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Keyed out HDV footage has noise

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Amit Ghildiyal
Keyed out HDV footage has noise
on Dec 11, 2009 at 12:55:47 am

Hi, I shoot 1080i 60i HDV footage on greenscreen for editing in FCP. Since the keys arent perfect in FC I then export unkeyed footage as pro res HQ (interlaced, chroma subsampling) before importing it into AE. My composition seetings match the video settings, the key (keylight) is good( unless there is a technique i am missing), but the keyed video has a lot of noise. I export from AE as prores hq too. I have tried the same process with HDV 1080i footage as well without luck. I have looked for solutions to deal with what i have ( an HDV camera, FCP and AE) but am confused on how to remove the noise and i have tried the noise filter in AE.

Kindly help.


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Chris Brearley
Re: Keyed out HDV footage has noise
on Dec 11, 2009 at 10:57:05 am

I've often found that keylight can leave behind some undesired noise. Try running the denoise filter before keylight and in the 'screen matte' section in keylight set the replace method to hard light or source and see if that helps.

The most likely cause may be that the noise you are seeing are tiny holes in your matte. Set the view to 'screen matte' and if it looks bad then you will have to bring down the clip white value. Depending on your footage you may want to autotrace your keyed footage to use on a duplicate layer, then shrink the mask slightly and feather it. Delete keylight then place the layer on top of the original. This way the only parts of the footage you will see with keylight applied will be the very edges. Hope that all made sense....

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Dave LaRonde
Re: Keyed out HDV footage has noise
on Dec 11, 2009 at 7:14:04 pm

Well, there are a couple of reasons for your lousy keys.

First, HDV is just about the WORST video you could shoot for chroma key work. It has the same God-awful color resolution as DV video. What's color resolution? Watch this and find out:

Second, Keylight is touchy when it comes to improperly-exposed shots. It likes good lighting. It likes uniformly-lit chroma key backgrounds. If you didn't do that, you've got trouble.

So get the key looking good in Keylight's Status view so you're convinced you have a good key, then use the layer as an alpha matte for a second instance of the footage. Use AE's built-in Spill Supressor if necessary. If you don't know what "Status View" means, you need to download the Keylight training materials from The Foundry.

But it's still going to be HDV. And if you left the video in its acquisition codec, your troubles aren't over. Read on:

Dave's Stock Answer #1:

If the footage you imported into AE is any kind of the following -- footage in an HDV acquisition codec, MPEG1, MPEG2, mp4, m2t, H.261 or H.264 -- you need to convert it to a different codec.

These kinds of footage use temporal, or interframe compression. They have keyframes at regular intervals, containing complete frame information. However, the frames in between do NOT have complete information. Interframe codecs toss out duplicated information.

In order to maintain peak rendering efficiency, AE needs complete information for each and every frame. But because these kinds of footage contain only partial information, AE freaks out, resulting in a wide variety of problems.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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Brendan Coots
Re: Keyed out HDV footage has noise
on Dec 11, 2009 at 10:29:31 pm

Dave's response is 100% spot on, and here's a few additional points:

1. You shot in interlaced. Bad idea. Always shoot progressive for greenscreen, and ideally at the highest frame rate possible (24p motion blur can be a disaster to key). With most cameras, 30p is the best format, even if that limits you to 720p (I sincerely doubt you're delivering @ 1080 anyway, right?). Bear in mind that virtually all prosumer cameras only do "fake" progressive. 24p isn't 24 progressive frames, it's 29.97 with some back-end trickery that makes it appear as progressive in your editing app. The solution is to shoot in 30p, and pull that footage into Apple's Cinema Tools (included with FCP). This tool will convert the footage to "Real" progressive that can then be used for proper keying.

2. HDV is not real HD. HDV cameras (any prosumer HD camera, actually) shoots at 1440x1080, then your editing/compositing app stretches it out to 1920x1080. In other words, it's fake HD - see a "fake" pattern here? Anyway, that massive amount of stretching reduces your image quality horribly, as you can imagine. This isn't as visually obvious when you're just watching regular HDV video, but keying is done on a pixel by pixel level so those details really start to matter.

3. All prosumer HD cameras suck badly when it comes to grain, especially in low light. It's a good idea to use more intense lighting and stop down the camera to get the look you want. This ensures that the camera is getting plenty of light, and modulating that after the fact. When your lens is wide open, or even close to it, there's gonna be a lot of grain.

In the end, "fake" HD just plain sucks for effects work, possibly even worse than traditional MiniDV because the "tricks" it employs are so much more extreme. It looks ok on screen if you shot it perfectly, but that perceived quality relies a lot of precarious back-end silliness that immediately breaks down under even the most basic color correction, much less effects work.

Brendan Coots

Splitvision Digital

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Chris Wright
Re: Keyed out HDV footage has noise
on Dec 12, 2009 at 9:44:36 pm

first off, turn your camera's auto gain off, and go full manual.

Back in the day before reelsmart motion blur and such, keying was jerky looking from shooting with a really high shutter. Now you can post-add the blur back in.

And don't go the other extreme; shooting with super bright lights and stopping down the camera to f/12. You'll get a problem called "circle of confusion", yes it's real!

You actually reduce image quality because light is bouncing off the diameter lens. That's one reason why movies are shot f/5.6. or less.

Another advantage is you get a shallow depth of field which blurs the green screen to make much better keys. If you have too, at least use a Neutral Density filter.

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