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Monday, January 17, 2000

Luminance Ranges in Video

Different video systems have differing internal definitions of “black” and “white.”

sidebar: Superblack

Videotapes, as well as a surprising number of digital video codecs, do not have embedded alpha channels. This is a bit of a shortcoming when you want a matte or key to travel along with your footage, such as when you are passing lower third titles on to a video editor to later composite over footage.

The most common solutions to this problem include using an extra tape or file that contains just the alpha matte, or to pre-composite the image over blue or green and then to key this color out later. However, some also use a technique known as superblack, which is a form of luminance-based keying.

In this case, “black” in the portion of the image area you want to keep is actually lighter than pure black. The background area you want to drop out is then set to pure black (or, depending on how you think about it, “blacker than black” - superblack). The equipment that receives this footage is then set up to key out any luminance values darker than “black” in the portion of the image you wish to keep.

To perform this digitally, you need a video editing system that works in the 601 luminance range (which virtually all do), and a codec that does not auto-stretch the 16-235 luminance range out to 0-255. In other words, you need a system that will allow you to preserve colors darker than “legal” black.

Either build your graphics using the 16-235 range, or build then at 0-255 and then apply a Levels effect with output black and white set to 16 and 235 respectively. Then composite this over a pure, “0 black” background. In After Effects, this happens automatically if your foreground image has a transparent alpha, and you have nothing behind it in the composition. When you import this render into your video editor, again, do not perform any luminance stretching, and tell its luminance keyer to matte out levels below 16.

You can also record an image with an embedded superblack matte to videotape. Prepare your image as above, taking care that your video system does not clip or remove black values lower than 16. Make sure the hardware has 7.5 IRE setup enabled. File values of 16 will go out at 7.5 IRE, but your background values of 0 will go out closer to 0 IRE, allowing them to be keyed downstream.

Superblack keys don’t look very good - they have essentially no antialiasing. However, they render fast inside a NLE, and don’t require a second synchronized tape deck in the linear world. Eventually, most editing will be performed digitally, and most NLEs will allow an embedded, real-time alpha channel matte - but until then, don’t be surprised if a client asks you for a superblack matte.

Special thanks to the numerous people who added to the knowledge base from which this article was assembled: Brad Pillow, Richard Jackson, Don Nelson, Greg Staten, Mike Conway, Mike Jennings, Jerry Scoggins, Peter Hoddie, Rob Sonner, David Colantuoni, Tony LaTorre, Tony Romain, Knut Helgelund, Frank Capria, and finally, Charles A. Poynton’s book A Technical Introduction to Digital Video.

Editing • Motion Graphics • Post Production • Visual Effects •

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Chris Meyer | 02/15- 07:45 PM

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