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

from [marschner03]

Kajiya and Kay’s model real hairresult of Marschner (03)

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 Scattering

by Zinke (08)

● Determined by a lot of Parameters● can be measured

7

Controllability

● physical parameters are unintuitive● change of visual features affect other

featureschanging 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

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

ExaminationGeneral Approach applied to Marschner‘s

model

a unit-area Gaussian function

β: the longitudinal widthθh : the half angle between incoming and outgoing lightα: the longitudinal shift

with

11

DecompositionGeneral 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 ControlsGeneral Approach applied to Marschner‘s

model (artist-specifical)

● the rim light:● color

● intensity

● azimuthal width

● longitudinal width

● longitudinal position

rim light

13

ReproductionGeneral Approach applied to Marschner‘s

model

● longitudinal controls

● azimuthal controls

● combining

a unit-height Gaussian function

14

RecombinationGeneral 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