The yetis had millions and millions of hairs, and that required a lot of rendering time. Imageworks found that once the hair density increased to a certain point, that it could take 30 minutes or more just in terms of ‘hair build time’ per frame. Sometimes it was even several hours per frame. That kind of render time was too long, and so a solution for this was also required.
“What we ended up doing instead,” detailed Karlsson, “was take the idea of our current system of calculating the camera distance from the hair but pre-baking some defined lower densities of hair which we would switch to based on the camera distance. This basically means that while we were still retaining the current system with the hair reducing in numbers as we go further away we would start at a much lower value in density.”
Things got even more complicated when there needed to be ‘hair on hair’ interaction. But Imageworks planned ahead for this. “Each clump of hair had at least one curve driving it,” said Karlsson. “This gives us enough fidelity to handle pretty much anything needed in terms of collisions. For example, Migo has over 1,500 curves being simulated on just his chest alone. The character Meechee has almost 4,000 in her shawl.”