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

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

Shading of Hair Very complex in general Kajiya and Kay’s model real hair result of Marschner (03) + multiple scattering effects especially in bright hair there are translucent features from [marschner03]

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

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

Physical Based Models Single Scattering Multiple Scattering Marschner et al. (2003) Multiple Scattering Dual Scattering by Zinke (08) Determined by a lot of Parameters can be measured there are a lot of other hair shading models

Controllability physical parameters are unintuitive change of visual features affect other features changing the refraction index influences other optical features increasing the width of the highlight depletes the brightness of the highlight due to energy preservation goal: decoupled behaviour top row: influences several features middle row: energy preserving causes the highlight to go down bottom row: increasing width of highlight doesn‘t influence the brightness of the highlight

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

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

a unit-area Gaussian function 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 and the NX functions in the normal plane to the hair fibre

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)

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

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

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

What is missing here? Multiple Scattering Effects basically the same steps for details please have a look at the main paper BUT: they are important! if dual scattering relies on single scattering -> single scattering needs to be normalized for multiple scattering , otherwise either scattering vanishes or blurres up [moon&marschner]

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

Results Walt Disney‘s Tangled pay attention to primary and secundary highlights scattering effects Walt Disney‘s Tangled

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