkgdB);
      // Add these differences to the running tally
      presenceSum += diffR + diffG + diffB;
      // Render the difference image to the screen
      pixels[i] = color(diffR, diffG, diffB);
      // The following line does the same thing much faster, but is more technical
      //pixels[i] = 0xFF000000 | (diffR << 16) | (diffG << 8) | diffB;
    }
    updatePixels(); // Notify that the pixels[] array has changed
    println(presenceSum); // Print out the total amount of movement
  }
}

// When a key is pressed, capture the background image into the backgroundPixels
// buffer, by copying each of the current frameÕs pixels into it.
void keyPressed() {
  video.loadPixels();
  arraycopy(video.pixels, backgroundPixels);
}