Shadow AlgorithmsShadow Algorithms

Gerald MatzkaGerald Matzka

Computer Science SeminarComputer Science Seminar

2 / 22G. Matzka, 2001.06.20

OverviewOverview IntroductionIntroduction Shadow AlgorithmsShadow Algorithms

Fake ShadowFake Shadow Vertex ProjectionVertex Projection Shadow Z-BufferShadow Z-Buffer Shadow VolumeShadow Volume Shadow Volume Reconstruction Static Shadow

Use CasesUse Cases

3 / 22G. Matzka, 2001.06.20

IntroductionIntroduction Why Shadow?Why Shadow? What is Shadow?What is Shadow? Anatomy of ShadowAnatomy of Shadow Umbra vs. PenumbraUmbra vs. Penumbra Shadow VolumeShadow Volume

4 / 22G. Matzka, 2001.06.20

Why Shadow?Why Shadow? Objects appear not Objects appear not

anchoredanchored Shadow provides additional Shadow provides additional

informationinformation Second view of an objectSecond view of an object Without shadow object Without shadow object

seems to floatseems to float State of the artState of the art

5 / 22G. Matzka, 2001.06.20

Definition: ShadowDefinition: Shadow “An area that is only partially irradiated or

illuminated due to blockage of light by an opaque object.”

A point is in shadow relative to a given light source if rays from that light source cannot directly reach the point. Shadowed points cannot see the light.

6 / 22G. Matzka, 2001.06.20

Umbra vs. PenumbraUmbra vs. Penumbra Area light sources Area light sources

produce soft shadowproduce soft shadow

Umbra: area in full Umbra: area in full shadowshadow

Penumbra: boundary Penumbra: boundary area; gradient from full area; gradient from full shadow to lightshadow to light

7 / 22G. Matzka, 2001.06.20

Definition: Shadow VolumeDefinition: Shadow Volume Shadow planesShadow planes Volume formed from Volume formed from

shadow planesshadow planes Open and infiniteOpen and infinite Inside in shadow - Inside in shadow -

outside in lightoutside in light Must be clipped and Must be clipped and

cappedcapped

Point light source

Shadow Volume

objectlight vertex

8 / 22G. Matzka, 2001.06.20

Shadow AlgorithmsShadow Algorithms Fake ShadowFake Shadow Vertex ProjectionVertex Projection Shadow Z-BufferShadow Z-Buffer Shadow VolumeShadow Volume Shadow Volume Reconstruction Static Shadow

9 / 22G. Matzka, 2001.06.20

Fake ShadowFake Shadow No exact calculationNo exact calculation Approximation of Approximation of

shadow position and shadow position and shapeshape

Estimated by center or Estimated by center or anchor of objectanchor of object

Pro: simple, fastPro: simple, fast Contra: flat ground, only Contra: flat ground, only

ground shading, not ground shading, not exact, rotate limitationsexact, rotate limitations

10 / 22G. Matzka, 2001.06.20

Vertex ProjectionVertex Projection Object projected to Object projected to

groundground Exact mathematical Exact mathematical

calculationcalculation

Pro: still simple, exact, Pro: still simple, exact, no rotate limitationsno rotate limitations

Contra: flat ground, only Contra: flat ground, only ground shadingground shading

11 / 22G. Matzka, 2001.06.20

Shadow Z-BufferShadow Z-Buffer Based on hidden surfacesBased on hidden surfaces Light’s point of view Light’s point of view

rendering into Z-Bufferrendering into Z-Buffer Camera’s point of view Camera’s point of view

rendering, lookup in Z-rendering, lookup in Z-BufferBuffer

Transformation between Transformation between camera and light viewcamera and light view

Z-value compare - Z-value compare - shadowed or litshadowed or lit

12 / 22G. Matzka, 2001.06.20

Shadow Z-Buffer cont.Shadow Z-Buffer cont. Problems: Problems: inaccuracies in calculations

self-shadowing: small fudge factorself-shadowing: small fudge factor point-sampling: area-samplingpoint-sampling: area-sampling

