I have the Processing sketch below, which scans through an image, storing the X and Y coordinates of each black pixel into 2 respective ArrayLists - xList, and yList. This sketch currently prints out these coordinates, but I now need to send each element of each of these arrays to the Arduino, and have the Arduino store them in it's own array (or arrays). How?

1. import javax.swing.*;
   
   PImage img;
   //Threshold determines what is considered a "black" pixel
   //based on a 0-255 grayscale.
   int threshold = 50;
   
   void setup() {
    
     // File opener
     try{
       UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
     } catch (Exception e) {
       e.printStackTrace();
     }
     final JFileChooser fc = new JFileChooser();
    
     int returnVal = fc.showOpenDialog(this);
    
     if (returnVal == JFileChooser.APPROVE_OPTION) {
       File file = fc.getSelectedFile();
       if (file.getName().endsWith("jpg")) {
         img = loadImage(file.getPath());
         if (img != null) {
           size(img.width,img.height);
           image(img,0,0);
         }
       } else {
         println("Please select a .jpg image file.");
       }
     }
     //I added noLoop() here, because the program would output
     //the coordinates continuously until it was stopped.
     noLoop();
   }
   
   void draw() {
     //Here it scans through each pixel of the image.
     ArrayList xList = new ArrayList();
     ArrayList yList = new ArrayList();
     for (int ix = 0; ix < img.width; ix++) {
       for (int iy = 0; iy < img.height; iy++) {
         //If the brightness of the pixel at (ix,iy) is less than
         //the threshold
         if (brightness(get(ix,iy)) < threshold) {
           //Add ix, and iy to their respective ArrayList.
           xList.add(ix);
           yList.add(iy);
         }
       }
     }
     //Print out each coordinate
     //For this statement, I use xList.size() as the limiting
     //condition, because xList and yList should be the same size.
     for (int i=0; i<xList.size(); i++) {
       println("X:" + xList.get(i) + " Y:" + yList.get(i));
     }
   }

I have an ArrayList in Processing that I need to send to my Arduino, and have the Arduino store each element of this ArrayList in an array. Or, if I could send the ArrayList as a single entity to the Arduino, which would then store each of the ArrayList's elements into a corresponding position in an array, that would work as well. The main problem I'm having is that I don't know how I can tell what elements are in the Arduino's array, and Serial.println won't work because the Processing code is using the serial bus. Here's what I have:

Processing Code:

1. import processing.serial.*;
   
   //The serial port that the Arduino is on
   Serial port;
   //The ArrayList
   ArrayList ar;
   
   void setup() {
     size(200,200);
     //Initialize the serial port at 9600 baud
     port = new Serial(this, Serial.list()[0], 9600);
     //Set initial size of ArrayList ar
     ar = new ArrayList(10);
     //Add numbers 0-9 to ar
     for (int i=0; i<10; i++) {
       ar.add(i);
     }
   }
   
   void draw() {
     //Loop through each element of ar
     for (int i=0; i<ar.size(); i++) {
       //Print the current element out below
       println(ar.get(i));
       //Convert the object cast by ar into an integer
       int curInt = (Integer)ar.get(i);
       //Write that integer to port
       port.write(curInt);
     }
   

I've tried a combination of other posts, the Learning section, and the book Processing, by Reas and Fry. Try as I might, I just can't figure out how to do this. I need Processing to load a black and white image, and store the coordinates of all black pixels in 2 arrays: 1 for their x-axis, and the other for their y-axis. How can I do this?

Here's what I have so far:

1. PImage img;
   int[] xArray;
   int[] yArray;
   
   void setup() {
     size(100,100);
     img = loadImage("image.jpg");
   }
   
   void draw() {
     background(0);
     image(img,0,0);
    
     //Load the pixels
     loadPixels();
     //Loop through each pixel
     for (int ix = 0; ix < pixels.length; ix++) {
       for (int iy = 0; iy < pixels.length; iy++) {
         //Set col to the color at that pixel
         int col = color(get(ix,iy));
         //If the color is less than 20 on an RGB scale (to include dark gray)
         if (col < 20) {
           //Add the X and Y coordinates to their respective array
           append(xArray,ix);
           append(yArray,iy);
       }
       }
     }
   }