Highlighting differences between consecutive video frames in Phaser

[rgoldsto]

I would like to highlight differences from one video frame to the next using Phaser's built in video streaming library. So functionally, as a person moves their body, the changes from one frame to the next will be highlighted and all of the unchanging image parts will become black. The important code snippet currently looks like:

function create() {
 video = game.add.video();
 video.onAccess.add(camAllowed, this);
 video.startMediaStream();}

function camAllowed() {
 bmd = game.add.bitmapData(video.width, video.height);
 bmd.addToWorld(game.world.centerX, game.world.centerY, 0.5, 0.5);
 game.time.events.loop(50, takeSnapshot, this);};

function takeSnapshot() {
 video.grab(false,1,'difference');//params clear true or false, alpha, blend mode
 bmd.draw(video.snapshot);}

The code almost works, but causes flashing video even for the parts of the scene that do not change from moment to moment. I played around with different globalCompositeOperation operations, many of which were trippy, not none which had the intended effect of making a completely black image except for where there was something different from the preceding to current frame.