Latest post Sat, Feb 7 2015 12:56 PM by Bruno M. 1 replies.
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  • Fri, Feb 6 2015 10:09 PM

    Best Chroma Key clean up method

    I'm not a VFX editor, but I'm currently working with problematic green screen footage with a lot of spill and where the green BG color somehow faded when the Alexa files were transcoded to DNX36HD. It's for a rough cut and not final, but Spectramatte (and to a lesser degree) Boris FX are having real difficulty pulling clean keys with this material, leaving tons of noise or bleed through and other artifacts. I've even tried painting/colorizing the BG to intensify the key color, but there's a lot of movement in the shots and I don't have time to keyframe each one individually.

    What strategies/techniques could I use to clean these up in a reasonably fast or not too intense way? I notice a greater variety of Boris keys (ie Make Alpha) but don't know enough about them. Nor am I familiar with AvidFX. I'm willing to do some extra work, but since since the cut is constantly changing I can't afford anything too complicated.

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  • Sat, Feb 7 2015 12:56 PM In reply to

    • Bruno M
    • Top 200 Contributor
    • Joined on Thu, Feb 11 2010
    • London, UK
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    Re: Best Chroma Key clean up method

    The whole business of compositing difficult greenscreen material can become quite complex. There are plenty of  tutorials on the internet covering this topic and quite a few plugins from different manufacturers that attempt to find ways around the problems of pulling a good key from poor material.

    However, I suspect that you're looking for some quick 'get-arounds', since you are at the offline stage, so no one is really expecting you to pull the perfect key at this stage.

    Cattle Prod:
    green BG color somehow faded when the Alexa files were transcoded to DNX36HD.

    First things first - Don't try doing this with DNX36 transcoded material. Re-transcode the green-screen material at an online resolution - the higher the better, but obviously within the playback capabilities of your system. Having a less compressed image will instantly give you better edges, which means better keys.

    I think it would be wise to try and establish why your colours faded on transcode, as this doesn't sound right, and whoever does do the final composite will thank you if you can sort this out. Was the Alexa material shot Log C? If so, you need to add a LUT (if you're on a version of MC that supports this) to bring it back up to REC 709, which is what I assume you're working in. Failing that, you could colour correct the material to get it looking right, then do a video mixdown (at a high quality DNX resolution) and work with that. Remember, you won't be able to link back to the original material from a mixdown, so you'll need the original source material placed back into your timeline before you create your final EDL, AAF etc.

    Spectramatte is a pretty good keyer if your greenscreen has a decent amount of saturation and level, and there are controls to supress spill. If you're unsure how to do this, there are tutorials out there on Youtube/Vimeo etc that will guide you.

    If you have Symphony, you could use the secondary correction to try and improve the green of your background. The trick here is to try and only pick out the green of the background without making the spill any worse.

    Hopefully, some of this will help you pull a better key without too much extra work. At the end of the day, the client will accept a less-than-perfect key at the offline stage as long as you assure them that the final composite will be perfect. So it might be a good idea to bring the VFX person in at this stage, who will have a better idea of the problems involved.



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