Virtual Light Field Groupvlfproject@cs.ucl.ac.ukUniversity College London

GR/R13685/01

Research funded by:

Propagating the VLF - Problems and Solutions II

Insu Yu

i.yu@cs.ucl.ac.uk

VLF Project

Overview

Specular Transfer– Determination of reflected Direction– Forward/Backward Transfer– Discontinuity and Resampling

Caustics– What is caustics– Holes & Jittering

Propagation Issues – Progressive propagation – Effect of Parameter Adjustment

VLF Project

Specular Reflection

What is Ideal specular reflection model ?

– All the incoming Energy (L) is reflected off the direction(R) of a surface

– Incidence angle (θi) equals the angle of specular reflection (θr)

– A Mirror Reflection

How can we adapt this model on VLF ?

N

L R

θi θr

VLF Project

Determination of reflected direction

Discrete ‘Uniformly Sampled Directions’– Due to the Representation of Finite Directions, an Ideal

reflected PSF direction can hardly be found

N

L R

PSF Directions (Quadrant) VLF Specular Reflection

VLF Project

Nearest Vs Tri-linear Directions

Nearest direction – Select closest direction to the

reflected direction– Works better on High

Resolution Directions

PSF

Tile

Finding Nearest PSF Direction

VLF Project

Nearest Vs Tri-linear Directions (Cont’)

Tri-linear – Select Three PSF direction which

enclose the reflected Direction – Transfer Incoming Energy to three

Directions according to barycentric weights of each direction

– Glossy not perfect specular – Tri-linear interpolated transfer introduce

excessive blurring

P1

P2

P3

K1

K3K2

Tri-linear Transfer

VLF Project

Forward Vs Backward Mapping

Forward mapping– Diffuse to Specular Transfer– Shooting Energy from Sender to

Receiver– Miss-alignment of source to

destination leads holes

PSF

Tile

Forward Mapping

Holes

VLF Project

Forward Vs Backward Mapping (Cont’)

Backward mapping– Trace a ray in Reflected

Direction in back order– One-to-One mapping– UV uniform subdivision on

receiver planes– Need to use bilinear

interpolation scheme to pull the radiance values

Backward Mapping

PS

F

Reflected PSF direction ’

QC

(Diffuse Sender)

P (Specular Receiver)

LU (

’,s’,t’,P)

Vis

ibili

ty e

xcha

nge

buffe

r

VLF Project

Discontinuities & Resampling

Specular Reflection is not jittered– Diffuse Surfaces are jittered multiple time– Due to Directional Energy Transfer– URM is updated only once where Diffuse to

Specular Transfer occurs

Blocky discontinuities appear despite Tri-linear transfer

Resample all TRM at end of propagation by backwards ray tracing

Resampling

VLF Project

Relation to Directions

Accuracy of Specular Transfer is proportional to the number of directions

Light Field Ray TracingSpecular Reflection (ES*D)

VLF Project

Caustics

VLF Project

Caustics

Caustics comes free as in Specular to Diffuse Transfer

VLF Project

Specular to Diffuse Transfer

Caustics present where Diffuse surface gathers energy from specular senders

LU(,s,t,Q)Temporary

Radiance Tile

Q(Specular Sender)

PN

( Diffuse Receiver)

Vis

ibili

ty e

xch

an

ge

bu

ffe

r

PS

F

VLF Project

Holes and Jittering

Holes Artefact– Due to representation of

Discretisation Directions– Transfer from small reflector

to large receiver– Long distance between

reflector and receiver Increase sampling density

of directions More jittered samples for

caustic

VLF Project

Filtering

Filtering to achieve accuracy and avoid aliasing

Caustics on Diffuse map can be excessively Blurred & introduce light leaking

– Require Adjustment of Gaussian Kernel Sigma

Possible to have higher resolution diffuse maps on caustic receivers

Can exploit additional map for caustics

VLF Project

Propagation

Propagation Process over various iterations

VLF Project

Progressive Propagation

Progressive propagation framework– Estimating unshot radiance– Selecting shooting sources & managing swapping

of maps– Purging of unshot energy– Zero-energy surfaces are never senders

Other issue– Capping of scenes

VLF Project

Scalability Test (Effect of parameters)

Various Polygons Various PSF Directions Various PSF Size &

Tile Size

Propagation time and on memory

Dual Xeon 1.7Ghz

VLF Project

Effect of parameters (Polygons)

Propagation time varies quadratically with the number of polygons (513 Direction, 8x8 Tiles, 64x64 Cells)

The memory grows linearly with the number of polygons TEST Scene

– One Emitter– polygons 224 to 1736– 5:1 ratio of diffuse to

specular surfaces

0 200 400 600 800 1000 1200 1400 1600 1800-50

0

50

100

150

200

250

300

350

400

450

Number of Polygons

Pro

pagation T

ime f

or

4 C

ycle

s (

min

s)

VLF Project

Effect of parameters (Directions)

Increasing PSF Directions– Pros

The greater accuracy is achieved Less jittering is necessary (overcome

missing holes)– Cons

More memory usage Longer Propagation Time

Linear Relationship between the number of direction and propagation time/memory Office Test Scene

VLF Project

Effect of parameters (PSF/Tile Resolution)

Size of Tile/PSF resolution determine the speed of propagation & rendering

Increasing Resolution results in faster intersection searching but more memory & propagation time

VLF Project

Question

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