Pro: Pro: accurate, medium effort Contra: blocky, self-shadowing, compare

with neighbor, drawn twice

13 / 22G. Matzka, 2001.06.20

Shadow VolumesShadow Volumes Count of Shadow Count of Shadow

Volumes at viewing Volumes at viewing positionposition

front plus front plus back minus back minus

Three steps:Three steps: silhouette generationsilhouette generation drawing of shadow drawing of shadow

volume(s)volume(s) rendering the shadowrendering the shadow

14 / 22G. Matzka, 2001.06.20

Step 1: Silhouette generationStep 1: Silhouette generation boundaries between adjacent front-facing

and back-facing polygons silhouette adding light vertex to silhouette vertexes

shadow planes shadow planes together with object

shadow volume

Shadow Volumes cont.Shadow Volumes cont.

15 / 22G. Matzka, 2001.06.20

Shadow Volumes cont.Shadow Volumes cont. Step 2: Drawing of Shadow volume

rendered twice in stencil buffer (cull-mode)

Point light source

Camera

12

1 0

Objects

16 / 22G. Matzka, 2001.06.20

Shadow Volumes cont.Shadow Volumes cont. Step 3: Rendering the ShadowStep 3: Rendering the Shadow

stencil buffer holds shadowstencil buffer holds shadow render a polygon using stencil buffer bitsrender a polygon using stencil buffer bits shadow for one light createdshadow for one light created

Repeat for next light from step 1Repeat for next light from step 1

17 / 22G. Matzka, 2001.06.20

Shadow Volumes cont.Shadow Volumes cont. Pro: Pro:

greatly improved realismgreatly improved realism hardware support of stencil bufferhardware support of stencil buffer

Contra:Contra: high requirement on fill-ratehigh requirement on fill-rate sharp shadowsharp shadow additional scene managementadditional scene management

18 / 22G. Matzka, 2001.06.20

Shadow Volume ReconstructionShadow Volume Reconstruction mixture of Z-Buffer and Shadow Volumemixture of Z-Buffer and Shadow Volume light’s point of view renderinglight’s point of view rendering edge filter to create silhouette imageedge filter to create silhouette image silhouette image used as shadow volumesilhouette image used as shadow volume clip and cap shadow volumeclip and cap shadow volume draw shadow volumedraw shadow volume apply shadow accordinglyapply shadow accordingly

19 / 22G. Matzka, 2001.06.20

Shadow Volume ReconstructionShadow Volume Reconstruction Pro:Pro:

smoother than Z-Buffersmoother than Z-Buffer faster than Shadow Volumefaster than Shadow Volume

Contra:Contra: additional step (edge filter, reconstruction) inaccuracies (edge filter, reconstruction)

20 / 22G. Matzka, 2001.06.20

Static ShadowStatic Shadow not a real-time Shadow Algorithmnot a real-time Shadow Algorithm none moving or none animating objectsnone moving or none animating objects pre-calculated shadow mapspre-calculated shadow maps rendered as normal polygonsrendered as normal polygons

21 / 22G. Matzka, 2001.06.20

Use CasesUse Cases Games and 3D EnginesGames and 3D Engines Ray-Tracers (no real-time shading)Ray-Tracers (no real-time shading) Trade-Off between speed and quality!!!Trade-Off between speed and quality!!!

Outcast: derived Z-Buffer Outcast: derived Z-Buffer algorithm, caused by algorithm, caused by voxel and polygon voxel and polygon mixturemixture

22 / 22G. Matzka, 2001.06.20

ConclusionConclusion Shadow gives large amount of informationShadow gives large amount of information Shadow generation is a classic computer Shadow generation is a classic computer

graphics problemgraphics problem Several basic algorithms with advantages Several basic algorithms with advantages

and disadvantagesand disadvantages State of the art in modern computer gamesState of the art in modern computer games

The EndThe End

Thanks for listening!Thanks for listening!