Performance analysis: more than 70% computational cost is on hair dynamics, especially the detection and resolution of collisions

There are two types of parameters that can be tuned: physical parameters such as surface tension coefficient, liquid viscosity, elastic modulus of hairs, etc, which comes from real world physics; and artistic parameters such as the multipliers applied on cohesive/collision effect. For a moderate number of hairs our method can be interactive. In our test we found about 70 percent computational time is spent on the resolution of hair collision, where our framework shares the bottleneck with the state-of-the-art frameworks for hair simulation. We currently implemented our method on a full hair model, parallelize for multi-core CPUs, mainly targeting at the high-quality offline simulation for VFX industry. In the future, it’s possible to generalize our method to reduced/guide-hair settings (which is the most popular hair simulation scheme currently used in video game industry), and parallelize on GPU to reach real-time performance.

How close is it to real life?

We are still distant from a world where the simulation has no difference from real life. We indeed had to sacrifice some effects to complete the simulation within the time budget given the computational power we have in this age. For example, we modeled the liquid height field as a cylinder around the centerline of each strand, but in real life the water on hairs form liquid bridge while the hairs approach each other, which may have different dynamics than our cylindrical approximation. Another example is handling collisions between hairs, which is the Holy grail problem in hair simulation. Limited by the computational power we have, we used a spring model for the collision handling: it is a simple model but we cannot guarantee that no penetration happens between hairs.

The major difficulty for simulating wet hair effects lies on the fact that the liquid around hairs can be very thin (moving in microscopic scale), but with millions of hairs these thin liquid films determine the overall appearance and dynamics of the wet hair. As the consequence of this challenge, we proposed a multi-scale model for liquid simulation.