The CG VFX of Troy - Part I: Framestore CFC's Armada

Image Based Lighting & Rendering

The traditional way of obtaining environment maps and high dynamic range maps is to use a chrome ball and a camera. The problem was that we were going to have a variety of difficult locations on set. We were shooting on beaches in Mexico and Malta as well as out in the open sea. Thus, we needed a quick way of capturing HDR images and environment maps. In our research, we found that the Canon 1DS digital SLR was particularly good as it has a full 35mm CCD back plate. This meant that we could attach a Sigma 8mm fisheye lens to it and photograph the full image with negligible artifacts. The Canon camera also allowed us to program the bracketing, so that it automatically took five shots at varying F-stops with just one shutter release. This entire setup was fixed on an iPix rotator that could rotate to take photos for an entire hemisphere on each side. The result is a rig that took just 20 seconds to capture a full HDR environment map at any given location. We used Panoweaver to stitch the images together and integrated all the HDR images and environment maps into shaders designed to incorporate Image-Based Lighting and standard CG lighting if necessary.

One of the issues we encountered was the open environment itself – and the challenge to attain the right contrast ratios from the fill (the sea and sky) to the burning hot sun. We performed some tricks with the HDR maps to generate additional lighting breakdown for the compositors where we threw away the color values and created intensity values split into the three channels – Red: very hot sun spot, Green: sky, Blue: sea. We then rendered with this RGB intensity map, as one of the many arbitrary outs, and let the compositors tweak the contrast ratios to get the image-based lighting just right.

Ambient occlusion and bent normal maps on the boats were pre-baked as textures and not generated in relation to any specific environment map. We placed a plane slightly beneath the boat and rendered to obtain slight subtleties such as crevice detail.