Dr' Scott Schaefer

1
Shadows

• Dr. Scott Schaefer

2
Shadows

• Shadows provide clues about depth
• Make scenes appear
• more realistic

3
Shadows

• Shadows provide clues about depth
• Make scenes appear
• more realistic

4
Shadow Algorithms

• Simple/Planar Shadows
• Shadow Maps
• Shadow Volumes

5
Simple/Planar Shadows

• Projection of an object onto a planar
• surface (floor/wall)
• Build projection matrix
• from light to wall
• Draw object in black
• using projection matrix

Image taken from http//developer.nvidia.com/objec
t/robust_shadow_volumes.html
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Simple/Planar Shadows

• Fast and simple
• Does not account for self-shadowing
• Only works for planar
• surfaces (nothing else
• has shadows cast on it)

Image taken from http//developer.nvidia.com/objec
t/robust_shadow_volumes.html
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Shadow Maps
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Shadow Maps

• Render scene from lights perspective

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Shadow Maps

• Render scene from lights perspective

Shadow Map
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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective

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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective
• For every pixel
• Transform to world
• space
• Compare distance
• to value in shadow map

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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective
• For every pixel
• Transform to world
• space
• Compare distance
• to value in shadow map

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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective
• For every pixel
• Transform to world
• space
• Compare distance
• to value in shadow map

In shadow!
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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective
• For every pixel
• Transform to world
• space
• Compare distance
• to value in shadow map

In shadow!
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Shadow Maps

• Render scene from lights perspective
• Render scene from
• viewers perspective
• For every pixel
• Transform to world
• space
• Compare distance
• to value in shadow map

Not in shadow!
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Shadow Maps
Image taken from http//www.cse.ohio-state.edu/ha
leyb/Hardware/ggDepthBuffer.jpg
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Shadow Maps

• Advantages
• Simple to implement
• Does not depend on scene complexity (except to
  render shadow map)
• Disadvantages
• Fixed resolution image leads to artifacts
• Omni-directional light sources require 6 shadow
  maps to cover every direction

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Perspective Shadow Maps

• Shadow maps are limited by screen resolution and
  depend on object distance!

Small distance
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Perspective Shadow Maps

• Shadow maps are limited by screen resolution and
  depend on object distance!

Huge distance
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Perspective Shadow Maps

• Distort viewing transformation from lights
  perspective to create a more uniform sampling
  from viewers perspective

Image taken from Perspective Shadow Maps
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Perspective Shadow Maps

• Distort viewing transformation from lights
  perspective to create a more uniform sampling
  from viewers perspective

Image taken from Perspective Shadow Maps
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Anatomy of a Shadow
Surface outsideshadow volume (illuminated)
Shadowingobject


Lightsource
Shadowvolume (infinite extent)
Eye position (note that shadows are independent
of the eye position)
Surface insideshadow volume (shadowed)
Partially
shadowed
object
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Shadow Volumes

• Build polygons for shadow
• volumes explicitly
• Render shadow volume
• polygons from viewers
• perspective and count
• inside/outside shadows

Image taken from Practical Robust Stenciled
Shadow Volumes for Hardware-Accelerated Rendering
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Shadow Volumes
Lightsource
Shadowing object

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Shadow Volumes
Lightsource
Shadowing object

zero
1
zero
2
2
1
3
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Shadow Volumes
Lightsource
Shadowing object

zero
1
zero
Unshadowedobject
2
2
1
3
Shadow Volume Count 111-1-1-1 0
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Shadow Volumes
Lightsource
Shadowing object

zero
1
zero
Shadowedobject
2
2
1
3
Shadow Volume Count 111-1 2
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Shadow Volumes
Lightsource
Shadowing object

zero
1
zero
Unshadowedobject
2
2
1
3
Shadow Volume Count 0
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Implementing Shadow Volumes

• For each surface, find silhouette edges
• Build shadow volume (viewer independent) by
  extending away from light

Image taken from Practical Robust Stenciled
Shadow Volumes for Hardware-Accelerated Rendering
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Implementing Shadow Volumes

• Use stencil buffer to count intersections with
  shadow volume
• Render front faces and increment if closer to
  viewer
• Render back faces and decrement if closer to
  viewer
• Dont update color or depth values!!!!
• If stencil buffer is non-zero, then pixel in
  shadow

Stencil value 1
Stencil value 0
Image taken from Practical Robust Stenciled
Shadow Volumes for Hardware-Accelerated Rendering
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Shadow Volumes Examples
Images taken from Practical Robust Stenciled
Shadow Volumes for Hardware-Accelerated Rendering
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Shadow Volumes Examples
Image taken from Doom 3
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Problems with Shadow Volumes
Far clipplane
zero
1
1
2
zero
3
2
Near clipplane
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Problems with Shadow Volumes
Missed shadow volume intersection due to near
clip plane clipping leads to mistaken count
Far clipplane
zero
1
1
2
zero
3
2
Near clipplane
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Problems with Shadow Volumes
Lightsource
Shadowing object

Shadow test fails!
Shadow Volume Count 0
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Solution Invert Depth Test
Lightsource
Shadowing object

Shadowed object
Shadow Volume Count -11-1-1
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Solution Invert Depth Test
Lightsource
Shadowing object

Now have problem with far clipping plane
Shadowed object
Shadow Volume Count -11-1-1
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Shadow Volumes

• Advantages
• Omni-directional light sources
• Proper self-shadowing behavior
• Pixel perfect shadows
• Disadvantages
• Surfaces must be connected closed
• Surfaces can only use planar polygons
• Silhouette computation uses CPU
• Heavy on fill-rate
• Near/Far clipping planes lead to problems

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Soft Shadows

• Point lights cause hard shadows
• Lights are not infinitely small points in reality
• Area light sources
• cause soft shadows

http//graphics.ucsd.edu/henrik/images/cbox.html
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Soft Shadows

• Point lights cause hard shadows
• Lights are not infinitely small points in reality
• Area light sources
• cause soft shadows

http//graphics.ucsd.edu/henrik/images/cbox.html
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Soft Shadows

• Simulate area lights with lots of points lights

Expensive
The cluster of point lights.
Image taken from Practical Robust Stenciled
Shadow Volumes for Hardware-Accelerated Rendering
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Soft Shadows

• Simulate area lights with lots of points lights
• Blur shadows in image space

Cheap, inaccurate
http//www.gamedev.net/reference/articles/article2
193.asp