A Conversation about Green screen with Daniel Land, Co-Director of Dirty Trousers
This is part two of an ongoing series with Dirty Trousers filmmakers as we follow them through the post production profess (See the Interview part 1).
Today we specifically talk about shooting green screen and the keying process. Daniel Land gives us some insight on what it is like to work on a film filled with effects with a limited amount of time and budge, and what it takes to bring their vision to the big screen.
iChat transcripts
MY: Hello. How is it going?
DL: Excellent....
MY: How is the film coming?
DL: Really well, at the moment. I just spent a week out in L. A. with Mr. Horvath working on the edit and we made some excellent progress.
DL: We've been putting a lot of effort into streamlining our workflow, which is already paying off. It's always better for us to work face-to-face, especially on some of the more complex sequences and we recently were able solve some nagging VFX questions.
MY: Oh, absolutely.
DL: How are you doing? it's been a little while.
MY: Doing good.
MY: NAB is only a couple of weeks away and I'm looking forward to it.
DL: I certainly wish I were going.
MY: It is always a good time.
MY: So, I really wanted to talk to you today about is the trial and error of shooting green screen and keying.
DL: Trial and error is an extremely accurate way of putting it.
MY: Shall we start at the beginning? What sort of studio set up did you use?
DL: When we started the project we were still in our senior year at CCS, so we had the use of the school's production studio. It was a great space, with the curved floor pieces and everything -- although, it was still mostly white at that point.
DL: At one point we took the liberty of extending the green screen ourselves in an overnight painting session, but that was a fantastic space to work with.
DL: We also had four portable boards that had to be created, 4'x8' sheets of foam insulation, for use on locations or when the studio wasn't available. I would say that we used those approximately half of the time.
DL: Just green latex house paint was used on those, which needed to be touched up periodically since they were always getting battle-damaged during the shoots.
MY: A shade of green that I'm guessing isn't very popular for houses!
DL: Not in Detroit, anyway.
MY: Ha ha.
MY: What type of camera did you use (I know you've told me before), and what type of settings did you use? I am wondering if you shot in native format or used HD settings?
MY: I have heard some horror stories about shooting in a non-native format and trying to key that footage.
DL: Primarily {about 90%of the time} the Panasonic DVX-100 with the Anamorphic lenses in standard-def 24P, though we've also used the Sony FX-1 in 1080i HDV format. We never had a problem with the Panasonic footage, but we've certainly had to tangle with the interlacing on the FX1.
DL: Depending on the size of the shoot there would be anywhere from 1-3 cameras shooting, and we naturally had to make sure they were all on the same page, settings-wise.
DL: We kept the shutter @ 1/60th for the most part, but there were certainly deviations from that from time to time. In general we just had to be aware of motion blur in the action sequences.
DL: ... avoiding it, that is, when dealing with green screen.
MY: Absolutely.
MY: Can you talk a little bit about your lighting setup?
DL: Thomas was definitely the lighting expert, but I'd been working with green screen for a few years already. Most of the time we had access to professional light kits, but since neither of us actually owns one, there have been more than a handful of shoots where we made do with construction work lights and various homemade barn doors and reflectors.
DL: In terms of lighting for green screen, there are really only a few things to bear in mind, the most important of which being to always light the subject and the background green separately.
DL: Very few of our scenes take place in fully green screened environments, there is almost always a partial set or location that sets the stage. That could be a challenge because we would light the scene or location and then have to find a way to get the green boards lit properly without negatively affecting the mood that had been set.
DL: That's probably a good time to mention that a large portion of our compositing is in enhancing real environments and locations, and that we really tried to avoid keying wherever possible. There's certainly a good deal of it in the film, but we steered clear of that stagy "chroma key" look that plagues a lot of composited projects as much as possible.
MY: I know what you mean.
MY: So, in post production, how are you keying?
DL: Keylight in AE. The level of control is fantastic and you can't beat it for ease of use.
MY: I have used it quite extensively as well. It's a great tool.
DL: Very...
MY: I noticed some of your footage had tracking marks on our green screen. Can you talk a bit about how they are used?
