Non-Photorealistic Rendering

Mike Wade

April 22, 1999

What is Non-Photorealistic Rendering?

• Produces images which are non-photorealistic

• “NPR”

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

How?

• Pen and Ink

• Toon shading

• Painterly Rendering

Paint By Numbers:Abstract Image Representations

Paul Haeberli

SIGGRAPH 1990

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

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

Painterly Rendering for Animation

Barbara J. Meier

SIGGRAPH 1996

Goal

• Eliminate “shower door”– but not too much

“Particle System Approach”

• Particle set represents geometry of a surface

• Particles are rendered as 2d brush strokes

Generating Particle Set

• Surface is known

• Decompose into triangles that represent surface

• Randomly distribute particles into triangles

Particles Rendered Painterly

• Based upon particle:– orientation– color– size– position

• Transformed to screen space

• Furthest from viewpoint rendered first

Animation

• Keep track of where the particles move

• Draw each frame as above

Maintaining Hand-drawedness

• Use randomness– perturb brush stroke attributes– user defined– use same randomness for each frame for each

particle in animation

A brief interlude of fun

Processing Images and Video for An Impressionistic Effect

Peter Litwinowicz

SIGGRAPH 1997

Goal

• Automatically generate impressionistic animations from videovideo

Generating Strokes

• Strokes have:– position– length– radius– orientation– color

Injecting randomness

• Used to create hand-drawn look– All previous stroke attributes may be slightly

perturbed– Also: order drawn– User-defined limits

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

Orientation

• May use a “fixed” orientation for stroke

• Or: Angle lines normal to gradient of intensity image– Be careful with small gradients

Rendering Video

• First frame: as described above

• Optical flow is used to find subsequent stroke centers

Optical Flow

• Avoid sparseness in image– Add stroke centers– DeLaunay triangulation– Mix new strokes in with old

• Avoid clumping– Remove strokes which are “too close”