Though all keyers are different, it is possible to lay down some general rules about their use. in this section (and in this entire Wiki) FG (foreground) is used to denote the top layerer in the composite (i.e. the green screen footage) and BG (background) to denote the layer over which it is merged.
- 1 Denoise
- 2 Don't output the composite from the keyer
- 3 Don't color adjust the FG before it is fed into the keyer
- 4 A soft matte is soft
- 5 A keyer operates on the entirety of the image
- 6 Don't blur the matte
- 7 Test the matte
- 8 Don't waste a keying operation on junk
- 9 Despill with HueCorrect
- 10 Blur edges of composite
- 11 See also
Apply a Denoise node before the keyer. This will remove noise from the green, which will produce a better matte.
Don't output the composite from the keyer
Most keyers have the option to output the footage in several ways: premultiplied, unpremultiplied and composited with the background. Generally outputting directly to a composite is a bad thing. The reason: it is a rare composite that does not require the colors, grain etc of the FG to be adjusted in order for it to match the BG. Outputting a composite precludes such adjustment. Therefore: the keyer should be considered as a 'matte puller' not a merging tool.
Don't color adjust the FG before it is fed into the keyer
Any color adjustment done to the FG before it is keyed will effect the performance of the keyer.
A soft matte is soft
The most important aspect of a matte is the edge. In getting rid of noise artifacts in the black background of the FG it is tempting to be over-enthusiatic and push the matte so far the black to white edges become too hard.
A keyer operates on the entirety of the image
Remember, a keyer operates on the entirety of the image: anything that you do to one part of it is done to all of it. Do not be working away to harden up one part of the alpha, unaware that you are over-hardening another part.
Don't blur the matte
If the matte is too sharp a solution is not to blur it. This damages the quality of the matte and increases unwelcome edge artifacts. Better instead to refine the quality of the matte.
Test the matte
You can test the matte in a number of ways: the 'gamma' and 'f-stop' sliders at the top of the Viewer window can be adjusted to 'stress test' both the black and the white regions. You can also do a temporary comp using a black, white or bright color as the background. This will make the edge value shortcomings more apparent.
Don't waste a keying operation on junk
A keyer need not be the only tool employed to separate the white FG object from its black background. On easily isolated areas a roto can be used to 'keep in' the white of a matte (a holdout mask) or to 'punch out' the black of a matte (a garbage mask). It is a waste of rescources to use a keyer to get rid of such junk information, and is also likely to over-harden the matte. Directions on how to use junk and holdout masks are here.
Despill with HueCorrect
No matter how well you light your subject, the green screen will often (always) cast a bit of green light on the figure. Though some keyers offer the tools to do this within the keyer, the tool of choice for getting rid of this is the HueCorrect node.
Blur edges of composite
In order to integrate a foreground over a new background, the edges where they meet sometimes require a very slight blur. This technique is described here. It emulates the way that the edges of objects in photos are mildly 'fuzzy'. A technique called light wrap performs a similar function, but I have found the edge blur more effective.
Tips on green screen use here.
It is often required that the output from two or more keyers need combining. Here's how: