interactive shadowspellacini.di.uniroma1.it/.../04_Shadows_Web.pdf · Topics in Computer Graphics...

Topics in Computer Graphics • Interactive Shadows © 2008 Fabio Pellacini • 1

interactive shadows


direct illumination

• point lights

• area lights

• distant lights

• environment lights

),(cos),,()(),( SPPP VLL ioiilighto !!!"!! =

),(cos),,()(),( iioiilighto VLL !!!!"!! PPP =

! "=A

lighti

oilighto dVr

LLS

SSPPSP ),(coscos

),,()(),(2

####$#

! "#=

iiiioiilighto dVLL

$$$$$$%$$ ),(cos),,()(),( PPP

note: more on this in Comp. Graphics course slides


why shadows?

[Akenine-Möller/Haines]


where do shadow come from?

no shadow shadow


soft vs. hard shadows



soft vs. hard shadows

point source

umbra

area source

umbrapenumbra



interactive shadow algorithms

Shadowvolumes

Light maps

Projectedplanarshadows

Hybridapproaches

[NV

IDIA

/Kilg

ard

et a

l.]


interactive shadow algorithms

• raytracing– not interactive … yet will not cover

• shadow mapping– image space algorithm

• shadow volumes– object space algorithm

• projective shadows– cannot do self shadowing

• light maps– not dynamic


shadow mapping


shadow mapping principle

• render image from the light



• store distance to closest surface



• render the image from the view



• project in shadow map: shadow if distance < map



• otherwise not shadow


shadow mapping algorithm

• pass 1: render scene from light view• pass 1: store distance from the light (in texture)

• pass 2: render scene from eye view• pass 2: transform each sample to light space• pass 2: compare current light dist. to stored one• pass 2: if current < stored then shadow


shadow mapping visualization[N

VID

IA/E

verit

t et a

l.]

camera view light view shadow buffer


shadow mapping visualization

[NV

IDIA

/Eve

ritt e

t al.]

camera view light distance projected shadow buffer


shadow mapping limitations

• not enough resolution: blocky shadows– pixels in shadow buffer too large when projected

[Fer

nand

o et

al.,

200

2]


shadow mapping limitations

• soft edges: percentage closer filtering– cannot average a shadow map then compare to it– perform all comparisons and then average results– used for reducing flickering, “incorrectly” blur edges

[NV

IDIA

/Kilg

ard

et a

l.]


shadow map limitations recap

• hard shadows– due to render light for one point of view only– use fake blurred edges, but does not look right

• blocky shadows: shadow map aliasing– due to too large shadow pixels– increase shadow map resolution

• shadow “acne”: shadow map biasing– due to floating point comparison– use epsilon, also known as shadow map bias


shadow mapping extensions

• shadow map aliasing– new parameterizations (PSM)– hierarchical algorithms (ASM)

• handle non-transparent objects / hairs– store multiple distance values per shadow pixel (DSM)

• soft shadow edges– lots of hacks, none works well enough in all cases


shadow volumes


shadow volume principle

• objects blocking a light …



• objects blocking a light define shadow volumes



• all points in the volume are in shadow


shadow volumes principles

• count enter/exit to determine if in the volume


shadow volumes principle

• ok for multiple objects; internal edges cancel out


shadow volumes algorithm

• [pass 0: compute silhouettes for shadowpolygons]

• pass 1: draw depth in camera buffer• pass 2: draw font-facing shadow polygons

– increment buffer

• pass 3: draw back-facing shadow polygons– decrement buffer

• pass 4: draw scene only if count == 0


shadow volumes visualization

shadowed scene shadow volume

light

[NVIDIA/Everit et al.]




light





light



shadow mapping/volumes comparison

• shadow mapping– image space algorithm: blocky shadows, …– efficient use of hardware– scales linearly with geometry– can blur shadow edges

• shadow volumes– object space algorithm: sharp shadows– inefficient use of hardware

• lots of wasted memory bandwidth– require silhouettes computations– very hard to make it robust (will not cover in details)


shadow volume demo


shadow mapping demo


shadow mapping is the winner!

interactive shadowspellacini.di.uniroma1.it/.../04_Shadows_Web.pdf · Topics in Computer Graphics...

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