CS-184: Computer Graphics

Lecture 23: Non-Photorealistic Rendering

Maneesh AgrawalaUniversity of California, Berkeley

Announcements

Final Project: multiple due dates• Project proposal due Wed Nov 17, 11pm• Progress report 1 due Mon Nov 22, 11pm• Progress report 2 due Wed Dec 1, 11pm• Final report due Wed Dec 8, 11pm

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Conveying ShapeLines

Shading

From Gooch2

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Topics

Photographs vs. drawings

Types of lines

Lines of curvature

Silhouettes and contours

Graphical drawing conventions

Texture and tone

Effects of drawing style

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A photographic depiction captures the exact appearance of the object as we actually see it

Subtle, complex details of coloration and texture are fully represented, with great accuracy

Photograph of the right hip bone (lateral aspect).Johannes W. Rohen and Chihiro Yokochi.

Color Atlas of Anatomy:A Photographic Study of the Human Body,Igaku-Shoin, 1993.

A drawing offers thepossibility to clarify structural or conceptual information that may be difficult to perceive in even a very good photo.

Color drawing of the same subject.

Sobotta Atlas of Human Anatomy, 11th English edition, vol. 2, edited by Jochen Staubesand, translated and edited by Anna N. Taylor,Urban & Schwarzenberg, 1990.

Photo vs. Drawing in Archaeology

James B. Porter. “Relief Monuments”, inThe Student's Guide to Archaeological Illustrating, Brian D. Dillon, ed., Institute of Archaeology, University of California, Los Angeles, 1981

Photo vs. Drawing in Archaeology

James B. Porter. “Relief Monuments”, inThe Student's Guide to Archaeological Illustrating, Brian D. Dillon, ed., Institute of Archaeology, University of California, Los Angeles, 1981

Doorty Cross, Kilfenora Cathedral12th Century

Photo vs. drawing

Hand-drawn illustrations are routinely used to emphasize important features that are difficult to capture in a photograph, while minimizing secondary detail

Drawings are also useful to portray information that cannot be captured or represented photographically, such as hidden surfaces

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Types of Lines

Lines Signify Features

Geometric features

Creases

Boundaries

Self-intersections

Silhouettes

Isoparametric lines

Parabolic lines

Principal directions of curvature

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Classic Geometric Line Types

Discontinuities Boundaries Silhouettes

Isoparametric

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Lines in Images

Photoshop “Find Edges …”16

From Dan Kersten17

Causes of Image Discontinuities

Lines Signify Features

Material features

Texture features

Material boundaries

Lighting features

Attached and unattached shadows

Highlights and highlight boundaries

Isoluminance contours

Luminance extrema

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How to Create Drawings?

Picasso, Portrait of Igor Stravinsky, 1920. Graphite and charcoal, Musée Picasso, Paris, France

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Which lines to draw?

How to draw the lines?

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Two Big Issues

Lines of Curvature

Normal Curvature

Curvature applet: http://www.ies.co.jp/math/java/calc/curve/curve.html22

Space Curve

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Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

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Curvature of Surfaces

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Curvature of Surfaces

Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

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Curvature of Surfaces

Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

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Curvature of Surfaces

Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

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Curvature of Surfaces

Principal Curvatures

Hilbert and Cohn-Vossen [1952]Geometry and the Imagination

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Lumbosacral and Sacro-iliac fusion . Russell Drake, medical illustrator, Mayo Foundation, 1932.

Russell Drake’s “single line system of shading” the flow of the shape is conveyed

through the directions of the carefully drawn strokes

Artistic Inspiration

V. Interrante

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Principal Directions

Klein bottleFrom Hertzmann and Zorin

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K1 = curvature in first principal direction

K2 = curvature in second principal direction

Gaussian curvature: K = K1 K2

Mean curvature: H = (K1 + K2) / 2

K > 0 : elliptic, convex or concave

K < 0 : hyperbolic, saddle-shaped

K = 0 : parabolic, cylindrical or planar

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Guassian Curvature

Gaussian Curvature

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Parabolic Lines

Felix Klein: Apollo35

Silhouettes and Contours

Occluding Contour

From Koenderink, Solid Shape

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Definitions [Koenderink 84]

Rim – the closed space curve on the shape that makes up the silhouette; the space curve is smooth and has no discontinuities except when the surface is discontinuous; the rim is not a plane curve!

Contour – the projection of the rim; the projection may have singularities

Silhouette – the visible part of the contour

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Occluding Contour

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DeCarlo, Finkelstein, Rusinkiewicz, Santella, Suggestive contours for conveying shape, SIGGRAPH 2003

DeCarlo, Finkelstein, Rusinkiewicz, Santella, Suggestive contours for conveying shape, SIGGRAPH 2003

Suggestive Contours

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Suggestive Contours - DEMO

Silhouettes alone Silhouettes + suggestive contours

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Graphical Drawing Conventions

Drawing Parameters

Haloed lines

Taper near t-junction

Control of line weight

Highlighting

Eye-lashing

Sketchiness

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Illustration Rules (Dooley & Cohen)

Importance• Low• Medium• High

Types• Boundaries• Creases• Silhouettes• Isoparametric

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Line Weight

From Martin (reproduced in Gooch and Gooch)

Single weight Two weights Distance weighting

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Highlighting

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Texture and Tone

Arthur L. Guptill (1976) Rendering in Pen and Ink, Watson-Guptill Publications, figure 94

Stroke Collections

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Prioritized Stroke Textures[Salisbury et al. ’94][Winkenbach et al. ’94]

Art Maps[Klein et al. 2000]

s

cale

tone

Pen and Ink Illustration [Winkenbach and Salesin 94]

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tone

te

xtur

e

Pen and Ink Illustration [Winkenbach and Salesin 94]

Collection of stroke images

Will blend → design with high coherence

Stroke nesting property

tone

s

cale

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Pen and Ink Illustration [Winkenbach and Salesin 94]

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Pen and Ink Illustration [Winkenbach and Salesin 94]

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Assessing the effect of non-photorealisic rendered imagesin CAD, Schumann, Strothotte, Raab, Laser, CHI 96

Effects of Drawing Style

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Comparison:CAD

Comparison: Shaded

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Comparison: Sketch

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Draft vs. Presentation

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Affect vs. Cognition

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Summary

Illustrations often better than photographs

Enhance important features

Deemphasize unimportant detail

Grand challenge

Produce a good line drawing

What lines, not just how to draw lines

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