Blank/Grey screen (frame differencing)

My code seems like a little bit buggy. Sometimes, i need to refresh few times to get the ideal results. This is very annoying and you need to keep on restart the processing and execute the code again. And there is no message in console. 3 problems happen in my code:

a) Grey screen (can hear video sound)
b) Normal video with sound (No frame differencing function)
c) Frame differencing + video (ideal)

import processing.video.*;

int numPixels;
int[] previousFrame;
Movie video;

void setup() {
  size(1280, 770);

  video = new Movie(this, "human.mp4");

  // Start capturing the images from the camera
   video.loop();
   video.read();
  numPixels = video.width * video.height;
  // Create an array to store the previously captured frame
  previousFrame = new int[numPixels];
  loadPixels();

}

void draw() {

  if (video.available()) {
 image(video, 0, 0);
    // When using video to manipulate the screen, use video.available() and
    // video.read() inside the draw() method so that it's safe to draw to the screen
    video.read(); // Read the new frame from the camera
    video.loadPixels(); // Make its pixels[] array available

    int movementSum = 0; // Amount of movement in the frame
    for (int i = 0; i < numPixels; i++) { // For each pixel in the video frame...
      color currColor = video.pixels[i];
      color prevColor = previousFrame[i];
      // Extract the red, green, and blue components from current pixel
      int currR = (currColor >> 16) & 0xFF; // Like red(), but faster
      int currG = (currColor >> 8) & 0xFF;
      int currB = currColor & 0xFF;
      // Extract red, green, and blue components from previous pixel
      int prevR = (prevColor >> 16) & 0xFF;
      int prevG = (prevColor >> 8) & 0xFF;
      int prevB = prevColor & 0xFF;
      // Compute the difference of the red, green, and blue values
      int diffR = abs(currR - prevR);
      int diffG = abs(currG - prevG);
      int diffB = abs(currB - prevB);
      // Add these differences to the running tally
      movementSum += diffR + diffG + diffB;
      // Render the difference image to the screen
      pixels[i] = color(diffR, diffG, diffB);
      // The following line is much faster, but more confusing to read
      //pixels[i] = 0xff000000 | (diffR << 16) | (diffG << 8) | diffB;
      // Save the current color into the 'previous' buffer
      previousFrame[i] = currColor;
    }
    // To prevent flicker from frames that are all black (no movement),
    // only update the screen if the image has changed.
    if (movementSum > 0) {
      updatePixels();
      println(movementSum); // Print the total amount of movement to the console
    }
  }
}