Monthly Archive for October, 2006

This is a quick test I made regarding background-substraction in Flash (which is also called Scene building etc.). I wanted to test this because of another experiment which I have in mind. The result is very nice, and it is acceptable fast.

A simplified description of the used technique. First of all I have the WebCam and a Video. A (grayscale) Snapshot of the scene will be taken which is called background. The background should not include the things you want to extract. From that moment on you create a grayscale picture from the webcam and substract the background from it. I draw the video onto the screen and for every pixel that has a difference greater than 8 I copy a pixel from the original webcam image.

Have a look
[as]var frame: BitmapData = webcam.getCurrentFrame();
videoBitmap.draw( videoDisplay );

//– create grayscale image of webcam frame
buffer.applyFilter( frame, frame.rect, new Point, bt709 );

//– substract the background
buffer.draw( background, null, null, ‘difference’ );

//– apply threshold
buffer.paletteMap( buffer, buffer.rect, new Point, null, null, alphaMap );

//– create mask using alpha channel
frame.copyChannel( buffer, buffer.rect, new Point, BitmapDataChannel.BLUE, BitmapDataChannel.ALPHA );

//– draw
smallScreen.draw( videoBitmap );
smallScreen.draw( frame );
screen.draw( smallScreen, scale );[/as]

I updated the way the resulting picture is drawn (thanks André!). Therefore the alpha-channel is abused as a mask applied with some thresholding. Instead of BitmapData.threshold I use BitmapData.paletteMap with a pre-cached table because it is a little bit faster. It is much better using the alpha-channel as a mask because of smooth edges. One last problem: Using only the Y-channel of a picture results in to many holes in the resulting image. I will try the same with three dimensions instead of one.