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Stereo Conversion: The Art Of

Stereo conversion, or dimensionalization as it is sometimes called, is the process of making stereo images from non-stereo traditional 2D images.

Many people argue that if you want a film in stereo you should shoot it in stereo. Yet many studios are warning post houses that if they work on stereo projects, even ones shot in stereo, there may well be a need to convert some footage and that high quality conversion is an important tool in the box of any effects house.

Stereo conversion is also needed for converting older films – such as the Star Wars franchise. John Knoll (ILM) is overseeing the stereo conversion of every Star Wars film for director George Lucas and in a recent AWN article he expressed the view that, “You can’t rush it and it’s an iterative process, and if you’ve got a gun to your head and you’ve got eight weeks to convert a 2,000-shot show, it’s not possible to maintain the level of quality control that you need.” In the case of Star Wars conversions for which Lucasfilm “will be (mostly) using outside vendors,” he also pointed out that he had been vocal in saying that past efforts “were victims of a too rushed production schedule and a too low budget.” Few people doubt that Knoll will deliver anything but cutting edge results.

3D animated films, such as Toy Story 3, Tangled and others find it easy to correctly generate stereo imagery from either stereo-RenderMan use or just rendering the entire scene from two similar but offset virtual cameras. For everything else live action, normal stereo production is hard. Even Avatar required tiny amounts of stereo conversion, for example the opening macro eyeball shot was far too close for a stereo camera rig to film, and this first shot of the film was converted stereo from 2D.

One of the leading company in this area today is Prime Focus, who after a battering of critical opinion on the conversion of Clash of the Titans, recently converted the film The Chronicles of Narnia: The Voyage of the Dawn Treader, which stands today as perhaps the best stereo conversion done thus far. For the last Narnia film, the decision to go stereo was made after principal photography but before the visual effects were completed. This allowed a partial hybrid approach of the some 1900 edits in the film. Some 10% or 190 odd shots were fully stereo generated 3D shots. The rest were converted mainly by Prime Focus. Interestingly, for some shots this conversion was not done by Prime to the final shot, but in partnership with the relevant effects house.


The problems with generating a second view or second ‘eye’ for stereo conversion are:
•  The parallax effect. That means a second eye will see around things the original eye won’t and thus there is missing background information to be replaced.
• A depth map is needed of the scene to determine the correct distribution of the objects for the second eye. While amazing work has been done with programs such as Ocula by the Foundry (see below), the process is far from automated.
• Cardboard cutouts and the need for roto. Not only is a roto required for the outline of any character in shot, if they are closer than say a wide shot, internal mattes are also required to generate different depths for different parts of their bodies. A character could easily have 7 rotos in addition to their outline for features such as nose, eyes etc and all of these must be conceptually and logically correctly placed based on z depth.
• Projection. While some shots respond well to re-projecting or ‘camera mapping’ the mono footage over 3d models and then filming the stereo by rendering the 3D scene from two virtual cameras, this rarely works well for people in movement as the difficulty of generating accurate 3d models to map onto, renders the approach extremely expensive. The technique works well for building, hall ways and other regular and mostly rigid body solutions but most films are about people – normally with soft edges such as hair etc.
• Shot design. Most stereo films are shot with an understanding and consideration of the stereo nature of the experience. A mono film may be poorly composed from a stereo point of view. For example, most staging of a stereo scene would have a distribution of the objects over the immediate foreground, avoiding the clumping that may seem odd when all the props are at the back of a room. Yet in mono although this shot may be extremely creatively valid, it is only when the shot is converted does it seem empty or oddly distributed.
• Singular depth resolution. One of the trickiest problems is say glasses on a live action character. While the glasses have a depth from the camera, the eyes behind them are further away. If incorrectly dealt with, the eyes of the character would appear to be printed on the glasses, not behind then glasses, but conversely the reflections on the glasses should not appear to be drawn on the actual eyes of the character. Add to this one tends to focus on the eyes of character in a close up and one could guess that this is perhaps a contributing factor as to why there was scheduling concern that led to a film starring a certain famous glasses wearing boy not being stereo converted recently. Similarly, hair is extremely complex and in a single close up may require extremely complex and fine roto work.

Key to good conversion

• A strong working relationship with the overall supervisors.
• A strong relationship with the vendors, so that relevant files such as keys and mattes can be provided.
• Pre-production planning. As with all vfx work, the earlier the conversion company can be involved the better.
• Time. Given the huge volume of work, ensuring the schedule allows time for roto and volumizing is vital.
• Speed of camera moves, framing and staging can all affect conversion. If you can isolate the key stereo moments in the script and shoot those knowing how they will be converted, you can produce a more impactful final conversion.
• Good asset management and asset tracking.

If Titans taught the industry any overall lesson, it is that stereo conversion should not be a process ‘tacked’ on the end of a film’s production. Sometimes a film is converted years after principal photography, but if the conversion team can be involved before principal photography it can aid the creative, schedule and the budget.

•Make a good film in stereo not a good stereo film.

This article has been written by Robert Germosa, the editor at Cinemagic; thank you for contributing :)

  • article is excellent..informative all the best..

    suresh nair
    Avid specialist
    Kingdom of Bahrain


    April 19, 2011

  • thanks! super kudos to you Suresh! hope u are doing well there in Bahrain…


    April 19, 2011

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