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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...

Digital Definitions



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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Comments

bill: | February, 17, 2008

hey
good article,
but i’m in the HD world (specifically HDV)
so how about an article on the HD colorspace (SMPTE 292M?), and the whole SUPERWHITE thing, and how FCP handles superwhite ...
thanks!
ps
love your new website… all the old favorites from DV magazine…

Chris Meyer: | February, 17, 2008

Hi - glad you found us in our new home!

The luminance issues with HDV are the same for DV. In other words, the internal signal is capable of recording numbers darker than “black” and brighter than “white.”

The superwhite issue has been made more interesting by the way Final Cut Pro handles luminance. They shift down signal-black to digital-black, preserving the extra latitude in the whites for you to play with. I like FCP’s 3-way color corrector as a way of recovering the overrange whites; their auto white button in the corrector does this for you.

Of course, the _real_ desire is to not shoot out of range luminance values in the first place, so you don’t have to correct them later! I hate to make gross generalizations, but when took in tapes from people shooting on Beta or BetaSP, we didn’t have this issue; only when we got more tapes from shooters using less-expensive DV and DVCAM cameras (including our own footage) did we start to have these problems. I can only assume it was an overgenerous acceptance of luma ranges while in auto mode (remember: DV was originally supposed to be a consumer format, so who cares about broadcast-legal ranges; they just want the images to POP…). So a lot can be said for a nicely adjustable camera that allows you to monitor these things (or at least one with zebra patterns to show where you’re going out of range).

That said, I also like specular highlights, so it’s nice to know about out-of-range whites and how to recover them, so you can tame them later.

take care -
Chris

Graeme Nattress: | February, 26, 2008

The main reason for the 16-235 range is not for cameras, but for digital decks dubbing old analogue tapes where the defined voltage range for the output signal can be a little loose. The headroom and footroom is for that, to ensure no signal is lost off the top or bottom on transfer.

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