Detect and locate webcam motion in Flash 8

In this post I want to discuss the possibilities we have in Flash 8 to detect and locate motion. There are many sources online telling us how to detect motion (it's rather easy) but how can we locate where the movement exactly is going on? I will explain a few possibilities like so you can choose yourself which one works best…

Detecting changing brightness:

As Guy Watson is showing in his article on the Adobe website brightness changes are a good manner to detect motion. And by combining this with the difference blend mode, you can more or less detect where the motion is going on. If you want to know more about this approach you can see Guy's tutorial here. Ones you have isolated the changed pixels you can easely find the location of the most centered pixel value by using some Mathimatical expressions.

Detecting changing average color values

Another possibilty I came up with while experimenting this matter is tracking the webcamimage for changed color values with a given offset. Because we cannot track the change of every pixel (to slow and Flash will probably crash) we are going to track the average color change in a specific region. So we have to divide our webcam image in different parts, actually a grid.

So suppose we divide the webcam image into a grid of 100 cells. Each cell will have a rectangle of rect(0,0,16,12). So we have something like this for a webcam stream with dimension 160×120 pixels :

We can calculate the average colour for each of the cell in the webcam image grid using following code:

function computeAverageColor(bm:BitmapData):Number{

var totalR:Number = 0;
var totalG:Number = 0;
var totalB:Number = 0;
var count:Number = 0;

for (var j:Number = 0; j<bm.height; j++) {
	for (var i:Number = 0;  i<bm.width; i++) {
		var col:Number = bm.getPixel(i, j);
		var red:Number = (col & 0xFF0000) >>> 16;
		var green:Number = (col & 0×00FF00) >>> 8;
		var blue:Number = col & 0×0000FF;
		totalR += red;
		totalG += green;
		totalB += blue;
		count++;
	}
}

var targetR:Number = Math.round(totalR/count);
var targetG:Number = Math.round(totalG/count);
var targetB:Number = Math.round(totalB/count);

//combine the color and return target color value
//…

}

As you can see in the funcion computeAverageColor we are adding up the separate color channel values and at the end we divide each sum with the total number of pixels in that specific tile "bm". Now we have three separate average channel values so next thing we have to do is combining this in one average color value by writing this code at the end of the function:

var targetCol:Number = targetR << 16 | targetG << 8 | targetB;

return  targetCol;

To divide the actual webcam image in those separate tiles I made a custom class BitmapBlock (extending on the Bitmapdata class) where I use the copyPixels method to extract a tile from the webcam image with the specific source rectangle as parameter.

Next thing to do is actually compare the tiles color every time the Camera.onActivity() handler is invoked. So we must store the tiles in an Array where I also hold a reference to the position of the tile on the Stage. We know when something moves in the camview the average color value will change. By measuring the change (if the difference is huge enough to be considered as a motion) we can know in wich region the motion is detected and by example make a snapshot of that specific region.

I did a few tests but I have to say that Flash 8 is a bit to slow for actual commercial use but it's a challenge to work around this and I hope by experimenting on this issue you will get a better understanding about the BitmapData and Camera class possibilities. If you want to make something that is actually usable, try experimenting with Adobe Director and the Webcam Xtra you can find here. Another possibility is offcourse Actionscript 3.0 where it all goes much faster :-).

If you have other possibilities to track and locate motion, feel free to comment this post

Happy coding,

Koen

PS: I started with this experimenting because I was thinking about a smart shooter game where the computer always follows you and tries to shoot you down

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