approximate soft shadows on arbitrary surfaces using penumbra wedges tomas akenine-möller ulf...

Approximate Soft Shadows onArbitrarySurfacesusingPenumbraWedges

Tomas Akenine-MöllerUlf AssarssonDepartment of Computer Engineering,Chalmers University of TechnologySweden

Copyright 2002 Tomas Akenine-Möller

Why shadows?Why soft?


Our goal is plausible softshadows in real time

• Soft shadow generation is a fundamental CG problem

• Many algorithms exist…• Not a single one that can be used in, e.g.,

a game• We (all) want

– Good performance (use hardware)– Arbitrary receivers and casters– Plausible, or even better, physically-correct

shadows

• Our paper is one small step in the right direction


Previous work on real-time shadows

• Lots…• Most important algorithms for

shadows on arbitrary surfaces:– Shadow Mapping [Williams]

• See ”Perspective Shadow Maps”, SIGGRAPH 2002

• Heidrich et al, EGRW2000, soft version

– Shadow Volumes [Crow,Heidmann]• See Everitt and Kilgard’s report on how to

robustly render shadow volumes


Some previous work on real-time soft shadow generation• Heckbert/Herf [HH]:

– average a high number of images from point samples on light source, use as a texture on a plane

– Uses many passes: 64-256 (or fewer, and banding artifacts)

– Not practical for dynamic scenes

• Soler/Sillion [SS]:– For planar configurations, convolve hard shadow with

shape of light source– Fast– Hierarchical error-driven algorithm– Image-based can use any shadow casting geometry

and light source [same for HH]

• Haines’ plateaus:– Planar as well– Overemphasize the umbra– Few passes


Our idea• Extend the shadow volume algorithm

– And use ideas from Haines’ work

• In the shadow volume algorithm, each silhouette edge shadow quad

• For soft shadows, instead each edge a wedge-like shape: penumbra wedge


In 3D, things are more complicated…so make some simplifying assumptions• Simplifications:

– Spherical light sources

– Only use silhouette as seen from center of light source

– Bound the penumbra volume with 4 planes, sharing a silhouette edge

• Also, use a hires stencil buffer (we use 16 bits) – called light intensity buffer here


How to rasterize a wedge…• Init light intensity (LI) buffer to 255 before• 255 = full light, 0 = no light, 0 < x < 255 penumbra


wedge

Front planebackplane

Examples of rasterization in 2D

• Next, describe missing pieces:– Construction of wedges– Light intensity interpolation

255

LI-buffer =255

0

+ ( 0 – 255 ) = 0 (umbra)

255 0255 100


Penumbra wedge construction


Light intensity interpolation• To make it possible to implement

using programmable hardware:– Our only requirement was C0 continuity

across wedges (side planes)

• First attempt:– Compute positive intersection distances

(tf and tb) to front and back plane– Then s=tb/(tf+tb)– Occasional artifacts

• 2nd try:


Implementation• Need pretty large vertex & pixel shaders

in order to implement using graphics hw• So currently:

– Render scene using hw-accelerated OpenGL– Read back Z-buffer– Software wedge rasterization into LI-buffer

• (some optimizations not covered here – see paper)

• For each pixel (x,y) get z and form the world-space coordinates

• Point inside wedge compute light intensity

– When all wedges rasterized• Clamp to [0,255] and modulate specular + diffuse

image, and then add ambient image


Results: simple scene, comparison with Heckbert/Herf and Soler/Sillion

Our algorithm2 fps using SW

Heckbert/Herf128 samples20 fps using HW

Soler/Sillion[worst case for thisalgorithm]


Results: increasing light source size, and complex shadow receiver

1.8 fps on a 1.5 GHz PC


Fractal landscape as shadow receiver100k triangles


Two lights

Demo


Problems with our algorithm• Acute angles in shadow casting geometry

• Only used spherical light sources• We use silhouette from center of light source (like Haines)• Due to our construction of wedges, we can only render

soft shadows for silhouettes that form closed loops– Limits the types of shadow casting geometry

• Non-robust creation of side planes when silhouette edge is near-parallel to incoming light– Hack to avoid: remove such edges, and shorten & reconnect

neighbors

Original result Expected result [HH]Our algorithm using 6extra wedges per acute angle


Future and Current Work• We’ve added occlusion

culling…• …and dealt with eye-

in-shadow problem

Demo

• Late, late, late results– Use a penumbra wedge to

bound the exact penumbra volume

– Much more accurate light intensity computation

– Allows us to use arbitrary shadow casting geometry

– Most artifacts disappear


Future work

• Qualitative analysis of errors– When do they occur?– And when do we care/not care?

• Accurate comparison to other algorithms

• More light source shapes• Implementation using

– New graphics hardware (large pixel shader program)

– Use HILO texture as LI buffer


Thanks for listening!

approximate soft shadows on arbitrary surfaces using penumbra wedges tomas akenine-möller ulf...

Documents

tomas akeninemller wedge

penumbra slide

penumbra wedge slide

light intensity buffer

hard shadow

shadow volume algorithm

shape of light source

plausible soft shadows