Set specific pixels fully transparent in PGraphics

Hi everyone, I am doing a demo about background subtraction and people extraction. I already picked the foreground pixels out of the whole pixels through comparing the different pixels between the static scene and later a people enter in this scene. I use createGraphics to store the foreground pixels and I would like to get a fully transparent background png. However, I only get a series of png files as normal camera feed frame, not the foreground extraction. I think the key section is here: if (diff > threshold) { pixels[loc] = fgColor; } else { pixels[loc] = color(0, 0); } I always get the error "NullPointerException" for the line" pg.pixels[loc] = color(0, 0); " so that I couldn't set the rest pixels as fully transparent.

Is there anyone have any idea about that? I have been trapped for too long but have no idea. Any help will be appreciated.

Here is the whole code if necessary:

import processing.video.*;

Capture video;

PGraphics pg;

PImage backgroundImage;
float threshold = 30;

void setup() {
  size(320, 240);
  video = new Capture(this, width, height);
  video.start();

  backgroundImage = createImage(video.width, video.height, RGB);
  pg = createGraphics(320, 240);
}

void captureEvent(Capture video) {
  video.read();
}

void draw() {

  loadPixels();
  video.loadPixels();
  backgroundImage.loadPixels();
  //pg.noSmooth();
  pg.beginDraw();
  pg.background(0, 0);

  pg.image(video,0,0);
  for (int x = 0; x < video.width; x++) {
    for (int y = 0; y < video.height; y++) {
      int loc = x + y * video.width;

color fgColor = video.pixels[loc];
color bgColor = backgroundImage.pixels[loc];

float r1 = red(fgColor); float g1 = green(fgColor); float b1 = blue(fgColor);
float r2 = red(bgColor); float g2 = green(bgColor); float b2 = blue(bgColor);
float diff = dist(r1, g1, b1, r2, g2, b2);

//pg.loadPixels();


if (diff > threshold) {

  pg.pixels[loc] = fgColor;
} else {

  pg.pixels[loc] = color(0, 0); //color(gray, alpha)
  // pg.clear(); clears everything in a PGraphics object to make all of the pixels 100% transparen
}
    }
  }
  noFill();

    pg.updatePixels();
    pg.endDraw();
    image(pg, 0, 0);
    pg.save("image_" + millis() + ".png");
}

void mousePressed() {
  backgroundImage.copy(video, 0, 0, video.width, video.height, 0, 0, video.width, video.height);
  backgroundImage.updatePixels();
}