1 Models for Translucent Objects Models for Translucent Objects - PDF document

Translucent Objects Translucent Objects Realistic Materials Realistic Materials Translucent Materials – light is scattered through the object – incident illumination smoothed due to diffuse scattering inside media Inhomogeneous Inhomogeneous Inherent High Dynamic Range Inherent High Dynamic Range Translucent Objects Translucent Objects 500,000 500,000 500,000 500,000 486,000 486,000 486,000 486,000 5 – caused by material variation or internal structure 0 – required for realistic appearance Overview Overview Models for Translucent Objects Models for Translucent Objects • basic physical properties – models for translucent objects – e.g., absorption and scattering cross sections σ a – the BSSRDF and σ s [Ishimaru78] – dipole approximation – defined for the whole object volume • rendering possible with variety of techniques such as – finite element methods [Rushmeier90, Sillion95, Blasi93] 1

Models for Translucent Objects Models for Translucent Objects Models for Translucent Objects Models for Translucent Objects • rendering techniques (contd.) • specialized models – finite element methods [Rushmeier90, Sillion95, – BSSRDF [Nicodemus 1977] Blasi93] – dipole approximation [Jensen et al 2001] dipole approximation [Jensen et al. 2001] – bidirectional path tracing [Hanrahan93, • includes measurements of physical parameters for Lafortune96] homogeneous materials – photon mapping [Jensen98, Dorsey99] – Monte Carlo simulations [Pharr00, Jensen99] – diffusion [Stam95, Stam01] – precomputed radiance transfer [Sloan03a] Overview Overview The BSSRDF The BSSRDF – models for translucent objects • bidirectional scattering-surface reflectance – the BSSRDF distribution function [Nicodemus 1977] – dipole approximation – general model of light transport inside an object general model of light transport inside an object – (almost) equivalent to a reflectance field [Debevec et al. 2000] – ratio of reflected radiance to incident flux – 8 dimensional function The BSSRDF The BSSRDF The BSSRDF The BSSRDF • outgoing radiance computed by integrating over the whole surface and all incoming directions 2

Single Scattering vs. Multiple Single Scattering vs. Multiple Single Scattering vs. Multiple Single Scattering vs. Multiple Scattering Scattering Scattering Scattering • single scattering • multiple scattering contribution strongly (almost) independent of dependent on incoming p g incident light direction g light direction – example: alabaster block illuminated by a laser from – example: honey pot the left illuminated by a laser from the left Single Scattering vs. Multiple Single Scattering vs. Multiple BSSRDF Approximation BSSRDF Approximation Scattering Scattering • often modeled • BSSRDF too complex for many application independently, e.g., – acquisition, storage, … – single scattering using ray single scattering using ray – all combinations of directions and positions all combinations of directions and positions tracing – multiple scattering using a less complex model with diffuse approximation Diffuse Scattering Approximation Diffuse Scattering Approximation Diffuse Scattering Approximation Diffuse Scattering Approximation • neglect directional dependence • approximate BSSRDF by diffuse reflectance – frequent scattering events in optically dense media – only 4 dimensions lead to diffuse scattering inside the media g – requires Fresnel terms at incoming and outgoing requires Fresnel terms at incoming and outgoing locations – simplifies handling drastically – commonly used 3

Diffuse BRDF Approximation Diffuse BRDF Approximation Diffuse BRDF Approximation Diffuse BRDF Approximation • neglect directional dependence (no Fresnel) • approximate BSSRDF by diffuse BRDF • assume incident and exitant location near – assume incident and outgoing locations are very close to each other – neglect Fresnel effect k d Overview Overview Dipole Approximation Dipole Approximation – models for translucent objects • [Jensen et al. 2001] – the BSSRDF • infinite half-space of homogeneous material – dipole approximation • optically dense, modeling of multiple ti ll d d li f lti l scattering Dipole Approximation Dipole Approximation Dipole Approximation Dipole Approximation 4

Dipole Approximation Dipole Approximation Determining Physical Parameters Determining Physical Parameters • example: marble from [Jensen et al. 2001] • required for dipole approximation – scattering and absorption coefficient – relative index of refraction relative index of refraction • also required for evaluation of single scattering term Determining Physical Parameters Determining Physical Parameters Results Results • image-based measure- Light Source ment setup [Jensen et al. 2001] – surface point illuminated by focused beam of white light Camera – object observed by digital 50 o camera – parameters determined via photograph photograph rendering rendering Sample diffusion solution Overview Overview – models for translucent objects – the BSSRDF – dipole approximation 5