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'Slither': Meet the CG Parasites

Alain Bielik wraps himself around the CG creatures and other creepy vfx that inhabit Slither.

The 261 vfx shots in Slither included creature extensions, parasites, an alien world, stunt enhancements, matte paintings and an asteroid collision. Courtesy of Digital Dimension. All images © 2006 Universal Studios. All rights reserved.

The man just loves monsters. Before embarking upon writing and directing Slither (released on March 31 by Universal), James Gunn had written Scooby-Doo and Scooby-Doo 2: Monsters Unleashed and Dawn of the Dead. This experience came handy when he started preparing Slither, the story of a small town being taken over by an alien plague that turns residents into zombies and all kind of creatures. The many creature shots were split among prosthetics effects crafted by Todd Masters and digital imagery created under the supervision of production visual effects supervisor John Gajdecki (The Santa Clause 2).

We created 261 shots of which I think about 10 were easy Gajdecki says. The rest was CG creature extensions, CG parasites all over the place, the usual number of wire removals, a flashback on an alien world, stunt enhancements, matte paintings and even an asteroid colliding with the earth! Nine different vendors produced about half of the shots; the rest was created by an in-house unit that I set up. They included:

  • In-house unit: 117 shots of tentacles, cat scans, CG creature, general effects.

  • Meteor Studios: 23 shots of impregnated a character exploding into 25,000 parasites.

  • Image Engine: 26 shots of slithering parasites.

  • Digital Dimension: 14 shots of tubules, heads being split open.

  • Technicolor Creative Services: 15 shots of alien creatures on an alien world.

  • Kleiser/Walczak: 14 shots of tentacles wrapping around actress, CG deer. This was a last minute help that was very much appreciated.

  • The rest of the shots were created by Rocket Science, Invisible Pictures and Switch Studios.

Models for Grants tentacles were built in Maya 6.5. Once the tentacle animation was approved, each shot was rendered up to 17 layers in mental ray. Courtesy of Digital Dimension.

Do It Yourself

Having run vfx companies in the past, Gajdecki was comfortable with setting up a production visual effects unit. Louise Barkholt and Joan Collins Carey joined the team as vfx producers, Geoffrey Hancock as digital effects supervisor; Steward Burris was the animation director and Nick Hsieh the lead compositor. At the peak of production, the unit comprised of 17 artists working on two shifts, as well as several co-ordinators to support them.

While the in-house department covered about half of the shots of the movie, the majority of its effort was focused on Grants tentacles. Grant is a human that has mutated into a nightmarish creature. We built the models in Maya 6.5, based on textures of the physical prosthetics that we shot in a light tent, Gajdecki explains. We photographed them with a Cannon Ds1 using a 100mm lens to keep a flat perspective on the tentacles. As the shots were locked, the plates were directly sent for clean up, having the prosthetic placeholders, their rigs, their rig shadows and occasionally the puppeteers painted out. Once the tentacle animation was approved, each shot went into the production pipeline where up to 17 layers were rendered in mental ray.

  • 1. Base color pass

2, 3. Vein pass and a splotch pass, both of which were used to color correct variety into the tentacles surface

4-7. Assorted ray-traced specular passes to create the wet sheen

8. A Fresnel pass provided pure white where the edges of the tentacle were 90 degrees to the camera, and black where the tentacle was dead on to the camera. This allowed the creation of translucent slimy edges.

9. Reflection Pass

10-11. Gradients and masks along the length of the tentacles to change their appearance from the tip to the joint with Grants body.

12. Z depth pass for aerial perspective

13-15. Key, fill and rim light passes. Different versions were baked into the RGB channels of each render

16. Shadow passes

17. Ambient occlusion pass to cut everything that needs to be removed out and what needs to come forward back in.

All the layers were finally composited in Digital Fusion.

For shots featuring 25,000 parasites crawling on the ground, Meteor used Houdinis crowd simulation engine, which allowed them to generate all the layers without adding any interpenetration. Courtesy of Meteor Studios.

