Submitted by , posted on 23 October 2003

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This IOTD show a ChromaKeyer function handling transparency extraction and color correction.

A ChromaKeyer purpose is to generate an alpha channel based on a colored backing to be used for compositing. Common backing color are Blue and Green, green being usually preferred for video.

The "Original image" show one frame from an NTSC DV clip. (DV compression is DCT/Huffman based at a fixed 6:1 ratio)

The "Final Composite" show the source image color corrected and overlaid using the generated alpha channel over a background image, in this case purple curtains.

On the right, the close-up of a region of interest detail how transparency is extracted even in a somewhat noisy situation.

In Figure A we see that if we only created transparency information we would endup with a green colored object. This need to be addressed by a process of color restoration. Figure B show how the result is color corrected, yet still leave other details like the orange/pink highlight/reflection in the bottom right of the bowl.

Because their isn't one solution to the problem (and a problem to solve on a per pixel basis), this process require some level of user interaction by giving some ranges to formula parameters. In practice, the cleanest the source the less user interaction is required.

Here is a link to watch a promo video that show how a ChromaKeyer can be used in video production:

Any inquiry and comments welcome,

Stephan Schaem

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