Please use the search form before asking questions! You would have found this by yourself :

http://processing.org/learning/libraries/framedifferencing.html

Have a look at the reference : there's a built-in library to load Quicktime (.mov) videos.

http://processing.org/reference/libraries/video/index.html

I guess you can use the same method as in the FrameDifferecing example on the Movie object (movie.loadPixels(), movie.pixels[i]).

This is my go at 'Frame differencing a Mov' - just gets a grey screen output - no error messages - any suggestions please?

import processing.video.*;

int numPixels;
int[] previousFrame;
Movie myMovie;


void setup() {
 size(640, 480); // Change size to 320 x 240 if too slow at 640 x 480
 // Uses the default video input, see the reference if this causes an error
 //video = new Capture(this, width, height, 24);
 myMovie = new Movie(this, "station.mov");

 numPixels = myMovie.width * myMovie.height;
 // Create an array to store the previously captured frame
 previousFrame = new int[numPixels];
 loadPixels();
}

void draw() {
 if (myMovie.available()) {
   // 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
   myMovie.read(); // Read the new frame from the camera
   myMovie.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 = myMovie.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
   }
 }
}