Hi guys,
I just wanted to share some information on the shots we did for White
House Down.
First off, there's an article in fxguide that explains a bit what we did :
http://www.fxguide.com/featured/action-beats-6-scenes-from-white-house-down/
And here is some more details about how we did it :
We built upon our ICE based City Generator that we created for Spy Kids
4. In SK4, all the buildings where basically a bunch of instances
(windows, wall, doors, etc.) put together using Softimage ICE logic to
build very generic buildings. ICE was also used to create the
streetscape, populate the city with props (lamp post, traffic lights,
garbage cans, bus stops, etc.), distribute static trees and car traffic.
Everything was instances so memory consumption was very low and render
times where minimal (20-30 minutes a frame in Mental Ray at the time).
The city in Spy Kids 4 was very generic and the cameras where very high
up in the sky so we didn't care as much about having a lot of details
and interaction on the ground level and we didn't really need specific
and recognizable buildings either.
The challenge in White House Down was the fact that it was Washington
and we needed to recognize very specific landmarks so it needed to be a
lot less generic. The action also happens very close to the ground so we
needed to have a lot more detail on the ground level and there needed to
be a lot of interaction with the helicopters that are passing by.
So we modeled a lot more specific assets to add more variation (very
specific buildings and recognizable landmarks, more props, more
vegetation, more cars, etc.). We updated our building generator to allow
more customizations. We updated our props and cars distribution systems.
They where all still ICE based instances, but we added a lot more
controls to allow our users to easily manage such complex scenes. We had
a system to automate the texturing of cars and props based on rules so
we could texture thousands of assets very quickly. Everything was also
converted to Stand-Ins to keep our working scenes very light and leave
the heavy lifting to the renderer.
Which brings me to Arnold.
We knew the trick to making these shots as realistic as possible would
be to add as much details as we possibly could. Arnold is so good at
handling a lot of geometry and we where all very impressed by how much
Arnold could chew (we where managing somewhere around 500-600 million
polygons at a time) but it still wasn't going to be enough, so we built
a deep image compositing pipeline for this project to allowed us to add
so much more detail to the shots.
Every asset where built in low and high resolution. So we basically
loaded whatever elements we where rendering in a layer as high
resolution while the rest of the scene assets where all low resolution
only to be visible through secondary rays (so to cast reflections,
shadows, GI, etc.). We could then combine all the layers through deep
compositing and could extract whatever layer we desired without worrying
about generating the proper hold-out mattes at render time (which would
have been impossible to manage at that level of detail).
In one shot, we calculated that once all the layers where merged
together using our deep image pipeline, it added up to just over 4.2
billion polygons... though that number is not quite exact since we
always loaded all assets as lo-res in memory except for the visible
elements that where being rendered in high resolution. We have a lot of
low res geometry that is repeated in many layers, so the exact number is
slightly lower then the 4.2 billion polygons reported, but still... we
ended up managing a lot of data for that show.
Render times where also very reasonable, varying from 20 minutes to 2-3
hours per frame rendered at 3K. Once we added all the layers in one
shot, then it came somewhere between 10-12 hours per frame.
We started out using Nuke to manipulate our deep images, but we ended up
creating an in-house custom standalone application using Creation
Platform from Fabric Engine to accelerate the deep image manipulations.
What took hours to manage in Nuke could now be done in minutes and we
could now also exploit our entire render farm to extract the desired
layers when needed.
Finally, the last layer of complexity came from the interaction between
the helicopters and the environment. We simulated and baked rotor wash
wind fields of air being pushed by those animated Black Hawks using
Exocortex Slipstream. That wind field then was used to simulate dust,
debris, tree deformations and crowd cloth simulations. Since the trees
needed to be simulated, we created a custom ICE strand based tree system
to deform the branches and simulate the leaves movement from that wind
field. Since the trees where all strand based, they where very light to
manage and render. We also had created a custom ICE based crowd system
for the movie Jappeloup that was updated for this project. We bought a
couple Kinect to do in-house motion capture and build a list of
animation cycles of agents reacting to helicopters flying over their
heads. We then procedurally analyzed the movement of the helicopters per
shot and randomly select an animation cycle from the list and time the
cycle in a way that the agents would react when the helicopters passed
within a specified distance of the agents.
These where definitely the most complex and technically challenging
shots we ever managed here at Hybride. It was achievable thanks to the
evolution of a lot of in-house tools we had created over the years and
obviously, thanks to a lot of very talented and hard working people in a
very short span of 3-4 months.
Mathieu Leclaire
Head of R&D
Hybride Technologies, a Ubisoft division
