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Simulating Drawing

• Start with line-drawing methods

• Modify algorithms to produce lines that look hand-drawn

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Simulating Drawing

• Simulating charcoal pencil

• Simulating colored pencil

• Simulating pen-and-ink

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Simulating Drawing

• Simulating graphite pencil

? Sousa and Buchanan (’99)

• Simulating colored pencil

• Simulating pen-and-ink

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Sousa and Buchanan

• Simulate graphite pencil.

• looked at physical interaction of drawing materials(pencil, paper, eraser and blender).

• Not physically accurate, but visually correct.

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Sousa and Buchanan

• consider techniques an artist must master.

• order pencil materials are used

• applied pressure

• how the pencil is shaped

• how the pencil is held

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Sousa and Buchanan

• Drawing primitives : build up tones and texturesusing the drawing materials.

• Rendering : Using the drawing primitives to outline,shade, shadow, or textures images based on refe-rence images or 3d objects emulating pencil dra-wings.

• High level tools : Allow user control of the drawingprocess.

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Sousa and Buchanan

example pen-cil/blender/eraser swatchesstrokes and marks on manydifferent types of paper.

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Sousa and Buchanan

Looked at these with a scanning electron microscope.

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Sousa & Buchanan: Pencil Model

• Degree of hardness

? 19 degrees, determined by graphite and lead con-tent, more graphite → softer and thicker.

? wax is a lubricant? % mass of graphite, clay, and wax particles

• Lead thickness

? Linear interpolation between 2mm and 4mm.

• Tip shape

• Pressure distribution coefficients

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Sousa & Buchanan: Pencil Modelnumber graphite clay wax thickness

9H .41 .53 .05 2mm8H .44 .50 .057H .47 .47 .05

. 2H .60 .34 .05H .63 .31 .05F .66 .28 .05

HB .68 .26 .05B .71 .23 .05

2B .74 .20 .053B .76 .18 .05

. 8B .90 .04 .05 4mm

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Pencil Model: Tip shape

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Pencil Model: Pressure

c, ci ∈ 0, 1c is the center pres-sure and the ci arefor the tip outline ver-tices.

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Paper Model

• Paper weight (thickness)

• Amount of lead to fill surface of grain

• Amount of lead to fill volume of grain

• Grain porous threshold

• Lead threshold volume

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Paper ModelDigital samples of paper textures

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Paper ModelPaper grain (roughness) is a height field.

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Pencil/Paper Interaction

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Pencil/Paper Interaction

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Pencil/Paper Interaction

• Pencil angle, paper grain, pressure, pressure distri-bution, pencil tip, etc.

• As pencil moves across paper the grain ”bitesthelead, depositing pencil materials onto paper.

• The paper may possibly be damaged as well. Theamou

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Blender and Eraser Modelabsorptive & dispersive properties of lead on paper

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Blender and Eraser Types and Tips

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Rendering

a) real, b) automatic with pencil only, c) eraser andblender as well.

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Rendering

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Rendering

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Rendering

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Rendering

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3D Rendering

• Render outline sketch using silhouette & crease li-nes.

• Render reference image in greyscale using Phongshading. Tone-match of each face rendered with si-mulated pencil strokes.

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3D Rendering

Using various pencils and pressures.

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3D Rendering

Using various tones

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3D Rendering

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3D Rendering

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3D Rendering

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3D Rendering

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Colored Pencil Drawings

• Takagi et al. ’99

• volumetric paper model(pulp & binding agent)

• color lead distributed onpaper

• colored particles redistri-buted by brushed water

• volume rendered.

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Colored Pencil Drawings

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Pen-and-Ink Illustration

• omit extraneous detail

• focus attention on relevant features

• clarifying and simplifying shapes

• exposing hidden parts

• can use less space

• can be easier to reproduce

• LOD often more natural

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Pen-and-Ink Illustration

• Stylized illustrations can be more effective than pho-tos

• Anatomy is almost entirely done with illustrations

• Architectural concept models

• assembly instructions

• repair manuals

• economy of expression

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Pen-and-Ink Illustration

• Winkenbach and Salesin (’94, ’96)

• Looked at how humans produced pen-and-ink illu-strations, then applied these rules and techniques to3D models.

? pen gives off no color or tone? color and shading suggested by combinations of

strokes? covering a large area with tone is time-consuming

and difficult? tone cannot be lightened once it is drawn.

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Pen-and-Ink Illustration

• Stroke - when the nib contacts the paper tracing outa path with thickness varied by pressure

? Too thin will look washed out, too thick detractsfrom delicate features

? frequently necessary to vary pen position and niboften turns

? strokes should look natural, not mechanical, stro-kes should vary in thickness

? wavy lines indicate an unfinished design.

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Pen-and-Ink Illustration

• tone - amount of visible light reflected to observerfrom a point

? This must be done using combinations of strokes.

• If the character of the strokes indicating tone are va-ried, they can also be used for texture.