CG Worms by the Thousands

Deemed one the most complicated visual effects in the entire movie, the sequence of Brenda exploding into 25,000 parasites was tackled by Meteor Studios. The team included vfx supervisor Paul Nightingale, executive producer Aaron Dem, CG supervisor Eric Clement, lead fx artist Dave Rand and texturing supervisor Christine Leclerc. In order to do some fluid simulation, we decided to use the RealFlow software, Clement remarks. Here are the different steps they took to make it happen:

  • 1. Created a CG Brenda that literally tore apart in Maya with soft body simulation.

2. Used that model to find the right timing for the explosion.

3. After the approval of the timing, they baked and exported this geometry in RealFlow to literally fill it with liquid.

4. After tweaking parameters like viscosity, gravity, etc., they generated a liquid mesh and a particle system.

5. They then exported this back to Maya, so they could render the mesh with RenderMan using a custom goo shader.

6. Since they needed to fill this liquid with parasites, they used the particle system that was generated with the liquid mesh as a target, and used Maya particle instancer to cycle the swimming parasites.

For shots featuring 25,000 parasites crawling on the ground, we decided to go with Houdinis crowd simulation engine, Clement continues. We came up with a custom setup that would generate random curves (used for the path of each parasites), and we were able to layer new sets of curves by projecting them on top of each other. What was really cool was that Houdini allowed us to generate all these layers without adding any interpenetration; parasite would naturally go over each other, thus creating the crowd simulation. Exporting ribs back to Maya, the lighting team picked up the animations, and rendered it out.

A spline IK-based system on motion paths, with some squash-and- stretch integrated into the rig were used to animate the worms. Courtesy of Image Engine.

Meet the Parasites

Most of the shots featuring CG parasites were created by Image Engine, a company that Gajdecki had collaborated with on the Stargate TV series. Image Engines team included resident Christopher Mossman, animation lead Adam de Bosch Kemper, modeling lead Ryan Jensen and lighting & compositing lead Brett Keys. John supplied us with a high resolution scanned worm, which we received along with a practical worm to have around for reference, Mossman recounts. It was quite disgusting having that thing lying around the office: you always hoped you didnt arrive to work in the morning to see it on your keyboard! We also built an outer skin mesh for an outer skin/slime layer to be used for refractions, and we employed Syflex to implement that. We tried a few different approaches to animating the worms before actually receiving any plates, and settled on a spline IK-based system on motion paths, with some squash-and-stretch integrated into the rig. It had its limitations, but for special situations, like the tub shot where the tail becomes detached from the surface, we modified the rig to make it do what James wanted to see. To make the worm look like it was actually stuck to the surface it was squirming on, we used a simple Maya soft body on the base mesh, then the Syflex slime layer was created based on that already flat bottomed mesh. Using this sort of system, we could quickly place paths where we wanted worms, block in animation to get approvals and then add in the animation detail by hand to match the speed and position of the worms.

The shot in which a police officers head gets blown in half was challenging on a couple of fronts. The team had no track marks, yet the matchmove had to be perfectly accurate. Courtesy of Image Engine.

In a particularly challenging shot, a police officers head gets blown in half. The tricky part was that the team had no track marks for the 3D track, no track marks on the actors face, and hair flopping all over the place. Yet, the matchmove had to be perfectly accurate. The actors head was modeled with skull, brain, muscle, fat, etc, with Maya. Then, the part of the model that was to be blown apart was removed, and the exposed brains sculpted to their damaged state. Textures were created using photographs of various meat products, as well as some hand painted portions. Within the head, a group of goo strings was animated using a Syflex cloth simulation. Finally, a dangling ear was hand animated to add some movement.

The most difficult part of the sequence was tracking the blown up head to the actors. Due to motion blur, the actors face deforming, and hair flying around, it had to be hand animated to match, Mossman recounts. The actors face was camera projected onto the head model and rendered as a pass to help the compositors blend between the real head and the CG replica. The compositors also had to paint in the background behind the removed portion of the head.

