1. Non-Photorealistic Rendering
Mike Wade
April 22, 1999

2. What is Non-Photorealistic Rendering?
Produces images which are non-photorealistic
“NPR”

3. Why NPR? Sometimes we don’t need/want photographs or rendered images
Photographs, renderings might be too “good”
Hand-drawn/painted animations are more energetic

4. How?
Pen and Ink
Toon shading
Painterly Rendering

5. Paint By Numbers: Abstract Image Representations
Paul Haeberli
SIGGRAPH 1990

6. Goal Convert synthetic or natural scene into impressionistic image
Created an interactive painting program
Painting: an ordered list of brush strokes
User could define:
location, color, size, direction, shape

7. Two Approaches to Animation
Generate a new sequence of stroke attributes for each frame
Coherent?
Create single array of brush strokes
Move scene behind that array
“Shower Door” effect

8. Painterly Rendering for Animation
Barbara J. Meier
SIGGRAPH 1996

9. Goal
Eliminate “shower door”
but not too much

10. “Particle System Approach”
Particle set represents geometry of a surface
Particles are rendered as 2d brush strokes

11. Generating Particle Set
Surface is known
Decompose into triangles that represent surface
Randomly distribute particles into triangles

12. Particles Rendered Painterly
Based upon particle:
orientation
color
size
position
Transformed to screen space
Furthest from viewpoint rendered first

13. Animation Keep track of where the particles move
Draw each frame as above

14. Maintaining Hand-drawedness
Use randomness
perturb brush stroke attributes
user defined
use same randomness for each frame for each particle in animation

15. A brief interlude of fun

16. Processing Images and Video for An Impressionistic Effect
Peter Litwinowicz
SIGGRAPH 1997

17. Goal
Automatically generate impressionistic animations from video

18. Generating Strokes Strokes have: position length radius orientation
color

19. Injecting randomness Used to create hand-drawn look
All previous stroke attributes may be slightly perturbed
Also: order drawn
User-defined limits

20. Stroke Clipping Used to show edges
1) Convert to grayscale, blur
2) Find the edges
3) Grow stroke from center until reaches an edge
Fall-off so the stroke is not perfect

21. Orientation May use a “fixed” orientation for stroke
Or: Angle lines normal to gradient of intensity image
Be careful with small gradients

22. Rendering Video First frame: as described above
Optical flow is used to find subsequent stroke centers

23. Optical Flow Avoid sparseness in image Avoid clumping
Add stroke centers
DeLaunay triangulation
Mix new strokes in with old
Avoid clumping
Remove strokes which are “too close”