Efectos especiales en Spiderman 3

Como suele ser habitual con algunas películas, aquí llego con el making of de los efectos especiales, esta vez de Spiderman 3.



Para la arena se creó Sandstorm, un programa específico para controlar los granos de arena.









Sandman
We see this villain in extreme close-ups, when he’s an 80 or 90-foot tall creature, when he’s the size of his alter-ego Flint Marko and resembles Thomas Haden Church and when he’s a dusty cloud zooming between buildings.

The sequence during which Sandman first rises from a sand pile begins with the camera focusing on a few grains of sand. “You can’t tell if they are rocks or sand,” says Ken Hahn, digital effects supervisor. “There’s no frame of reference.”

It’s a 2,700 frame shot, a powers of 10 type of shot. The camera pulls back, the rocks look like pebbles, and soon we’re in an environment that’s maybe 50 by 50 feet square. All of a sudden, the sand on the floor moves in a coherent manner and we see a creature rising out of it.

“The consciousness of the Flint Marko character is drawing the sand into a physical human form,” says Hahn. “Then the camera sweeps around and we see him from hips to head. That’s all one continuous shot. We had to make sure nothing popped.”

During the transformation, Marko falls back into a mound of sand at one point, then gathers strength, and begins to form into a human shape again. Some of the sand rises up into his shape, some drifts down as if you were to let a handful of sand slide through your fingers.

Animators used a basic human rig based on Church’s body with scaling controls so they could elongate his arms and otherwise change his shape. “We had a kind of squash and stretch capability,” says Spencer Cook, animation supervisor. “We had to be careful though to fit this fantastic character into the real world. In human form, he’s a burly criminal. As Sandman, when he robs an armored car, he forms his fist into a giant hammer that is five times the size of a normal fist and pounds his way through.”

To the audience, it looks like he’s pounding with a fist made of sand. Once the character animators shaped Sandman’s performance, the effects team turned the shape into sand. The process wasn’t simple.

“The tricky part about this character was that you couldn’t decouple animation from effects,” says Hahn. “Normally, artists are able to animate without having to think about issues with simulation, but on this show, we needed to think about the process differently.” As a result, character and effects animators often sat side by side, looked at every frame, and then modified the shapes so that the sand would flow, not suddenly shift from one place to another and wouldn’t hide facial expressions.

“We really tried to obey the physical characteristics and behavior of sand rather than fake something,” Hahn says. “When sand pours off his body, it collides with the ground appropriately and creates atmospherics appropriate for that volume. A few years ago, we would have done particle effects through a noise field for the microdust, but the computing ability nowadays has gotten to the point where we are able to do real simulations.”


By creating clumps of scaled down sand, the effects crew provided a way to accent Marko’s facial expressions and to form details in his fingers. “We also wanted to break up the surface,” says Bloom. “When you pick up sand in your hand, there are always larger grains, and if you shake it, it sorts itself out. So, the lighters covered the surface with sand using shaders that controlled the clumping and size of individual grains.”

One shader might scale every grain randomly. Another might multiply every fifth grain by a factor of two. A third might look at the curvature of the mesh around the eyes and eyebrows and scale the grains down to preserve the shapes.

Lighting the sand was particularly difficult. Because it was composed of micro-surfaces, it didn’t react like a continuous surface. That meant, for example, the facets in the grains of sand picked up hits from key lights and sprinkled light all through the face. To reduce the flickering, animators toned down the performance.

“Animators are used to seeing the skin and the skin is the final rendered element with textures added, but in our case, that wasn’t always true because sand had its own discrete motion,” Hahn says. “If the facial animation was too twitchy, we’d get flickering artifacts. We didn’t have continuous well-behaved surface normals. We had micro-facets always in motion, so any slight shift caused the lighting to change across the surface.”

To optimize the simulation calculations, the team devised a method for breaking the SphereSims into parts so that they could, for example, run ten simulations and then combine the results at render time. “Because we aren’t dealing with rotation or any non-spherical shapes, and because we simplified the rigid body equations, we could run multiple simulations with upwards of a million grains at a time,” says Bloom.

To optimize rendering, the programming team wrote a RenderMan plug-in that provided a proprietary method of memory management and could dice each scene into little buckets. Although RenderMan has such memory features built in, for Sandman, Imageworks needed finer control.

“We could push a maximum number of particles without having to slice the render in Z,” says Bloom. “We ran a pre-process before rendering started that looked at how many particles were in the scene, chopped them into boxes, and once done, released all the memory.”

As a result, the team rendered full 2K resolution film frames with motion blur and 480 million sand grains without additional memory optimizations.