Lessons learned: working with green screen and DVCProHD
HD Postproduction: Observations on green screen keys with DVCPro HD.
So I am caught in the quagmire of postproduction on my inaugural review of technology for Scribe – and I can’t help but cringe at my green screen… I know I should record this in uncompressed HD, but I don’t have a capture card and I certainly don’t feel like walking my Mac Pro into the studio (its heavy). So I have cringed at the chunky keys I have been getting. That is, until today. I have been doing some math (actually quite a lot lately as I am reading a new classic crypto based novel: Neal Stephenson’s “Cryptonomicon†that, like Asimov’s books on Biology and Physics, does incredibly funny things to my brain) and math being what it is, opens the mind to looking at problems in much more detached ways than brute force and constant fiddling with settings until things “just clickâ€.
The nice thing about thinking in mathematical terms is that it tends to illuminate certain day to day post production issues that arise more from obfuscation born of film community rumor mill ginning out spurious fact and innuendo or marketing hype rather than steadily manufactured fact governed by conscientious scientific rigor. The wholesale rejection of math in the filmmaking community is more likely to do with the necessity to focus on more complex issues, like making sure a client pays and that one’s vision is palatable to the viewer, rather than any sort of community deficiency derived from malnutrition or a predominant genetic predisposition towards mathematical incompetence. That said, math is still quite useful for the camera operator and doubly so for the postproduction wizards (the former to which I claim no formal affiliation, but would surely love to be invited to their secret parties - RSVP).
So, if you are burdened by the need to do some green screening and blessed with both math and the necessary tools (editing system, extra drive space, postproduction software) then all you have to do is jiggle a few handles and flush - its not as easy as a toilet in operation, but fundamentally similar enough to warrant the metaphor for our purposes. Okay, enough blathering, lets get into it.
The first problem is that even with well-shot DVCPro HD footage reinforced by a properly lit green screen (minimal green spill, lighting that fits the scene we will composite in to, a hint of separation from the background, etc.) backed up by good wardrobe selection and good camera work, we still can’t pull a great key with most applications due to the nature of DVCPro HD pixels. These pixels are fairly anamorphic (@ 1.333:1) and pose problems in the key because they tend to be pretty jagged in the final result. It looks like a wide step ladder walking down your picture whenever faced with the problem of a curve. Now, this is not to disparage the DVCPro HD codec – I love it, as it does what it was designed to do very well, but it wasn’t designed for keying applications – or at least I hope it wasn’t.
So the second part of our problem is how to get – and keep – an alpha channel for mixing gradients and adjusting lighting patterns in our final composition. Well, that is a problem since DVCPro HD – and most other video codecs – don’t do the alpha channel thingee. You can fake it in most post production applications, but it gets annoying having to do this every time. So what is the solution?
I have tried many different things (see previous statement regarding brute force and fiddling) but have – until now – had little success with the actual DVCProHD codec in this scenario. So, in thinking about the problems facing me (and many others, I’m sure) I decided to try something based in part on solving the specific problems of getting to square pixels with little or no recompression artifacts and producing a usable alpha channel.
The first part of the problem was solved using Final Cut Pro 6.0.2 as the initial editor of choice, since that is what I will ultimately be using to produce an output for the web. I import my green screen footage, lay it into a sequence with DVCProHD 720P 24fps as the sequence setting, as this is the same setting as my footage.
Next, I apply a 4:2:2 chroma smoothing filter (Effects>Video Filters>Key> Chroma Smoothing Filter – 4:2:2) to blur a bit of the chroma without noticeably changing the structure of the image. I then export the footage to a Quicktime movie – but with a few modifications. I change the overall size of the image to 1920 X 1080 pixels using square pixels and the Animation codec. In the Animation codec settings dialogue, I change the color to “Millions of colors +†in order to set an alpha channel.
This doesn’t actually extract a usable alpha channel, it just reserves the space for me. I now have a new media file that is full screen HD 1080p 24fps with square pixels and noticeably smoother chroma that my next application, Shake 4.1, will truly appreciate.
I render it all into a nice extremely hefty and very unwieldy file (4 minutes looks to be about 20GB! Yikes!) but now have everything I want and need to continue on in my work – without the annoying stair stepping of the DVCProHD pixels.
I open the newly rendered file with Shake and use Keylight several times in order to create a good key with a “hold out†mask that lines the image I am keying and helps in flushing the unwanted green away into oblivion, a garbage mask with the rotoshape tool, and a little bit of Dilate-Erode to shrink/grow the resultant outline to fit. This is now used to aid in creating my alpha channel (problem 2) so that if I want to do some goofy stuff with the alpha channel in another application (like Motion 3), it’s there for the taking.
That file is rendered out as DVCProHD 720P 24fps and brought back in to Final Cut Pro and allowed to render back in to the timeline. It is interesting to note that I have yet to get Apple’s Prores codec to work properly in this way – everything comes out as if it were covered in super opaque rainbows – very weird.
So why bump it up in overall resolution (which does not increase the resolution of the picture – yes I am aware of this)? It gives me a more discrete interpretation of the edges, and since I am also applying a Byte node to change the bit working space from 8 bit to 32 bit Float, I also now have very fine control over the edges I will be creating for the edge map. Yay Math! The rest is easy enough from there, and the result is a nice crisp key pulled from the DVCProHD footage.
If you have any suggestions, tips, tricks you would like to add, send them on! I’ll try them out and see what works. Also, check out the attached Shake 4.1 script and see if it helps you out.
Drew is a Producer/ Cinematographer/ Special Effects Geek at ScribeMedia. He has been making stuff for people to watch for over 10 years.
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