Making a 3D movie: understanding the workflow

Making a 3D movie has many challenges all of its own. No producer or cameraman sets out to produce 3D that will be difficult to watch but with the intricacies of managing a two camera 3D rig combined with the pressures of the shoot, problems happen. A rig that was set up at one point in time can easily become uncalibrated and the issues will only become apparent in the post production environment. Even if these accidents are avoided there are inherent imaging issues generated by camera geometry that need to be addressed before post production in a creative sense can start.

Avoiding headaches

Professional 3D camera rigs use high quality lenses whose size and weight often require the use of a mechanically complex mirror rig. These provide the ultimate in flexibility in shooting allowing the interaxial centres, the distance between lens centres, to be adjusted from zero to well above the distance measured between human eyes. They also allow for cameras to be converged, as our eyes do when looking at objects close up.

On these rigs, smaller cameras and lenses can be effectively used in a side-by-side mode with the minimum interaxial distance governed by the camera body and lens size. With both these shooting formats there is potential for artefacts that will need to be addressed; however planning your shoot and setting up your cameras carefully can save lots of headaches during post.

3D Post-production

At the front of the post process will be all the work needed to set up the left and right eye sequences to minimise viewer discomfort. The list of checks and corrections needed includes the following:

  • Basic orientation: left and right frames may need to be flipped or flopped to make them usable.
  • Colour imbalance: as we are using separate imaging systems we cannot guarantee that the colour balance, gains and gammas will match.
  • Basic geometry: correcting for size and camera axis rotation errors. No two lenses are the same; this is especially true for zoom lenses which add additional complexity to the basic geometry.

Getting these issues corrected will then reveal the finer geometric issues of keystoning created by the converging of the cameras.

Your eyes are able to tolerate quite severe errors for a short period of time but we are planning to produce a full programme or evening's viewing and leaving these issues uncorrected will cause headaches in your viewers, and often they will not know why.

The most common issues that cause discomfort are:

  • Excessive Depth Budget, the range of depth from the front to the back of the scene, causing the eyes to converge or diverge.
  • Vertical disparities between the left and right eyes causing the eyes to twist as they try to make visual sense of the scene.
  • Having the depth of the subject point switch rapidly at cuts causing viewers to change their point of convergence continuously.

If you get any of these set-up factors wrong, you may find your movie gives your audience a headache, but having addressed these we can move on to the creative task which was much more readily reached in 2D post production.

3D Post-production is a challenge - even with the best available equipment. Double the data volume in two hi-res streams needs to be handled in sync and in realtime. You want to have random access to any frame or any clip at any time, with 100% reliability and a toolset that will work seamlessly from ingest to export.

You also need equipment that enables you to move your edit between suites, yet keeps everything synchronised and at the stage you reached with your work.

Managing the Depth Budget

Before Stereo 3D, you never had to worry about the Depth Budget. What is the Depth Budget? It's the limits between negative parallax (in front of the screen plane) and positive parallax (behind the screen plane).

These limits are usually surprisingly small and can be expressed as a percentage of screen width for TV production (large screen is a different matter), such as the Depth Budget of 2% positive parallax and 1% negative parallax advocated by experienced stereographer Vince Pace and used by Sky TV in the UK. If you keep within the Depth Budget - with occasional short-term effects not exceeding 4% positive parallax and 2.5% negative parallax - then eyestrain will be kept to a minimum, and the production will be comfortable to watch for two to three hours.

Be aware of the target screen

There are different issues to consider during Stereo 3D post, depending on the screen size - whether the finished piece will be viewed on a TV at home or on a big cinema screen. 2% of a television screen is a lot smaller physical dimension than 2% of a cinema screen. In the latter case the 3D effect will be exaggerated and may cause the viewer's eyes to converge or diverge unnaturally causing strain.

Plan well and choose your tools for post carefully and you will end up with a high-quality end product that is comfortable and enjoyable for audiences to watch from beginning to end.

David Throup is Group Leader, R&D at Quantel. The company develops innovative, world-leading content creation systems for broadcast, post and DI. Quantel products deliver at SD, HD, 2K, 4K and Stereoscopic 3D.

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