Luminance Ranges in Video
Different video systems have differing internal definitions of "black" and "white."
By Chris and Trish Meyer | January 17, 2000
It would seem like a simple concept: "black" is the darkest color you can have; "white" is the brightest color. However, not all video hardware and software think this way. Quite often, systems can go "darker" than black and "brighter" than white, allowing safety margins for certain situations.
This means that some systems uses different values for black and white than others. This can cause a lot of problems for a video editor or artist who uses a variety of tools during a production, because images may shift in relative brightness and contrast for no apparent reason. Compounding this problem is a lack of accurate information about how to manage these shifts. But if you ignore them, the results can range from washed-out images to illegal color values.
Therefore, you will need to take it upon yourself to be aware of the black and white definitions that different systems are using, and to translate between them as needed. We will also discuss the oft-confused analog concept of "set up" and how it relates to these digital values. It initially requires a bit of a mind-twist, but will pay off in the long run. We will be using After Effects for some of the examples later in this article, but these concepts apply to all systems - so read on...
The majority of computer graphics software, including most paint, 3D, and compositing programs, use intuitive definitions: Black is 0% brightness, or an RGB color value of 0/0/0; White is 100% brightness, or an RGB color value of 255/255/255 (for an eight-bit-per-channel color definition, where each color channel can have a strength range of 0 to 255; scale accordingly for higher bit depth systems). You can't get darker than black or brighter than white. We'll call this system "computer" luminance, with a range of 0-255.
Most digital video systems internally define black as a value of 16, and white as a value of 235. However, most software treats black as 0 and white as 255.
However, many digital video systems use a different set of definitions: Black is just over 6% brightness, or an RGB color value of 16/16/16; White is just over 92% brightness, or an RGB color value of 235/235/235 (again, based on 8-bit-per-channel ranges). This system is defined by the ITU-R 601 specification used by most digital video equipment, from DV to D1 decks, and transmitted over digital video connections such as SDI and FireWire (IEEE1394). We'll call this system "601" luminance, with a range of 16-235.
The 601 system allows colors that are darker than black and brighter than white. This is especially important for cameras, because you may occasionally shoot an object that has a bright spot that is "hotter" than legal white, and might want a way to later recover the detail in this hot spot. Going darker than broadcast black is also used at times for synchronization signals, as well as some primitive keying applications such as "superblack" where systems mask or key out areas of an image "blacker" than black.
The next trick comes in knowing when and how to translate between these two worlds.
The Two Systems
Many hardware systems never expose you to the fact that internally, they are probably using the 601 luminance range. Externally, these systems present you with the computer luminance range. When they decode or decompress a frame for you to use, they automatically stretch 16 down to 0 and 235 up to 255, scaling the values inbetween as needed and squeezing out the values above or below the 16-235 range. Likewise, when they encode or compress a frame to later play back through their systems, they squish 0 up to 16 and 255 down to 235. Avid, Scitex, Digital Voodoo, BlackMagic DeckLink, and AJA are examples of digital video cards and codecs that can do these translations for you.
However, many systems pass their internal 601 luminance values directly to the user when they decompress their frames, and - surprise, surprise - expect you to have conformed your computer range images back into 601 range before you hand an image back. Aurora cards, the old Media 100, and many DV codecs are examples of these systems, as well as many DDRs (digital disk recorders). Note there is nothing wrong with the way these systems work, and when we get to some DV examples later on, you may be happy they expose these extended values to you. And if you stay entirely within these systems, the issue may never come up, anyway.
Problems occur when you start moving between different systems, and therefore, luminance ranges. For example, say you pulled up some archived footage captured on a Media 100 that you were going to combine with still images and graphics inside Adobe After Effects - your footage will be using a different luminance range than your stills, making the video appear more washed out and less contrasty than you expected. Or say you created a 3D render that you intend to play back through DV - your darkest and brightest areas may now get pushed darker and brighter than you intended, resulting in more contrast and some illegal values. These can be fixed, as long as you know what is going on in the first place.
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