With the tubules, the challenge was to create a translucent and organic look, which was accomplished by using subsurface scattering shaders in mental ray combined with several custom render passes. Courtesy of Digital Dimension.

Ugly, Nasty and Slimy

While Meteor Studios and Image Engine mainly tackled swarms of nasty parasites, Digital Dimension focused on creating ugly tubules and splitting heads in half. Vfx supervisor Benoit Girard set up a team that included compositing supervisor Erik Bruhwiler, lead compositor Ryan Smolareck and roto/paint supervisor Tammy Sutton. Models and animation were provided by production as Maya files that were transferred into 3ds Max using Filmbox as a bridge. Digital Fusion was the main compositing package.

With the tubules, the biggest challenge was to create a translucent and organic look. This was accomplished by using subsurface scattering shaders in mental ray combined with several custom render passes. We painted highly detailed textures to add realism and depth to the materials, such as veins, wrinkles and nodules, lead sequence 3D artist Mitch Gates observes. Subtle yet intricate displacement, along with ray-traced reflections for specularity really helped to enhance their realism and flesh-like quality. The finished elements were lit using HDRI techniques and rendered with mental rays Final Gathering to create a highly realistic look that integrated seamlessly with the live-action plates.

To create the motion of the CG slime that coated the tubules, Digital Dimension relied on 3ds Maxs integrated cloth simulation tools. For the sticky webbing of goo that stretches between the tubules, artists modeled sheets of cloth and constrained them at strategic points along the tubule surfaces during the simulation calculations. Their sinuous appearance was accomplished with shader tricks, using detailed textures for opacity and displacement. The more stringy drips of goo were simulated as long strips of cloth attached at one end to the tubules. Careful adjustment of the cloth attributes, gravity and drag forces allowed them to behave in a very slime-like manner as the tubules writhed about. The CG slime was finally rendered in two passes, refraction and reflection.

For this effect, a head was modeled. The head model was split into halves and anatomical details were added to the inside geometry. Courtesy of Digital Dimension.

The most gruesome shot featured a character getting split in half. Another house had already composited the shot, using practical elements provided by the client for the entrails spilling from his gut, but the director was not satisfied with vfx of the head splitting, Vfx producer Chris Del Conte explains. Everything from the neck down was already considered final, but we were asked to add the effect of the actors head splitting in two. We modeled a head in 3ds Max to match the actor and match-moved that to the original plate. The head model was broken into halves and anatomical details were added to the inside geometry. Camera projection techniques were used to apply the actors face from the plate back onto the 3D head, and textures were painted for the inside gore using medical images as reference. The splitting of the head was animated using lattice deformations, making sure the jaw line always lined up to the existing comp. Bits of connective tissue and hanging flesh were created with cloth simulation techniques similar to what we used for tubule slime. Practical fluid elements were added in the comp to create the illusion of blood flowing down inside the open head and neck.

When Worlds Collide on a Shoestring

Even with his limited budget, Gajdecki managed to tackle a sequence that one expects to see in much larger productions: a collision between an asteroid and Earth! The meteor was modeled and UV-textured in Maya, animated and lit in Studio Max. Flames were generated in Studio Maxs particle system with an Afterburn shader, while dust and debris were created in Maya and Studio Max. Finally, Earth was a 3D geometry derived from topographical data that was then textured and lit in Studio Max. The many elements were then rendered in Final Render and composited in Digital Fusion. Rocket Science handled the sequence.

Thanks to the enthusiasm of the director, and to the surprising range of effects shots, John Gajdecki found Slither to be a really good film. Its funny, well acted, well executed, and the visual effects support the story. Thats all I could want from one year of my life.

Alain Bielik is the founder and editor of renowned effects magazine S.F.X, published in France since 1991. He also contributes to various French publications and occasionally to Cinéfex. Last year, he organized a major special effects exhibition at the Musée International de la Miniature in Lyon, France.