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Pen-and-Ink Illustration• tones by lines of roughly equal weight and spacing• correct arrangement of adjacent tones more import-

ant than an individual tone• sometimes must force tone by contrast or shadows

to distinguish features/objects• Character of strokes important for conveying texture,

geometry and lighting.

? crisp straight lines for glass? horizontal surfaces hatched with horizontal lines? absence of detail for glare? sketchy lines for old & carful stippling for new

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Pen-and-Ink Illustration: Outlines• for contours & to mark important interior features

• outline stroke conveys texture: crisp straight lines forhard objects, a variety of lines for soft objects.

• Thick outlines for shadows or to bring foreward.

• Outlines should become ”haloed” and fade awaywhere one object passes behind another obect.

• Outlines must be introduced when omitting tones

• Indication for outlines as important as for tones.

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Pen-and-Ink Illustration: pipeline• 3D model• Texture - assigned to 3D surface as ”stroke textures”.• Lighting model (Phong)• Visible surface (BSP trees)• Shadows - object-space• Consider 2D adjacency.• “stroke textures” for hatching (no scan conversion)• clip the strokes, then apply waviness• outlines - boundary and interior outlines (suggest

shadow directions or give view-dependent accents)

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Pen-and-Ink Illustration: Strokescharacter by varying waviness and pressure

pressure even for grain varies between planks

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Prioritized Stroke Textures

• List of strokes with priority, draw strokes

• in priority order until desired tone reached

• allow the textures to scale well.

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Strokes Indicate Both Texture and Tone

• cross-hatching

• stippling

• bricks

• shingles

• grass

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Indication

• suggest texture without drawing every last stroke.

• can make illustration more powerful - suggestion

• economy

• just enough detail in the right places

• fading detail appropriately

• difficult to learn

• They use a semi-automatic way - user specifieswhere detail is needed.

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Indication: User specified detail

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Indication: Example without Indication

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Indication: Example with Indication

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Outlines

• boundary outlines - surround visible polygons, con-siders adjaceny info and textures of surrounding fa-ces

• interior outlines - used within polygons to suggestshadow dirrection and give view-dependent accents

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Outlines: View Dependent

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Texture from Boundary Outlines

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Texture from Boundary Outlines

Only draw an outline ifinsufficient contrast.

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Shadows

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Shadows

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Shadows

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Example

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Pen-and-Ink for Parametric Surfaces

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Controlled Density

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Controlled Density

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Variations in Lighting

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Examples

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Examples

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Examples

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Pen-and-Ink from Reference Image

Salisbury et al. (’94, ’96)first system (’94) allowed user to paint over referenceimage

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Pen-and-Ink from Reference Image

Salisbury et al. (’94, ’96)2nd system allows scale-independent illustrations &interactive editing of stroke texture orientation.

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Pen-and-Ink from Reference Image

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Pen-and-Ink from Reference Image

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Pen-and-Ink from Reference Image

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Pen-and-Ink from Reference Image

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Pen-and-Ink from Reference Image

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Watercolor Painting

Curtis et al. ’97

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Watercolor Painting

• three layer fluid model

? shallow-water - water & pigment flow above paper? pigment-depostion - pigment deposited onto and

lifted from paper? capillary - diffuse absorption via capillary action

• With a bit of physics, water and pigment is movedaround paper, each layer (glaze) is stored.

• glazes are composited using a lighting model thathandles absorption and scattering of light.

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Watercolor Painting: 3-Layer model

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Watercolor Painting: Example colors

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Watercolor Painting: Example colors

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Interactive Water Coloring (almost)

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Watercolorization

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Watercolorization

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Watercolorization

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Watercolorization

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Watercolorization

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Graftals - Kowalski et al. ’99

rendering fur grassand trees

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Graftals - Kowalski et al. ’99

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Graftals - Kowalski et al. ’99

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Graftals - Kowalski et al. ’99

movie kowalski.mov

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Painterly Rendering

• Meier ’96 - Disney

• Brush strokes stick to model, not viewplane

• frame-to-frame coherence for animation

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Painterly Rendering

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Painterly Rendering

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Painterly Rendering

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WYSIWIG NPR - Kalnins, et al. 2002

Draw strokesonto a3D model.

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WYSIWIG NPR - Kalnins, et al. 2002

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WYSIWIG NPR - Kalnins, et al. 2002

movie wnpr aud uncmp3.avi

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Coherent Stylized Silhouettes

Kalnins et al. 2003temporal coherence for outlines

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Coherent Stylized Silhouettes

movie css-final-divx.aviJot demo

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Magic Camera

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Magic Camera

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Magic Camera

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Magic Camera

NPR Camera!images by Farell Eaves.

http://www.ljworld.com/section/arts/story/85743/new image every time.

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Magic Camera

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Magic Camera

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Magic Camera

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Magic Camera

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Magic Camera

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Magic Camera

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Magic Camera

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Links

www.cs.utah.edu/npr/pages.cpsc.ucalgary.ca/ mario/

Farell Eaves