Keylight noise

[Matt Larson]

Adjust the spill and alpha bias.

[Jonathan Miller]

Sean,

If you absolutely cannot figure out how to get rid of this noise by tweaking Keylight, then there’s another fix for you. Create a track matte from that layer.

So, get the key the way you want it (ignoring the noise) and duplicate the layer. Next, on the lower of the two layers, remove the Keylight effect.

Then, making sure you can see the trackmatte settings in the timeline (click the Switches/Modes button at the bottom of the column in the timeline) place a check in the “T” box for the layer that still has Keylight.

Now chose the proper setting for your lower layer in the TrkMat drop-down (the first selection is the one you want, I believe.)

So now the keyed footage should cut the proper hole in the unkeyed footage giving you a great key, but no noise. You’ll probably have to remove some spill, so add the Spill Suppressor to the lower layer.

Are we close? Hope so.

Good luck!

Jon
TreeLine Productions
Fort Collins, CO USA

[Sean Cusson]

Hey guys, txs for the replies. In the end I used the method Jonathan described (which I had setup just before typing out the first post). I would still like to know why it’s happening though. Matt are you saying that the noise is caused by spill? If so that’s very strange because there is no spill on the part of the clip that’s now showing noise. The actor is actually in front of a hanging screen (green) so there is no spill in front of her. Is Keylight automatically applying some sort of spill suppression which is causing the noise?

Sean Cusson
Q media design
sean@qmediadesign.ca

[Jonathan Miller]

Man, I totally know what you’re experiencing. I haven’t had the time to figure out how to fix it since I’m always crankin’ on deadline. I’ve adjusted values like crazy but no luck. It doesn’t happen with every clip, though. I hope someone has a detailed answer.

Good luck!

Jon
TreeLine Productions
Fort Collins, CO USA

[X-claim]

While on the topic, can anyone simply explain how to use the alpha and spill bias’ in AE 7, I had it down when it was sliders, but now that they have changed to colour selectors I am lost.

I am keying a netball player (it is a female sport similar to basketball for those of you not from my corner of the earth) anyhow there is a girl wearing a singlet which is kind of redish purple and the damn thing is shot against bluescreen (grrr directors, they had a green screen on location too [/rant]). It is shot well on digibeta and lit well, but I just can’t get rid of the noise on her singlet as there is too much blue in the colour. I can’t even get a clean matte, anyone know of anything that I can try?

[Jonathan Miller]

Why not just try to key out that small part of the footage by itself, then layer it over the previous key. In my experience, I rarely get a perfect key using only one layer of footage. I usually have to key things out in pieces, then build them up until I have what looks like a finished key. By layering it all up, your final result should look like one piece of footage, and can act like one if you precompose it.

Good luck!

Jon
TreeLine Productions
Fort Collins, CO USA

Currently producing these popular podcasts:

[X-claim]

Yeah that sounds like a plan, i’ll give it a shot.
Thanks

[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

[Jonathan Miller]

Jack, thank you so much for the post here. Man, there’s nothing like going straight to the source for some tips.

Greatly appreciated. Of course, I have no projects right now that involve keying, but I’m going to print your explanation out and file it away.

Jon

Good luck!

Jon
TreeLine Productions
Fort Collins, CO USA

Currently producing these popular podcasts:

[X-claim]

I have printed that for future reference as well.
You mentioned a forthcoming version of the plugin… will this have fixed the problem where the bottom half of frame disappears upon rendering as mentioned here:

https://support.thefoundry.co.uk/cgi-bin/yabb/YaBB.pl?board=Report;action=display;num=1145631913

It is also mentioned somewhere in this forum

Thanks
Hayden
New Zealand