Unstoppable VFX for a Runaway Train
Since it was unclear which shots would require a CG train (and what details would be visible and from what angles), each car was carefully measured and photographed. The resulting models were constructed in Maya. The resolution had to also hold up from a mile away as well as from a few feet. Tools were generated to quickly build tracks and animate trains based on survey and tracking information.
Thus, with such attention given early in the process, most initial renders integrated very well with plates based on the lighting data that it had been possible to acquire. But, it hadn't been possible (within the production schedule) to acquire the detailed lighting data required to accurately illuminate every point along the train as it moved through varying lighting conditions. So, where needed using the available survey and lighting information, (and available reference as a guide) environment representations were constructed in 3D and incorporated as the trains were rendered in PRMan in order to lend local detail to the reflections and indirect diffuse. This allowed for more natural lighting interaction as each point along the quarter-mile length of the train reflects its own unique view of the world around it. Attention was given to the slightest bumps and defects along the varied surfaces to ensure that the rendered result was indistinguishable from reference.
Again, models were constructed in Maya and textures painted in Photoshop. Starting from a small footprint, the refinery nearly doubled in size to accommodate the requirements of specific shots and angles, pressing into the corners where tracks converge and even threatening to envelope the road. Lit in PRMan using HDRI and incorporating global illumination to accurately reflect specular and the subtle bleed of diffuse illumination, the tanks were fit seamlessly into the world of the Stanton Curve.
The final effect, according to O'Shea, "allowed Tony to have as much real photography in the frame and that's what he judges things by."
Bill Desowitz is senior editor of AWN & VFXWorld.