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Using Pre-vis in Post-Production
Post-production on visual effects movies can be extremely intensive and time consuming, with shots requiring a complex interaction of different elements from several sources. With some elements dependent on others, the trick is in avoiding bottlenecks that will hold up the entire production.
A basic, traditional post-production workflow looks more-or-less like this:
1. Capture your footage.
2. Create a rough cut of your project in your NLE, so you know which shots you’re using and don’t waste time adding effects to stuff that will get cut out later.
3. Export the effects shots to whichever programs you’re using and generate your effects.
4. Render out the completed effects and slot them into place on your NLE timeline.
This workflow is fine if you have a handful of localised effects shots but can quickly become rather convoluted if you’re dealing extensively with greenscreen sequences or composited characters, whereby crucial parts of shots might be ‘missing’. How do you create even a rough edit when most of your shots don’t yet contain the correct timing and framing?
I’ve recently been dealing with these exact problems during the creation of the next FXhome ‘Learn’ DVD, which is tentatively titled Learn: Sci-fi Adventure. Following in the footsteps of the Learn: Beach Landing & Superheroes DVD, the new one focuses specifically on the sci-fi genre, taking its cues from all your favourite space operas and guiding you through the creation of some classic visual effects.
My god, it’s full of greenAssault on Igneos, the short film that accompanies the tutorial, is a 2m30s actionfest set on an alien planet populated by evil cloned soldiers (played by Chris Puttock of Beach Landing fame) and follows an unsubtle infiltration by a highly skilled, monk-like character (played by stuntman Adam Kirley, who you may have seen in such films as Casino Royale, Indy 4, Terminator: Salvation and, er, FXhome’s Superheroes).
Although we’d initially intended to shoot on a real location, access issues resulted in us filming the entire short in a makeshift greenscreen studio. While I’ve obviously shot a large amount of greenscreen over the years, it was the first time I’d had an all-greenscreen project. In the end it was quite appropriate as it largely mirrors the way the Star Wars prequels were shot, but it has led to some challenges in post.
Guesswork editingEvery project I’ve ever worked on, I’ve followed the workflow mentioned above. Once I embarked on compiling a rough edit of Assault on Igneos, however, it quickly became apparent that it wasn’t going to be so easy.
Some shots that would eventually contain three characters only featured a single actor, making it largely impossible to time the editing of the shot before completing some of the compositing. A lightsword fight halfway through had no sense of geometry, every angle cutting to a near identical green room, with the actors seeming to shift positions in a disjointed manner. Shooting greenscreen also tends to result in a lack of good coverage, reducing the amount of improvisation that can occur on a real set or location.
A very, very rough edit was assembled, partly through guesswork. It made for surreal viewing, everything lacking context and any sense of pacing or excitement. It was impossible to judge whether the edit was successful.
The conundrum is that the finished backgrounds won’t be ready for a considerable time, as work on them could only start once the rough edit was available. Working with just the green background wasn’t a viable option – from effects, sound, music and editing standpoints it’s too vague.
World placeholdersKeying out the green wasn’t the solution either – in that case the short became even more abstract, with character floating in a black void. Some kind of background was needed, which is why I turned to the idea of pre-visualisation for the first time.
Pre-vis uses rough, temporary elements to depict the framing and atmosphere of a shot, in the absence of the final pieces. It’s normally used in pre-production to outline a scene prior to filming, which can be useful if lots of visual effects are involved, or if timing is crucial. The pre-vis can show actors and crew exactly what is supposed to happen in the scene far more effectively than the bare script. In this case it would help all those working on the film get a better idea of the finished product, and would provide valuable reference to the artists creating the final backgrounds.
Although I originally intended to sketch the backgrounds by hand, I realised early on that even this would be too time consuming and inaccurate, given the number of different angles involved. While the hand-drawn approach might have been useful to give the short some context, it wouldn’t be of any use to the background artists.
Google SketchupIf you’ve never used Google’s free 3D software Sketchup, now’s the time. Even if you’re already familiar with more fully featured 3D software, Sketchup still provides an incredibly quick and efficient workflow for creating quick mock-ups. Meanwhile, if you’ve never touched 3D before, Sketchup provides an extremely easy interface that works off intuitive architectural design concepts rather than standard 3D modelling techniques.
Using Sketchup’s tools I was able to create a low detail, 3D versions of the film’s locations. Once the basic geometry was created I could then move and rotate the camera to any position, allowing for all the different angles to be created easily and vastly faster than having to draw each one from scratch.
The Sketchup renders were then imported into VisionLab along with the greenscreen video and composited together. Additional elements of photographic reference were used to complete the shots, resulting in a rough cut that finally made sense to a viewer.
All the other effects in the shots can be worked on to their final detail level, while the pre-vis is handed over to the artists responsible for the final, full quality backgrounds. Once the finished backgrounds are ready they can simply be dropped in to replace the pre-vis versions.
ConclusionsI’ve definitely found this to be a very efficient way to work with extensive greenscreen projects, providing the level of detail you need to complete your rough cut and to give context to shots, without having to wait for the final versions of background elements.
When we were at NAB last month several filmmakers were singing the praises of Sketchup for use as a set and lighting guide. The DP on the television show 24 frequently uses it to test out lighting setups, for example.
We’ll have more about Assault on Igneos and the Learn: Sci-Fi Adventure DVD in the near future!