An Artist Friendly Hair Shading System

1. An Artist Friendly Hair Shading System Stefan Kinauer KAIST (Korea Advanced Institute of Science and Technology)

2. Overview • Physical based hair shading models • Reformulation for better artist controllability and Application to a Single Scattering Model • Results

3. Shading of Hair • Very complex in general Kajiya and Kay’s model result of Marschner (03) real hair from [marschner03]

4. Physical Based Models • Single Scattering • Marschner et al. (2003) • Longitudinal scattering functions M(θ) • Azimuthal scattering functions N(Φ)

5. Physical Based Models • final result • primary highlight • secondary highlight • transmission component • backscattering • forward scattering

6. Physical Based Models • Single Scattering • Marschner et al. (2003) • Multiple Scattering • Dual Scatteringby Zinke (08) • Determined by a lot of Parameters • can be measured

7. Controllability changing the refraction index influences other optical features • physical parameters are unintuitive • change of visual features affect other features increasing the width of the highlight depletes the brightness of the highlight due to energy preservation goal: decoupled behaviour

8. Reformulation • goal: • preserve the quality of hair shading • enable the artist to create non-physical correct effects • provide controls for the artist to change hair appearances • in exact the way the artist likes to • without influencing other features of the hair • at least do not slow down the rendering process

9. Reformulation • Examination • Decomposition • Defining AFCs (Artist Friendly Controls) • Reproduction • Recombination

10. Examination General Approach applied to Marschner‘s model with • a unit-area Gaussian function • β: the longitudinal width • θh : the half angle between incoming and outgoing light • α: the longitudinal shift

11. Decomposition General Approach applied to Marschner‘s model (artist-specifical) visual distinctive components to control primary highlight (R) secondary highlight (TRT) glints (TRT) rim light (TT)

12. Defining Artist Friendly Controls General Approach applied to Marschner‘s model (artist-specifical) the rim light: color intensity azimuthal width longitudinal width longitudinal position rim light

13. Reproduction General Approach applied to Marschner‘s model longitudinal controls azimuthal controls combining a unit-height Gaussian function

14. Recombination General Approach applied to Marschner‘s model summing up all terms

15. What is missing here? • Multiple Scattering Effects • basically the same steps • for details please have a look at the main paper • BUT: they areimportant! [moon&marschner]

16. Results • In their tests it was significantly easier to produce wanted results • The rendering is about 1.3 times faster than the corresponding physical shader • Is currently used for Walt Disney Animation Studios

17. Results Walt Disney‘s Tangled

18. Questions and Answers • sources for this presentation: • [marschner03] Light Scattering from Human Hair Fibers by Stephen R. Marschner et al. • [moon&marschner] Simulating Multiple Scattering in Hair Using a Photon Mapping Approach by Jonathan T. Moon and Stephen R. Marschner • The main paper: An Artist Friendly Hair Shading System by Iman Sadeghi, Heather Pritchett, Henrik Wann Jensen and Rasmus Tamstorf