my sketch wont work with an external webcam i get nullpointerexception error

edited April 2014 in Library Questions

My sketch tracks the amount of motion from the webcam to then outline that movement the sketch works with my isight camera but when i try to change to an external webcam i get nullpointerexception error this is my code here

// Variable for capture device
Capture video;
// Previous Frame
PImage prevFrame;
// How different must a pixel be to be a "motion" pixel
float threshold = 50;

void setup() {

    String[] devices = Capture.list();

  video = new Capture(this,width,height, devices[1],30);
  // Create an empty image the same size as the video
  prevFrame = createImage(video.width,video.height,RGB);

void draw() {

  // Capture video
  if (video.available()) {
    // Save previous frame for motion detection!!
    prevFrame.copy(video,0,0,video.width,video.height,0,0,video.width,video.height); // Before we read the new frame, we always save the previous frame for comparison!


  // Begin loop to walk through every pixel
  for (int x = 0; x < video.width; x ++ ) {
    for (int y = 0; y < video.height; y ++ ) {

      int loc = x + y*video.width;            // Step 1, what is the 1D pixel location
      color current = video.pixels[loc];      // Step 2, what is the current color
      color previous = prevFrame.pixels[loc]; // Step 3, what is the previous color

      // Step 4, compare colors (previous vs. current)
      float r1 = red(current); float g1 = green(current); float b1 = blue(current);
      float r2 = red(previous); float g2 = green(previous); float b2 = blue(previous);
      float diff = dist(r1,g1,b1,r2,g2,b2);

      // Step 5, How different are the colors?
      // If the color at that pixel has changed, then there is motion at that pixel.
      if (diff > threshold) { 
        // If motion, display black
        pixels[loc] = color(0);
      } else {
        // If not, display white
        pixels[loc] = color(255);


  • Answer ✓

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