[Jack Binks]

Hi All,

Some good tips here, perhaps I can help clarify some of the other points.

-The two bias controls essentially govern how the source footage is colour corrected before being processed. The alpha bias alters the source used to pull the matte, and the despill bias alters the source used to despill the footage. A number of users (myself included!) found the sliders to be somewhat unintuitive – essentially it was a case of suck it and see with regards to the settings. The colour pickers use a different scheme – all you have to do is pick the predominant colour of the foreground image (generally skin tones). My general method is to pick from a few different regions in the foreground to get an idea of the most aesthetically pleasing region to choose.
One thing to bear in mind is that in the vast majority of cases you want to have the alpha and the despill bias identical – in fact we’ve only seen two or three shots here at the office where we needed to have them different. Remember that link checkbox that was in the old AE Keylight? The fact that it’s not in the new one is down to an overzealous simplification of the control panel and is in fact a bug! This will be fixed in a forthcoming version that will be available off our website, in the meantime, just pick the same colour for both, unless you have extremely impure screen colours in your shot.

-During the matte pulling and despilling phases Keylight uses the two none-screen colour channels (so say R & B for a greenscreen) in order to sensibly soft limit, and to a certain extent reconstruct, the data in the screen colour channel (G in previous example). This occurs across the image, not just in the regions of spill, as it is both implicit in the algorithm and, as a useful side effect, minimises edge artifacts that would be otherwise seen.
This means that if your input footage has noise evident on a single colour channel its characteristics may well be duplicated (and possibly amplified) in the other colour channels. It generally becomes most evident in regions where the source is most heavily processed, i.e where the colour is close to the screen colour. This is very footage dependant, and your best bet for judging whether this is the root cause is to have a look at the individual colour channels of the input footage close up, and compare noise characteristics with the output you’re seeing from Keylight.

There are a number of methods for dealing with this, a couple of which have been already mentioned here.

-As a slight modification to Jonathan’s first method you can instead apply Keylight and set its output to ‘Intermediate Result’ this gives you the unprocessed foreground with the generated matte in its alpha. Now pop your spill suppressor of choice onto the fx stack and let it do its magic.
-Jonathan’s second method involves keying the offending section of the image separately. As he suggests, this is a very versatile technique. A little aside; most of the time I find the easiest way to get a good key is to use two layers – the lower one to pull a good matte for the edges, and the upper one as a hard clipped binary matte, eroded in to provide solid insides. Keylight provides a number of methods for defining multiple regions, although the easiest is often simply to use two layers and let AE do all the combining. Check out the manual for more details of Keylight’s inner matte/mask functionality, which essentially allows you to either draw a mask or pull a key in a precomp to define an area of the source which you wish to leave out of the keying process, before applying one of a variety of post processes to govern how it’s reintegrated into the image.

Hope that helps! If you’re still running into problems and want us to have a look at the footage, scoot a couple of frames and an aep over to support at thefoundry.co.uk and we’ll have a look to see if perhaps we can come up with any improvements for you.

All the best
Jack

The Foundry, UK

[Jack Binks]

Hi Walkingduck

If you want we’ll set you up an ftp account on our server where you can upload the footage in question. We’ll have a look at the problem and see if we can come up with some good settings for you? If this suits you, pop an email over to support at the foundry.co.uk and we’ll get the account set up.

Cheers
Jack

The Foundry, UK