1

Shadow Algorithms

Non-photorealistic

Rendering

2

Shadow Algorithms

•Simple Planar Shadows

•Shadow Maps

•Shadow Volumes

2

3

Shadows

•Shadows provide clues about depth

•Make scenes appear

more realistic

4

Shadows

•Shadows provide clues about depth

•Make scenes appear

more realistic

3

5

Ray-tracing

light

light

s-ray

s-raye-ray

6

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

4

7

Simple Planar Shadows

•Fast and simple

•Does not account for self-shadowing

•Only works for planar

surfaces (nothing else

has shadows cast on it)

8

Shadow Maps

5

9

Shadow Maps

•Render scene from light’s perspective

Shadow Map

10

Shadow Maps

• Then render scene from Viewer’s perspective

6

11

Shadow Maps

• Render scene from light’s perspective

• Render scene from

viewer’s perspective

• For every pixel- Transform to world

space

- Compare distance

to value in shadow map

12

Shadow Maps

• Render scene from light’s perspective

• Render scene from

viewer’s perspective

• For every pixel- Transform to world

space

- Compare distance

to value in shadow map

7

13

Shadow Maps

• Render scene from light’s perspective

• Render scene from

viewer’s perspective

• For every pixel- Transform to world

space

- Compare distance

to value in shadow mapIn shadow!

14

Shadow Maps

• Render scene from light’s perspective

• Render scene from

viewer’s perspective

• For every pixel- Transform to world

space

- Compare distance

to value in shadow mapIn shadow!

8

15

Shadow Maps

• Render scene from light’s perspective

• Render scene from

viewer’s perspective

• For every pixel- Transform to world

space

- Compare distance

to value in shadow map

Not in shadow!

Shadow Maps

9

17

Shadow Maps

Shadow Maps

•Advantages

- Simple to implement

- Does not depend on scene complexity (except

to render shadow map)

•Disadvantages

- Fixed resolution image leads to artifacts

10

Shadow Maps

18

Low ResSmall Pixel

Shadow Maps

•Advantages

- Simple to implement

- Does not depend on scene complexity (except

to render shadow map)

•Disadvantages

- Fixed resolution image leads to artifacts

11

21

Anatomy of a Shadow

Shadowing

object

Partiallyshadowed object

Light

source

Eye position

(note that

shadows are

independent of

the eye position)

Surface inside

shadow volume

(shadowed)

Surface outside

shadow volume

(illuminated)

Shadow

volume

(infinite extent)

22

Shadow Volumes

•Build polygons for shadow

volumes explicitly

•Render shadow volume

polygons from viewer’s

perspective and count

inside/outside shadows

12

23

Shadow Volumes

Shadowing objectLight

source

Eye

position

24

Shadow Volumes

Shadowing objectLight

source

zero

zero

+1

+1

+2 +2

+3Eye

position

13

25

Shadow Volumes

Shadowing objectLight

source

zero

zero

+1

+1

+2 +2

+3Eye

position

Unshadowed

object

+ ---+ +

Shadow Volume Count = +1+1+1-1-1-1 = 0

26

Shadow Volumes

Shadowing objectLight

source

zero

zero

+1

+1

+2 +2

+3

Shadowed

object

+ -+ +

Shadow Volume Count = +1+1+1-1 = 2

Eye

position

14

27

Shadow Volumes

Shadowing objectLight

source

zero

zero

+1

+1

+2 +2

+3

Unshadowed

object

Shadow Volume Count = 0

Eye

position

28

Implementing Shadow

Volumes

•For each surface, find silhouette edges

•Build shadow volume (viewer

independent) by extending away from

light

15

29

Shadow Volumes

30

Problems with Shadow Volumes

zero

zero

+1+1

+2

+2

+3

Near clip

plane

Far clip

plane

16

31

Problems with Shadow Volumes

zero

zero

+1+1

+2

+2

+3

Near clip

plane

Far clip

plane

Missed shadow

volume intersection

due to near clip

plane clipping; leads

to mistaken count

32

Problems with Shadow Volumes

Shadowing objectLight

source

Shadow test fails!

Eye

position

17

33

Solution: Invert Depth Test

Shadowing objectLight

source

Shadowed object

Shadow Volume Count = -1+1-1=-1

Eye

position

- +

-

34

Shadow Volumes

• Advantages

- Proper self-shadowing behavior

- Pixel perfect shadows

• Disadvantages

- Surfaces must use planar polygons

- Clipping plane lead to problems

- Eye in shadow problematic

18

35

Soft Shadows

•Point lights cause hard shadows

•Lights are not infinitely small points in

reality

•Area lights yield

soft shadows

36

Soft Shadows

•Point lights cause hard shadows

•Lights are not infinitely small points in

reality

•Area lights yield

soft shadows

19

37

Soft Shadows

•Simulate area lights with lots of points

lightsExpensive

The cluster of

point lights.

38

Soft Shadows

•Blur shadows in image space

Cheap, inaccurate

20

Non-photorealistic rendering

(NPR)

Non-Photorealistic

Rendering

21

41

Motivation

What is our ultimate goal in graphics?

•Photorealism- Makes synthesized pictures like

- photographs of real objects

- Includes artifacts

- (e.g. depth of field, lens flare)

•Communication- Graphics for transmitting

- information into the brain

Gray’s Anatomy

42

Non-Photorealistic

Rendering

• Uses concepts from art instead of physics

• Two fundamental visual cues

- Silhouette – the visible edges of a surface

- Hatching – the use of texture to

- indicate the local orientation

- (shading) of a surface

22

• Pen-and-Ink Illustrations

• Painterly Rendering

• Cartoon Shading

• Technical Illustrations

Pen-and-Ink Illustrations

23

Pen-and-Ink Illustrations

Pen-and-Ink Illustrations

24

Pen-and-Ink Illustrations

Painterly Rendering

25

Painterly Rendering

Painterly Rendering

26

Painterly Rendering

Painterly Rendering

27

Cartoon Shading

Cartoon Shading

28

Cartoon Shading

Technical Drawings

29

Technical Drawings

• Level of abstraction

– Accent important 3D properties

– Dimish or eliminate extraneous

details

• Do not represent reality

Scientific Visualization

30

Summary

• Many styles to choose from

• Automatic vs User Input

• Cartoon Shading temporal

• Acceptance in Art