Difference Matting within the Avid

COWIn this tutorial, Ra-ey Saleh will show you how even without shooting Green Screen you can replace the background of a shot whilst keeping the moving foreground undamaged. He'll use a technique called 'Difference Matting' and do it all within the Avid! What is Difference Matting? Without getting too in depth, a Difference Matte Key creates transparency information by comparing 2 images and then "keying out" pixels in the Source image that matches both the position and colour in the Difference image. Typically, it's used to key out a static background behind a moving object, which can then be placed on a different background. Often the Difference image is simply a Freeze-Frame of the Source image before the moving object has entered the scene. For this reason, the Difference Matte Key effect is best used on static shots. If it's so simple, why doesn't everyone use them? Well, Difference Matte Keys are notoriously unpredictable. All "differences" between the two images show up, even minor lightning changes, and the results can at first look very unimpressive. However, although Difference Matte Keys are no Green Screen replacement, if used to create a rough first pass and then touched up, they can provide a valuable time and cost saving alternative to sending it to GFX! read more...

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