Cinefex Weekly Update | Issue 20

The three years between Shrek and Shrek 2 would have been insignificant in the live-action realm; but in the rapidly advancing arena of computer graphics, three years was ample time to give birth to new tools and applications. The advantage of a second time at bat with the same characters also gave PDI/DreamWorks artists and programmers opportunities to finesse what they had wrought so beautifully the first time out. "In the gap between Shrek and Shrek 2," said head of effects Arnauld Lamorlette, "we looked at what we did in Shrek, what had worked and what could be improved."

Among items on the 'improve' list were the rendering techniques used on the first film. PDI/DreamWorks rewrote all of its rendering tools and took advantage of global illumination technology to imbue Shrek 2 with warmer, more realistic and richer computer generated images. Though global illumination -- which creates real-world bounce light in the digital realm -- had been around for quite a while, and was certainly around at the time the first movie was in production, it had only recently been used in film production. "Global illumination was cumbersome to use," said Lamorlette. "It was computation intensive and slow. To use it on Shrek 2 wasn't a matter of inventing a new technology; it was about taking a technology that had been around for 20 years and making it usable in a production pipeline. We made it faster and created controls that would allow artists to use it easily on a day-to-day basis." Similarly, the PDI/DreamWorks team used subsurface scattering shaders -- which emulate the way light travels through and reflects off of skin and other translucent materials -- to give Shrek 2's characters more realistic, warmer skin. "Like global illumination, subsurface scattering was out there when we did the last movie, but it wasn't fast enough to use in a production pipeline."

The company invested equal effort into improving its hair simulation, not only to make it more workable, overall, but to accommodate the furry new character, Puss In Boots, and a range of more complex hairstyles on the hero characters. The upgrading of the hair pipeline involved developing hair dynamics and rewriting all of the existing hair shaders. PDI/DreamWorks also rewrote fabric shaders to afford greater variety in character costumes. "The new shaders gave us the capability of doing a very wide range of fabrics," said Lamorlette. "As a result, the costumes in the movie are unbelievably varied and rich."

Variety and richness characterized Shrek 2's crowds, as well, most prominent in a scene in which the citizens of Far Far Away greet the royal family in a red carpet parade. PDI/DreamWorks made great strides in its mob system, largely through the development of Dynamic Crowd Character, which gave individual figures within a crowd the same range of dynamics exhibited by hero characters. "Our generic crowd characters were just like our hero characters," Lamorlette explained. "They had the same character setup, the same properties in the hair, the same clothing simulation, the same animation controls. That meant that we could do very sophisticated things within the crowds. We could switch them from crowd character to hero character very easily." Character complexity levels were determined during rendering. "If the crowd character was far away, we would compute less hair and reduce the quality of the rendering; but, when we got close to that crowd character, we would use all the same complexities as in a regular character."

Crowd characters could be manipulated either by animators or the mob system. "The mob system would assign animation cycles to crowd characters," said Lamorlette, "adapting them according to the size of the character. It also assigned behaviors, based on a complex set of rules we set up. One of the rules might be, 'Look at the closest hero character.' So, if we had a hero character passing by on the red carpet, the crowd would look at the closest one. When that hero character moved further away, the crowd would turn their heads to look at the hero character that was now closest to them. That was all done procedurally, with rules in the mob system. The animator could still go in and override that -- but that wasn't usually necessary. Dynamic Crowd Character gave us a lot of variety within the crowd, rather than giving us something in which the whole crowd acted as a flock."