How good is the 3D tracking data?
The real time tracking data is quite good, but in cases with a very exposed live action/CG interface, you’ll be able to see a few pixels of slip. In closer shots, without ground contact, the data is basically final quality.
In cases with intense ground contact, there are a couple of routes to take. We built a pipeline to take our data into Boujou and Syntheyes (as well as Nuke, AE, etc.) so you can use the Previzion data as a ‘seed track’ to drive the Boujou or Syntheyes solve. If you’re using 3D Equalizer or Matchmover, it would be very straightforward to add an export for these as well. This usually helps the solve quite a bit, as our focal length measurement is very accurate.
How do you handle lens distortion for wide angle lenses?
We measure lens distortion as part of our lens calibration process. We render correctly distorted geometry in Previzion, to show as clean a match as possible. We store the distortion data along with the rest of the lens data on a per-frame basis in our Collada data take files.
Getting the distortion into post production, however, is a bit trickier. Each renderer has its own parameters for measuring distortion. Fortunately, we wrote a whole separate tool called Take Converter to extract the data from Previzion takes into a variety of external formats. Adding another exporter is very quick for us, so it’s a matter of targeting which specific pipeline is in use.
The basic question is figuring out where to apply distortion in the pipeline. It’s usually done in the compositing stage.
Can Previzion work with film cameras?
Previzion requires a HD video signal to work with; this can be generated by the new Arri HD video taps. The Arri taps record the full width of the imaged emulsion, which is critical for Previzion’s through-the-lens calibration system.
Most film cameras accept LTC analog linear timecode for audio synchronization; this same timecode can be embedded into the video tap’s HDSDI signal and used to match the motion of the camera move data to the film frames.
Standard definition video taps would need to be upconverted to HD before working with Previzion. There are some converters that should do this well (AJA’s FS1). Upconverting a SD signal may cause difficulties when trying to calibrate lenses; a workaround in this case would be to calibrate the lens on a 35mm digital camera, and then move the lens over to the film camera. The lens parameters remain identical when changing from one camera to the next.
Is Previzion’s keying final quality?
Under the right conditions, the keyer is capable of final quality results. However, in practice, the variation in background lighting is usually pretty large when the camera moves significantly, with multiple shades of green all over the place depending on the incident lighting angle. As a real time preview/editing aid, however, it’s great.
For most projects, it’s best to plan for traditional keying in post, but the great thing about the live preview is that it catches all the big greenscreen problems while there is still time to do something about it. When the DP can light the live action while seeing the whole composite come together, it makes for a radical improvement in matched composite lighting. If it works with our keyer on the stage, you’ll be OK in post, as all of the keyers use similar math.