Save frames of background subtraction capture

Hi everyone, I am doing a background subtraction capture demo recently but I met with difficulties. I have already get the pixel of silhouette extraction and I intend to draw it into a buffer through createGraphics(). I set the new background is 100% transparent so that I could only get the foreground extraction. Then I use saveFrame() function in order to get png file of each frame. However, it doesn't work as I expected.I intend to get a series of png of the silhouette extraction with 100% transparent background but now I only get the general png of frames from the camera feed. Is there anyone could help me to see what's the problem with this code? Thanks a lot in advance. Any help will be appreciated.

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() {
  pg.beginDraw();

  loadPixels();
  video.loadPixels();
  backgroundImage.loadPixels();

  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);


if (diff > threshold) {
  pixels[loc] = fgColor;
} else {
  pixels[loc] = color(0, 0);
}
    }}
    pg.updatePixels();
    pg.endDraw();


    saveFrame("line-######.png");
}


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