Convolution matrix script in Processing

gistfile1.pde

PImage img;

void setup() {
  img = loadImage("img.jpg");
  size(img.width, img.height);
}

void keyPressed() {
  if(key == ' ') saveFrame("frame-" + minute() + "-" + second() + "-#########" + ".png");
}

float[][] randMatrix() {
  int s = 3;
  float b = 0.00002;
  float[][] matrix = new float[s][s];
  int i, j;
  for(i = 0; i < s; i++) {
    for(j = 0; j < s; j++) {
      matrix[i][j] = random(-b, b);
    }
  }
  return matrix;
}

float[][] matrix = randMatrix();

void draw() {
  loadPixels();
  apply(matrix);
  updatePixels();
}

void mousePressed() {
  image(img, 0, 0, width, height);
  matrix = randMatrix();
}

color get(int x, int y) {
  int pos = y * width + x;

  if(pos > 0 && pos < pixels.length) {
    return pixels[pos];
  } else {
    return 0;
  }
}

void apply(float[][] matrix) {
  int mi, mj;
  int pi, pj;

  int c = matrix.length / 2;
  float sum;
  int b;
  int[] newPixels = new int[pixels.length];

  for(pi = 0; pi < width; pi++) {
    for(pj = 0; pj < height; pj++) {

      sum = 0;

      for(mi = 0; mi < matrix.length; mi++) {
        for(mj = 0; mj < matrix.length; mj++) {
          sum += get(pi + mi - c, pj + mj - c) * matrix[mi][mj];
        }
      }

      newPixels[pj * width + pi] = int(sum) | 0xff000000;
    }
  }

  for(pi = 0; pi < pixels.length; pi++) {
    pixels[pi] = newPixels[pi];
  }
}