‘Clash of the Titans’ tells a tale based on classical Greek mythology. Artists from Cinesite and MPC share the development stories behind the city of Argos and the fantastic CG creatures that populate the movie, bringing drama and excitement to the screen.From Digital Media World Magazine
Cinesite’s VFX Supervisor Simon Stanley-Clamp travelled down to Tenerife for four weeks of on-site supervision for the facility’s main sequence, the battle between Perseus’ band of soldiers and the Scorpiochs, just before the shoot there started in May 2009. They had received the award, estimated at about 90 shots, in December 2008 and started work on the project in January. Warner Bros had said it would be a relatively short assignment with a tight, short shooting schedule. But it ran out to a 26-week post, and they worked on 176 shots in the final cut.
At Tenerife “We took hourly HDRI photography for lighting at the three locations, knowing that the crew would often be shooting in bright sunlight. We figured if we had light for every hour of the day, we should be covered.” Once the shoot started, Simon also shot specific HDRI of each scene as reference. “Things can change on the day – an extra reflector, diffuser or even an extra light can appear on set.” Throughout the shoot, Cinesite’s team continued to record an immense amount of data, some of which they shared with specialist matchmovers in VFX Supervisor Nick Davis’ team. Into Previs It has happened to him before when the previs is inaccurate to get the first edit back and find it doesn’t match the previs. By giving them the correct model and rig, they were able to avoid this happening. “Obviously, we could have conformed, broken our rig and made the Scorpiochs do almost anything. But we were confident in the looks and behaviour of our design and wanted it to be used. They should be menacing powerful beasts that chunder around, not flighty or floppy. We’d built constraints into the rig for these characteristics.” Live Scorpions Body Building Colour Coding The colours were a late innovation but meanwhile dozens of new textures were invented and collected for the Scorpiochs. Simon was looking for a rhino or elephant skin quality, avoiding the look of a mollusc shell. Other kinds of reference came from insects, and the observation that yellow and black were common among dangerous animals like spiders and snakes. Weight and Speed “At that stage, it was like any animated feature – we do rough versions, look at them in dailies, cut them back in, see how it works with the shot before and after. We tried extremes of fast and slow, not looking at them in isolation but cutting them back into the current sequence and developing a scene. The full animation cycle of dailies, reviews, signing off, moving into lighting and more finessed versions all helped us understand how well it would integrate into the shots.” Dust Storm Circular Animation Urban Renewal From these references CG Supervisor Patric Roos and the CG team presented new artwork incorporating location based shots captured by the second unit crew in Tenerife, featuring dramatic cliffs and an old volcanic rock coastline. The location of the city was based on a real place, from where the production designers also supplied them with reference material. They chose what was most useful and retrofitted the material with the live action plates. “The city has a very specific role within the story as the place, set in the cliffs, where the Kraken can enter and prevent people from escaping” Patric said. Locating the Camera “We had some flexibility in being able to move things around to get the layout and composition of images right, but we tried to locate the action accurately as far as possible, and did a lot of previs. Fitting the live action into the CG was a major task for us,” said Patric. His team also aimed to determine in advance which assets they needed to build at high and low resolutions. Real Story “Our 3D pipeline is Maya-based. Many of our proprietary tools sit inside of Maya like plug-ins, others are external, usually the ones for simulations. All rendering is done with Renderman, matchmoving with Boujou and 3D Equalizer. About 25 per cent of compositing was done in Nuke, and the rest with Shake, although this balance is changing toward Nuke.” Specific to cityscapes, MPC has a layout department and layout tool. The workflow starts with previs and plates, while the layout department manages the assets, keeping them up to date regarding shot composition and the cameras to be used. For cities like Argos, this department is responsible for laying out the buildings into the streets. The layout tool was developed for ‘Prince of Persia’, which has a very large city, specifically for this task. The difference for ‘Titans’ was the steep terrain, which meant customising this tool and laying out parts of Argos by hand. But one tip they took from ‘Prince of Persia’ was dividing the town into different ‘postcodes’ with specific layout characteristics – high-class areas, a palace area, market, slums. Similar areas could be populated by similar rules. “But tweaking the elevation to give the terrain the necessary dramatic appearance, for example, changed the ground plain, sometimes creating ‘empty’ areas. Layout on steep terrain is tricky. This department’s work is really the backbone of CG shot production, ensuring each shot has all assets, updated and in place,” Patric said. Living City Parallel Production “From their library of building types, the layout department laid out representations of where certain buildings should be to use in previs. As they went into rendering shots and look development, they swapped out the placeholder buildings for high-res versions. As a result, previs happened parallel to layout, parallel to asset building – all happening simultaneously. It was a testament to our pipeline that this regime could work.” Colossal Creation “We did motion tests to make sure he could perform as a character. There's always a very collaborative effort between animation