Hippos on tightropes: perfecting physics

How do you control the physics of a new universe?
22 August 2016

Interview with 

Prof Alejandro Garcia, San Jose State University


Bouncing ball animation


Bouncing ball animationFilm animators need to be very well versed in, not only art, but physics. This is because our brains are very sensitive to the way in which the world around us should work. So if there's something slightly wrong with the way an object falls, or some water moves, it can trigger a mental reality check and break the spell of the movie for the viewer. Georgia Mills caught up with physicist Alejandro Garcia from the San Jose State University who has been a consultant for DreamWorks, and is a strong believer that fictional films need to obey the laws of physics...

Alejandro - In creating a film the director has to think about what kind of world is being presented and how things are in that world. And usually the audience will have more connection with a world that feels real, that is believable, and having believable physics goes a long way towards creating a believable world that can serve for the suspension of disbelief for the audience to actually be engaged.

Georgia - So how would you go about creating the sort of realistic rules of physics?

Alejandro - Well animators - they start out by studying basic motion and recreating basic motions and they realise that, in some cases, in films the rules of physics will be broken. But, for the most part they want to follow the laws of physics and for effects animation like fire and smoke, they want to have those look believable as well.

Georgia - Is it just a case of looking at this in the real world and copying it, or do you have like equations to make the perfect fire?

Alejandro - Oh, there's definitely equations. And I should mention that all the studios have scientists that design computer simulations (fluids), and usually these are folks who were classically trained with PhDs in physics or mechanical engineering, and so forth and their simulations are similar to what is done in regular physics research.

Georgia - So it really is a case of scientists at the drawing board, I guess?

Alejandro - Yes. Now there is one major difference in that the scientists that work in a studio, they have to be able to have a simulation which is what they call 'art directable.' For example, they might do a simulation of smoke and show it to the director and the director will say "well, I like that but I need the smoke to move this way and I also need the smoke to look friendlier - the smoke is looking too angry." Seriously, this is exactly the kind of feedback they get and so they have to go and adapt their simulation to fit with the vision the director has for the story at that point.

Georgia - ; I suppose, in a similar vein, you mentioned getting the physics right but a lot of animations have quite zany, whacky things going on, which completely defy the laws of things like gravity. How do you go about that?

Alejandro - Often the laws of physics are broken as a way of relieving tension because that removes often the danger that's possibly in a scene, whereas scenes that are more dramatic tend to have more realistic physics. So adjusting the accuracy of the physics can be used as a plot device in a film.

Georgia - So could you perhaps give me an example of a film you've worked on and how you (sort of)  worked on this process?

Alejandro - Yes, so this was Madagascar III. In the film there's a lot of circus acts so I worked with the animators in terms of doing trapeze work, tightrope. You know, if you have a hippopotamus and a giraffe on a tightrope, how do you make them move so that it actually looks believable when they're trying to maintain their balance?

Georgia - I love that - you've got to make a giraffe and a hippo on a tightrope seem believable?

Alejandro - Exactly, exactly! And I think we were very successful in that.

Georgia - In terms of Madagascar III - that's an example of a film, I guess, that was built up from the ground up.  But then there's these films that animation is layered on to real life like using things like motion capture - how does this differ from the processes you mentioned?

Alejandro - Motion capture is usually for films which are live action and they are trying to take a character that is mostly human but is modified...

Georgia - ; Like Gollum, for example?

Alejandro - Like Gollum, exactly - he's the classic example.  So the limitation of motion capture is that you are always going to have a character which is moving like a human. You can take that performance and the animators can manipulate it, but you basically start from that, a human performance. And one example is when you have a very large character, like in Jack the Giantslayer. So the giants were all done with motion capture but there was a minimal manipulation of the timing of them.  When you see them, sometimes they don't feel giant because they're moving exactly like a regular size human. One thing which helps in that regard is actually slowing down the timing - that was used in the most recent Captain America civil war, where one of the characters is very large at one point in the movie.  And they did motion capture for him but then they slowed the time down, I think by a factor of three, which then makes the character feel gigantic.

Georgia - And just finally - obviously crafting a story in a film is a very difficult thing. But just in terms of getting the physics right - what is the single hardest thing someone can ask you to do?

Alejandro - The one thing which animators worry about all the time is getting a sense of weight for characters. This is an enormous challenge because every point in the motion you have to determine what kind of forces are acting, and also what kind of reaction occurs when that character is interacting with another character. So the biggest challenges is multiple characters interacting with each other, or with elements in the environment.

Georgia - So an animated battle scene, for example - that would be a bit of a nightmare?

Alejandro - Exactly! That's probably one of the most challenging sort of shots to do.


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