If you caught this past weekend's big blockbuster, Marvel's The Avengers, then you may have watched it in 3D.  Many of you may or may not have been aware that the film was first shot in 2D and converted to 3D after the fact.  Wondering what that process involved?  Or why some films would shoot in 2D and then convert, vs. shooting stereoscopically from the start?  In a fascinating in-depth article, fxguide delves into the many challenges vfx artists face when converting 2D films to 3D -- also known as "stereo conversion" -- revealing the kind of pain-staking labor and ingenuity required as well as some of the aesthetic differences between the two formats:

Now, 2D-to-3D got a pretty bad rap after Clash of the Titans came out -- it was one of the first films rushed through a 2D-3D conversion job by a studio looking for that 3D cash-in.  But since then, studios and vfx companies have put significant resources into advancing the tools and workflows that make stereo conversion possible -- in the process making for some truly good-looking 3D films out of 2D shot films.  They have applied the process to older movies like Titanic and the Star Wars saga, and newer ones like Transformers: Dark of the Moon and John Carter -- either to your general entertainment or chagrin.  Regardless of how you may feel about the reasons for converting movies, reading through the guide you'll learn a lot about the sheer challenges of doing things like transforming a busy, rainy street scene -- a process that may involve rotoscoping individual drops!  Not only that, it also drives home some of the technical challenges of 3D in general -- whether it's a conversion job or not.  Take, for example, the idea of "floating windows":

One technique that Bob Whitehill has used extensively, but was not used on films like Avatar is ‘floating windows’. This technique aims to get around the limits in one’s stereo ‘budget’, in relationship to the edge of the screen. If a character is closer to camera – as in an over the shoulder shot – it is hard to have them sitting in stereo space closer the audience (ie. ‘in the space’ between the screen and the audience, since their body is cut off by the bottom of the real cinema screen). As our eyes can see the stereo image is floating closer to us than the edge, yet our minds ‘know’ the person must be on the other side of that screen or window, we mentally reduce the stereo effect.

In other words, the illusion is lessened since we know that someone cant be close to us AND further away behind the sharp real world edge of the cinema screen. The trick that Bob Whitehill did not invent but radically perfected at Disney was to float another dummy edge of screen between the viewer and the stereo violating back of the shoulder. This floating window is rendered in the footage, but tricks the audience into thinking the edge of the real world cinema is closer than it is – hence not violating the edge (this effect is known as an edge violation) and thus we regain some of the stereo budget. That is the useful working space one has to place things between a sensible close and a sensible far in stereo terms.

With a section that lists common techniques and tools of the trade, along with case studies of many of the movies mentioned above, this really is a great read if you're interested in stereoscopic film, period.  For the full guide, check out the article here.

I watched The Avengers in 2D, mostly because I get headaches with 3D movies (even ones purposely shot in 3D).  I thought it looked great.  Did you get a chance to watch The Avengers in 3D?  How about the Star Wars conversion?  How did they look?

[via fxguide]