and rigging in our internal pre-production phase.” New developments were made on the rig, primarily for the tentacles, including a new solver and tools allowing secondary jiggle and the ability to stick the tentacle on a surface and create drag on the skin. Dynamic Skinning “We also added a lot of photo-based procedural textures driven by mattes and a layered shader allowing us to add many levels of detail in the textures, so as you move closer it reveals finer detail. We were confident that it would hold up for any type of shot. Over the Ocean Water Interactions They had to find ways to increase the voxel grid density to allow enough detail to sell the scale where interaction occurred. They devised a hybrid method producing a generic ocean surface that let them add hydro fluids, or 3D voxel sims, locally and then blend them back with the ocean surface around the edges. The containers were smaller, and helped add complexity to the emitted elements from those surfaces. “We kept the approach to lighting, and made sure the water shader was compatible with the rest of the shader library we use. We used image-based lighting, or IBL, for the water reflections and for the scattering component we used physically correct, measured scattering data to produce the right colours, depth visibility and scale.” Black Pegasus “The black colour of the horse added a special problem,” Patric said. “Because Furtility’s shader tries to be physically accurate and the parameters are quite sensitive, the horse’s shape would be defined by the specular highlights on the hairs. Normally feathers could be made on cards and modelled. But in this case, we had very short black hairs, with a specific look, adjoining the feathers.” Focus on Highlights Applying reverse engineering, they knew how the hairs would behave from the reference footage of the horse and used this as the start point for their extension, allowing them to instance CG feathers made with the same geometric primitive as the short hair, and to use the same shader. So, when the hair and feathers were next to each other, they behaved naturally and when you saw the highlight moving across the horse, it didn’t change from one look to another. “The shader can be difficult to control,” said Patric. “But in this case we needed to let the feathers match the body. I’ve used feathers on cards before, shading and texturing them, but because the feathers and fur were black, dealing with the highlights became the focus of the task. The result shows that we can put a CG creature with a groom next to a live action character, and the two can be blended.” Thinking Backwards I tried the skinning solution instead, which could add a high level of detail at render time and carry the groom as well. We could increase the detail of the mesh – revealing the veins under the skin, for example, by refining the highlights and specular of the fur, all of which improved the close up shots. The same applied to the feathers. “From further away both appeared as simplified surfaces, but as the camera pushed in, the detail was all there. We could take it to as high a resolution as we needed and drop back to low-res easily, all handled in the pipeline. At render time we can dial the resolutions up or down pretty easily, and make approximations between fast and slow, close or far away moving shots.” Tracking Wings |
Sword Play Cinesite’s work was not confined to the Scorpiochs. At a critical moment in the story, Perseus’ sword transforms from a baton to a full sword when he touches it. Cinesite modelled the baton and the full sword, matching the practical props, and designed the transition for the transformation. “It definitely wasn’t to resemble a light sabre. It is much more organic. Tendrils of light grow and orbit the baton extending upwards, describing the final shape of the sword, which forms like hardening mercury within the light patterns. This all happens very quickly in 18 or so frames with movement in the plate, so the specific detail reads as a whoosh of light before the sword appears. Initially I hand painted the frames to get look sign off, before embarking on the full particle and fluid work used in the final shots.” |
Atmospheric Shader MPC’s team put a lot of effort into rendering. They built a physically accurate atmospheric shader placing a sun source where required in a scene, generating volumetric lighting and depth hazing – all physically correct. “That really helped us. Normally you might use compositing. But since everything was rendered CG anyway, and the tests looked great, it was just rendered as an extra pass,” Patric said. IBL with ray-trace supported sampling was used throughout most of the film even for the city rendering, which made a huge amount to render but helped the look tremendously. It included correct scattering properties, and the colour of the sky would change as needed. They designed the lighting angle shot by shot, as you would in camera. “From this atmospheric pass, we could output five or six passes of information to use in compositing. It gave the volumetric shafts from the Kraken casting god-rays down on the city, for example, and made similar tasks much easier. We could render a pass of everything and take the outputs pretty much as they were, put them together and get a decent image. Compositing in camera optical effects helped and loads of tweaks on top to make it filmic. We learned a lot from developing this shader, all using Renderman. “We specialised rendering the city by doing a lot of pre-computation based on IBL to generate global illumination to allowed faster iterations of the GI passes based on key views and merging of light data. We took about 20 minutes to an hour per frame once everything was processed in parallel on multiple machines. We did four to eight local bounces on the city. Characters use one bounce of diffuse and soft reflections.” |
Clash of the Titans is enteres in the AEAF Awards in Feature Film VFX catagory Words: Adriene Hurst |
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