DL: Those are for any of the moving camera shots in the green screen environments, which are allowing us to accurately position the digital environments behind the actors. That's something else we kept to a minimum as most of our green screen footage is from locked off camera positions... A decision that came as much from practicality as from the classical aesthetic of the cinematography.
DL: To answer the question more technically, we're able to track those points using the Imagineer software as reference to match the 3D camera in the CG scene to the movements of the real camera on set.
MY: You used Mocha?
DL: We're using Mocha.... I can't quite use the past-tense as those are some of the more complex shots in the film and are still underway.
MY: How is it working for you? Did you try other methods of tracking before Mocha?
DL: So far, so good. I'm still getting the hang of the planar tracker, but I love the way it operates. It doesn't give you full 3D data though, so for the couple of shots in which a more organic move is happening we'll still have to turn to pftrack or boujou to export full camera data. Most of the time Mocha seems more than capable, though.
MY: 2.5D!
DL: My favorite dimension...
MY: Back to keying, you had mentioned that you shot with green painted cards. What were your methods for matching the lighting in your footage to the lighting in your background plates?
DL: Eyeball, mostly... We would have print-outs onset of the frames we were trying to match, but it was hardly scientific. As long as the light sources lined up, the rest was largely intuitive.
MY: Did you make any big mistakes during the while shooting/keying process?
MY: Anything you'd like to reveal? :-)
DL: I can't dodge a direct one like that....
DL: Tons, and it's not like we're out of the woods yet... but I will say that I don't think we made the same mistake more than once. Or twice...
DL: There were no major disasters that come to mind, but there is one scene in particular that has been partially filmed on three separate occasions spread out over about the full three years of production...
MY: What happened?
DL: It was a scheduling situation where we were unable to get all four of the actors in the scene ever together at the location at the same time. They're all seated around a table and so we had to employ some green screen work to make it seem that they were all together... Add to that a surprisingly early sundown and you've got lighting all over the place.
DL: Scheduling is probably the most difficult part of production on a film like this where no one is getting paid, and I think any independent filmmaker would agree. Leroy works as a Detroit City Firefighter and he was on call, so we've got nothing to complain about... Hopefully it works well enough that no one will be able to tell which scene I'm talking about, though!
MY: That does sound like a rough shot.
DL: Thomas will be shaking his head... I have to take the heat on that one.
MY: So, how exactly do you match up a shot like that? That has to be a challenge.
DL: It's not incredibly complex, just more than you wanted to deal with on what was supposed to be an entirely practical shoot. Matching the angle of sunlight with the studio lights is crucial, as is the shot framing, but if you get those right than the post work is pretty straight forward.
DL: A dash of roto to clean up anything that overlaps and it can be entirely convincing.
MY: Did you learn any good tricks in the keying process?
MY: ...like tricks to clean up spill, edges or holes in your matte perhaps?
DL: There are little tricks that I've picked up over the years. I always leave a 1-2% transparency on the foreground layers; enough to allow some of the color/value to bleed through but not enough to notice... I guess the biggest thing is to keep your background in mind while shooting the foreground, specifically with regard to the edges. Do you want the light to wrap around the character? Is the environment darker behind his legs?
DL: Etc... It's those questions, coupled with being willing to subtly adapt the background to cooperate with the green screen footage that offer the most latitude.
DL: Over and over we find that if you get it right on set, the post is a breeze. The problems come when you don't, and try to patch it up on the back end.
MY: Right. That makes sense.
DL: And that naturally goes for everything; Editing, VFX, sound... everything. It seems obvious but sometimes you have to learn the hard way.
MY: It's certainly no fun going through frame by frame and hand-painting your matte. :-)
DL: Not. At. All.
DL: One last bit on the actual keying process, and this is something Thomas brought up... A lot of times it helps to start on the most difficult frame in the shot, one that has the worst shadows or holes, then working outward from there. Tackle the toughest part first and then it's all downhill.
MY: Well thank you so much for chatting with me today. This was really great.
DL: My pleasure.
DL: Catch you soon and Keep on Fighting the Good Fight!
MY: :-)