Archive for July 3rd, 2009

Alternative to adaptive thresholding

As cool as it is, adaptive thresholding generates too much redundant data. The only alternative implemented before seems to be the method called automatic thresholding, suggested by ARToolKitPlus folks:

ARToolKitPlus can do dynamic thresholding by looking at the marker content (pattern) and taking the average between the darkest and brightest pixels. If no marker is found the threshold value is randomized.

From this description alone, it is clear that there will be frames with marker undetected even though it is clearly visible in the image, so I thought, until now, that this method has to be quite bad. It turns out, however, that this method actually works very well and the number of lost frames is very small. In practice, you don’t even have to find pixel values range, it suffices to assume that current good threshold value lies close to last known one.

On code side, you no longer need to write or import some fancy filter classes, all you have to do is to modify your fixed thresholding main loop just a bit:

private var threshold:Number = 0.5;
private var confidence:Number = 0, confidenceThreshold:Number = 0.5;
private function onEnterFrame (e:Event):void {
	videoSnapshot.draw (video);

	if (confidence < confidenceThreshold) {
		// randomize threshold
		threshold *= 0.8;
		threshold += 0.2 * Math.random ();

	confidence = 0;
	if (detector.detectMarkerLite (raster, 255 * threshold)) {
		confidence = detector.getConfidence ();

		detector.getTransformMatrix (result);
		stuff.setTransformMatrix (result);

	scene.render ();

If you’re sandy fan, you can play with this method using this stub code.

Old stuff

July 2009
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Oh, btw…