Refining your Keys in Color's ColorFX Room

By Alex on September 14, 2009 12:01 PM | Permalink | Leave a Comment(0) | TrackBacks (0)

Color's ColorFX room gets a bum rap from a lot of folks used to "serious" node-based compositing and keying environments, but its native toolset, however limited, can unlock a tremendous new degree of control over Color's extremely powerful color-correction capabilities.

Many of the ColorFX nodes replicate pieces of the functionality exposed in Color's other rooms, except that ColorFX node trees allow you to "mix and match" them in ways far more nuanced than Color's (already-powerful) default workflow. One of the most useful isolation tools, which you'll use over and over in a typical node tree, is the HSL key. This node works exactly the same as the HSL key in the Secondary rooms â€“ except that you can use as many of them as you'd like to very precisely isolate portions of your image.

Let's say that your source video looks something like the image below. Ignore the overall color problems â€“ for now, your task is simply to isolate the subject's face, and do something different to it than to the rest of the image. It could be a skin smooth, a goofy Pleasantville effect â€“ doesn't really matter, we just need to isolate the narrow region corresponding to this particular actor's face in this particular type of footage.

I'm afraid I can't give you this source clip to work with, but I think you'll get more out of this tutorial by following along with footage of your own anyway. Ideally, your footage should feature an actor as well as some regions of similar color in the backdrop.

To get started, copy my basic node tree below, just so that we can see the impact of what we're doing.

Remember, although I just used a B/W node for now, we could use any of Color's extremely robust grading nodes to affect the different portions of the image. We're focusing on pulling a good solid key, so only worry about the matte part of the node tree (i.e. the HSL Key node). First, try using the HSL Key's eyedropper to initialize your key â€“ mine picks up parts of the chair as well as the actor's face.

By default, the HSL Key gives you the same weird partially-desaturated preview as it does in the Secondaries. To get a better look at the actual channel values of our key, let's pipe the HSL Key node into a Blur node -- since that'll probably be our next step to refine the key anyway.

Color doesn't ship with explicit matte refinement tools, like Shake's Dilate/Erode or Motion's Matte Magic. But we can fake some of the action of those tools by combining successive blur and curve nodes.

Play with your blur until your image reflects rough regions of high-intensity white in the desirable parts of your key, with dimmer greys in the noise regions that you picked up from your scene. Then place a Curve node in your tree. Use the Curves node to apply a strong contrast to your greyscale mask -- the steep part of this curve refers to the new "cut point" between whites and blacks. You can very precisely differentiate between shades of grey by adjusting the position of the steep part of the curve (and then, if you're still pulling portions of your backdrop, you can use additional blur nodes to smooth them out further, or combine additional keys using Add or Difference nodes).

It's not perfect, but it's a heckuva lot better than our original key. And resist your urge to say "just mask out that sliver of chair!" Once you've got a grasp of procedural techniques, you'll often be able to combine those faster than you could go through and roto the entire shot (or 75 similar shots, as the case may be). That's a huge advantage of Color's ColorFX: while they're a pain to build, they're awfully recyclable once you do build one.

If there's interest, I'll go over some more techniques to refine your mattes in the upcoming days and weeks ... and of course, we cover all of these and many more approaches in our Color classes. The two-day Color class has a chance to focus much more on the ColorFX room, while the one-day Color training spends so much time on the colorist's main workflow that we can't go into tremendous depth in the ColorFX room. But, of course, we're also happy to customize an on-site or Internet training to your specific needs and interests! Just call our friendly staff at 1-866-566-1881, or click Contact Us above.