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Difference Blend Mode

Difference Blend Mode

This solution came about when a friend suggested that I should take a look at the difference blend mode, one of 11 blend modes available in Flash Player 8. There are nine blend modes in Flash Player 8 and two special blend modes that require extra steps for them to function correctly. A blend mode can be used to create a composite bitmap formed by blending together the color data from two or more bitmaps with different calculations. Each different calculation is a different blend mode. I was interested in the difference blend mode, because it creates a negative effect based upon the difference in brightness of the pixels.

You can apply a blend mode to a BitmapData object with ActionScript by specifying the name of the blend mode as a string for the fourth parameter of the draw() method:

//draw the previous snapshot onto the current snapshot
//using the difference blend mode
now.draw(before, new Matrix(), null, "difference");

The resulting bitmap contained a negative effect; any pixels that were the same color in both the previous snapshot (“before”) and the current snapshot (“now”) were black. Finally, a good result; I was almost there, but I still had an ugly negative image. The next point of call was to change all the pixels that were not black to a different color. In this case, I chose green, which resulted in a bitmap that showed only the movement detected in the webcam as a green outline.