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Monday, January 17, 2000

Luminance Ranges in Video

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

Translation

Translating between these different worlds is actually quite easy, once you’re aware they exist. The questions you need to ask yourself, at every step of your production, are:

1. Which luminance range is this source using?
2. Which luminance range does the software I am currently using assume?
3. Which luminance range does my output codec and hardware expect?

Once we know these answers, all we have to do is make sure we keep the various black and white points aligned throughout our workflow.

Again, if you stay in a “closed” system, you don’t have to worry about these issues. The best example is a non-linear editing system where all your sources are either captured or created inside that system - it already knows which luminance range it is using, and will keep things consistent internally.

The confusion comes when you mix and match. Let’s walk through a few scenarios and see how they would need to be treated.

Case Study: Compositing

We used to have a Media 100 system, as did many of our clients at the time. Therefore, it was common for our captured footage, as well as our final renders, to be in Media 100 format, which used the 601 luminance range. We do most of our compositing inside After Effects, also drawing on an extensive stock footage library we have in house, plus additional 3D elements that we render.

When we ask ourselves The Three Questions listed above, the answers are:

1. Our captured sources use the 601 luminance range; all of our other sources use the computer luminance range.
2. Our software uses the computer range.
3. Our final output should be back in the 601 range.

Therefore, we needed to translate all of our 601 sources to computer range while working inside After Effects, and then translate our final After Effects render from the computer to the 601 range.

To perform this input translation in a program like After Effects, each Media 100 source must have a Levels effects applied to it, with the Input Black parameter set to 16, and the Input White parameter set to 235. Leave the Output Black parameter at its default value of 0, and the Output White at its default value of 255. An example of these settings is shown in the figure below:

To translate footage that uses the 601 luminance to the computer’s luminance range, we apply a levels-type effect, with Input Black set to 16 and Input White set to 235. The lack of color values in the source below 16 (where the cursor is pointing) is a hint the footage uses the 601 luminance range.

If you are going to be using a single source more than once, you might consider placing each source in its own “composition” with Levels applied, and then use this already-treated comp - not the source itself - when it is needed in other comps. Note that some systems, such as old Videonics’ Effetto Pronto, allow you to set a parameter in the options for each source that does the same thing (this feature also appeared briefly in After Effects 7, before being replaced by a more comprehensive color management system - lick herec for a column on these subjects). This is a good model for other programs to emulate.

Why not leave the Media 100 footage alone, and treat the other sources to match the Media 100’s range? Because After Effects is not inherently aware that it should keep black at 16 and white at 235. As you add treatments such as glows and transfer modes, it is easy to get hot spots that hit 255; when you want to fade a layer to black, you would have to fade it instead to another dummy source set to a color of 16/16/16; when you create “white” text, you would need to make sure you selected a color of 235/235/235 instead of 255/255/255. Yes, you can do it, but it’s a lot more work.

There are a couple of approaches to end up with 601 luminance range images when we render our final composite. The old way was to nest our “final” comp into a new “render” comp, and again apply Levels to it. At this stage, the input and output settings are reversed: Output Black is set to 16 and Output White is set to 235, while the input settings are left at their defaults of 0 and 255, respectively. Ever since Adjustment Layers were introduced in After Effects 4.0, we now place one of these layers at the top level of our final comp, and apply Levels as described. Our blacks will now be placed at 16, and our whites at 235.

To translate typical computer images to the 601 luminance range, again apply a levels-type effect, this time with Output Black set to 16 and Output White set to 235.

There is a “gotcha” to this way of working: The Levels effect will only translate the portions of the image that are not completely transparent. This is a problem if you are creating simple titles on black, for example. If you have any areas were the alpha channel is 0 in value, the background color will not bumped up from 0 to 16. Therefore, it is best to place a black, full-size dummy layer (such as a “solid” in After Effects) at the bottom of the stack in the final composition. This will fill in these transparent areas, and make sure they get translated properly. (There are, of course, exceptions to every rule; see the sidebar on the last page of this article on Superblack.)

Ultimately, we hope this treatment migrates to an Output Module option. It would make it easier when we’re rendering multiple versions of the same composition - for example, to both our Media 100, which operates in 601 space and therefore needs this translation, and a codec the client may require that happens to do this translation automatically. Moving it to the output module, after the alpha channel is calculated, would also eliminate the transparent/black background workaround above.

Editing • Motion Graphics • Post Production • Visual Effects •

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Favorite Technical References

Chris Meyer | 02/15- 07:45 PM

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

Posted by Chris Meyer  on  02/17  at  02:51 PM

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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.

Posted by Graeme Nattress  on  02/26  at  04:40 AM

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