What is InviziTrak?

InviziTrak is a piece of filmmaking equipment that allows green screen tracking markers to be recorded separately from the main footage, allowing the compositor to have a set of marker-free frames for chroma-keying and a separate set of frames with tracking markers for match-moving.

How does it work?

Footage is usually shot with a shutter angle of 180 degrees, which means the shutter is only open for half of the frame period. Mechanical film cameras needed to use the time when the shutter was closed to advance the film to the next frame, but digital cameras are actually able to acquire another frame when the shutter would normally have been closed anyway. This can be achieved by setting the camera to shoot at twice the frame rate, and setting the shutter angle to 360 degrees. InviziTrak uses lasers to project tracking markers during this additional frame, and turn them off during the normal frames. The resulting footage will have frames that alternate between having tracking markers and being marker-less.

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Post Production

In post production, the frames with tracking markers can be split off into a separate stream, leaving the footage identical to footage shot at the normal frame rate with a 180 degree shutter angle. There is no loss in quality, motion blur, or light exposure, except now there are no longer any tracking markers that need to be painted out. The tracking markers are all contained in the separate stream, which can be used to calculate the match-move. The match-move will be offset by half a frame, but interpolating the coordinates by half a frame will produce an extremely close approximation in most cases. For jerky shots, a select number of tracking markers can be left on for both set of frames, which improves the match-move but still greatly reduces the number of tracking markers that need to be removed. An additional benefit of InviziTrak is that the duration of the laser pulse can be as short as 1 millisecond, which greatly reduces the amount of motion blur on the tracking marker on fast camera movements.

 
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Hardware

InviziTrak contains a few pieces of hardware: a sync transmitter, one or more tracking marker projectors, and an iPad with a custom app for control. The sync transmitter extracts the shutter timing from the HD-SDI monitor feed or a TTL shutter sync pulse from the camera, and wirelessly synchronizes the tracking marker projector with the shutter (synchronization from genlock, analog video, and other camera sync outputs will become available in the coming months). The transmitter can synchronize multiple projectors within its radio’s range, typically a few hundred meters.

Each tracking marker projector can control six laser modules. Each laser module is mounted on an adjustable ball joint and can be fitted with different optics to generate dot grids or crosshairs to suit the environment. The brightness of the lasers is similar to that found on green laser pointers; bright enough for most lighting conditions, but not so powerful as to be hazardous. The lasers are also dimmable to adapt to various lighting conditions.

The tracking marker projector comes with several mounting options: Junior/Baby pin for stands, 100x100 VESA for monitor mounts, and Omega twist-lock clamps for overhead lighting grids. For power, it has a 110-220V Neutrik PowerCON input and output connectors, and it also has a Anton Bauer battery bracket for portable use.

InviziTrak comes with an iPad controller with a custom app that lets you adjust the projector’s timing and laser settings. Multiple projectors can be synchronized to the same transmitter, and each laser can be individually configured. When viewing on a monitor at the final desired frame rate (e.g. shooting at 48 fps and viewing at 24 fps), a switch on the iPad app makes the projector place the marker on the alternate set of frames, so that you can preview both sets of frames.

 

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