Shadow Algorithms

1
Shadow Algorithms

• Gerald Matzka
• Computer Science Seminar

2
Overview

• Introduction
• Shadow Algorithms
• Fake Shadow
• Vertex Projection
• Shadow Z-Buffer
• Shadow Volume
• Shadow Volume Reconstruction
• Static Shadow
• Use Cases

3
Introduction

• Why Shadow?
• What is Shadow?
• Anatomy of Shadow
• Umbra vs. Penumbra
• Shadow Volume

4
Why Shadow?

• Objects appear not anchored
• Shadow provides additional information
• Second view of an object
• Without shadow object seems to float
• State of the art

5
Definition 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
Umbra vs. Penumbra

• Area light sources produce soft shadow
• Umbra area in full shadow
• Penumbra boundary area gradient from full
  shadow to light

7
Definition Shadow Volume

• Shadow planes
• Volume formed from shadow planes
• Open and infinite
• Inside in shadow - outside in light
• Must be clipped and capped

8
Shadow Algorithms

• Fake Shadow
• Vertex Projection
• Shadow Z-Buffer
• Shadow Volume
• Shadow Volume Reconstruction
• Static Shadow

9
Fake Shadow

• No exact calculation
• Approximation of shadow position and shape
• Estimated by center or anchor of object
• Pro simple, fast
• Contra flat ground, only ground shading, not
  exact, rotate limitations

10
Vertex Projection

• Object projected to ground
• Exact mathematical calculation
• Pro still simple, exact, no rotate limitations
• Contra flat ground, only ground shading

11
Shadow Z-Buffer

• Based on hidden surfaces
• Lights point of view rendering into Z-Buffer
• Cameras point of view rendering, lookup in
  Z-Buffer
• Transformation between camera and light view
• Z-value compare - shadowed or lit

12
Shadow Z-Buffer cont.

• Problems inaccuracies in calculations
• self-shadowing small fudge factor
• point-sampling area-sampling
• Pro accurate, medium effort
• Contra blocky, self-shadowing, compare with
  neighbor, drawn twice

13
Shadow Volumes

• Count of Shadow Volumes at viewing position
• front plus ? back minus
• Three steps
• silhouette generation
• drawing of shadow volume(s)
• rendering the shadow

14
Shadow Volumes cont.

• Step 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

15
Shadow Volumes cont.

• Step 2 Drawing of Shadow volume
• rendered twice in stencil buffer (cull-mode)

16
Shadow Volumes cont.

• Step 3 Rendering the Shadow
• stencil buffer holds shadow
• render a polygon using stencil buffer bits
• shadow for one light created
• Repeat for next light from step 1

17
Shadow Volumes cont.

• Pro
• greatly improved realism
• hardware support of stencil buffer
• Contra
• high requirement on fill-rate
• sharp shadow
• additional scene management

18
Shadow Volume Reconstruction

• mixture of Z-Buffer and Shadow Volume
• lights point of view rendering
• edge filter to create silhouette image
• silhouette image used as shadow volume
• clip and cap shadow volume
• draw shadow volume
• apply shadow accordingly

19
Shadow Volume Reconstruction

• Pro
• smoother than Z-Buffer
• faster than Shadow Volume
• Contra
• additional step (edge filter, reconstruction)
• inaccuracies (edge filter, reconstruction)

20
Static Shadow

• not a real-time Shadow Algorithm
• none moving or none animating objects
• pre-calculated shadow maps
• rendered as normal polygons

21
Use Cases

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

• Outcast derived Z-Buffer algorithm, caused by
  voxel and polygon mixture

22
Conclusion

• Shadow gives large amount of information
• Shadow generation is a classic computer graphics
  problem
• Several basic algorithms with advantages and
  disadvantages
• State of the art in modern computer games

23
The End

• Thanks for listening!