Tonight I saw Piranha 3d. The name misled me into thinking that it was in fact filmed in 3d, but within seconds of it starting I could tell that it wasn't up to the same standard as Avatar, Shrek Forever After or How to train your dragon. 

Converting a film from 2d to 3d would certainly be technically challenging. There simply isn't enough visual information information available to do it correctly, so I think that they make compromises and not-a-little-bit of cheating to get the 3d effect. Unfortunately, it becomes glaringly obvious that things aren't quite right in certain situations. 

To convert a 2d film into 3d they must slice each frame up into different distances, or planes. The first really bad conversion effort I noticed was when there was a police motorbike in a scene. The bike has a clear wind shield at the front, through which you can see the background, in this case a shopfront. Now, because the shield is clear but you can still see it, it's not possible to separate the foreground and the background into different distance planes, so the converters must have had to make a decision about where to place the shield. They chose the logical place - in the foreground. Because the see-through shield also has the slightly out of focus background showing through it, this had the unfortunate and jarring side-effect of bringing the background into the same foreground distance plane. So, you can see a solid wall, then just one section of it behind the shield looks much closer, and then the rest goes back to the correct distance. Effectively it makes the shield look like a weird magnifying glass.

The other really noticeable parts where the conversion was difficult or impossible to do properly was whenever there was a "holey" object in the foreground. For example, there is another shot right at the beginning where the camera pans past a chain-link fence. This is quite a complicated object, so converting it 100% correctly should have been very time-consuming or impossible. It kind of looked like they roughly cut out the bits in between the fence and made them look further away. Looking through the credits they actually have painters and graphic artists on the conversion team, so they must make up the missing information for the other eye. The result was weird blockiness in between all of the diamonds of the fence. A similar thing happened whenever there was a shot through grass or other complicated surfaces/objects in the foreground with important things happening in the middle/background.

There were a lot of moments like these, and a lot of other situations that also looked wrong, i imagine simply because the combination of budget and time constraints plus high technical difficulty made it impractical to get really perfect results. On the upside, all of the parts that were cgi animated looked great :) 

I kind of think that perhaps it would be possible for a computer to automatically convert a panning 2d shot into a correct 3d shot, even in the situations described above. There should be enough information in the set of frames taken together to reconstruct the 3d objects enough for a film. It would probably take some impressive mathematics, but I reckon it must be possible, or perhaps it has even been done already. I have seen software that can build a 3d model from a fee photographs at different angles, so it shouldn't be that much of a stretch to do it for moving pictures.

Anyway just wanted to get that off my chest so I can go to sleep :) the film itself was hilariously bad in a good way, definitely worth seeing if you like b-films or horror comedies - just I would recommend the 2d version :p