Wysiwyg npr drawing strokes directly on 3d models
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WYSIWYG NPR: Drawing Strokes Directly on 3D Models Robert D. Kalnins Lee Markosian Barbara J. Meier Michael A. Kowalski Joseph C. Lee Philip L. Davidson Matthew Webb John F. Hughes Adam Finkelstein Princeton University Brown University Outline Related Work Introduction Objective

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Wysiwyg npr drawing strokes directly on 3d models l.jpg

WYSIWYG NPR:Drawing Strokes Directly on 3D Models

Robert D. Kalnins Lee Markosian Barbara J. Meier

Michael A. Kowalski Joseph C. Lee Philip L. Davidson

Matthew Webb John F. Hughes Adam Finkelstein

Princeton University Brown University


Outline l.jpg
Outline

  • Related Work

  • Introduction

  • Objective

  • An Example

  • Paper

  • Toon Shaders

  • Stroke

  • Silleuette

  • Hatching

  • Results


Related work 1 2 l.jpg

Markosian et al. 1997

Salisbury et al. 1997

Deussen & Strothotte 2000

Meier 1996

Related Work 1/2

  • NPR approaches

    • Producing still images


Related work 2 2 l.jpg

Markosian et al. 2000

Praun et al. 2001

Related Work 2/2

  • NPR approaches

    • Producing still images

    • Interactive rendering (with temporal coherency)


Introduction l.jpg
Introduction

  • Direct WYSIWYG painting

    [Hanrahan & Haeberli 1990]

    • Directly painting texture maps onto 3D models


Introduction6 l.jpg

billboard

bridge

ground strokes

terrain editing

Introduction

  • Providing direct drawing interfaces for creating stylized scenes

    [Cohen et al. 2000]


Objective l.jpg
Objective

  • A system that Provides direct drawing interfaces for creating stylized scenes

    • Starting with a 3D model, and

    • Drawing directly on it


An example l.jpg

Loading a model.

Displaying the model in a silhouette style.

An Example


An example9 l.jpg

Choosing a toon shader.

Creating background color and texture.

Adjusting the lighting.

An Example


An example10 l.jpg

Choosing a brush. (black pencil)

Selecting silhouette style.

Drawing creases.

An Example



An example12 l.jpg

Drawing hatching.

An Example


Where we are l.jpg

Choosing a toon shader.

Creating background color and texture.

Adjusting the lighting.

Where we are…


Paper 1 2 l.jpg
Paper 1/2

  • Background color or image


Paper 2 2 l.jpg

at h = 1, t(a) = clamp(2a)

at h  (0, 1), t(a) = h clamp(2a) +

(1 – h) clamp(2a – 1)

at h = 0, t(a) = clamp(2a - 1)

Paper 2/2

  • Media simulation for a paper effect

    • Paper texture encoding a height field, h  [0, 1]

    • Compositing incoming color into the framebuffer with transparency t(a)


Toon shaders l.jpg

Generating texture coordinates from L . n

Toon Shaders

  • Drawing each patch of the object

    • Using custom 1D texture maps [Lake et al. 2000]


Toon shaders with media simulation l.jpg
Toon Shaders with Media Simulation

  • Rendered over a coarse paper texture


Where we are18 l.jpg
Where We Are…

Crease strokes

Decal strokes

Silhouettes


Strokes 1 2 l.jpg
Strokes 1/2

  • Stroke primitive

    • [Northrup and Markosian 2000]

    • Represented as a Catmull-Rom spline

    • Elements under user control:

      • Color

      • Alpha

      • Width

      • Degree of taper at endpoints

      • “halo”


Strokes 2 2 l.jpg
Strokes 2/2

  • Visibility

    • Using an ID reference image:

      [Northrup and Markosian 2000]

      rendering mesh faces, crease edges, silhouette polylines – each in a unique color


Decal strokes l.jpg

this work

[Hanrahan & Haeberli 1990]

Decal Strokes

  • For drawing directly onto a surface

    • Represented as spline curves

    • Benefits:

      • No necessity for a parameterization of the surface.

      • No sampling artifacts

      • Consistent screen-space width of strokes


Crease strokes l.jpg
Crease Strokes

  • Creases:

    • Chains of mesh edges following sharp features

  • When a crease is oversketched, …

    • Recorded as a crease path + a set of displacements

Base Path


Stylized strokes l.jpg
Stylized Strokes

  • Rubber Stamping

    • Repeating a sequence of example offsets

  • Synthesizing by example

    • Adopting ‘video texture’ to generate less repetitive offsets [Schödlet al. 2000]


Silhouettes 1 4 l.jpg
Silhouettes 1/4

  • Applying a sketched prototype stroke to all silhouettes

  • Issues:

    • Silhouette detection

    • Temporal coherency


Silhouettes 2 4 l.jpg

nv

nv

v

v

cv

cv

f(v) = nv. cv

Silhouettes 2/4

  • Silhouette detection:

    • Checking randomly sampled faces

    • Tracing neighboring triangles containing any silhouette


Silhouettes 3 4 l.jpg
Silhouettes 3/4

  • Temporal coherency

    • Silhouettes are view-dependent


Silhouettes 3 427 l.jpg
Silhouettes 3/4

  • Temporal coherency

    • Using arc-length parameter of the previous frame


Silhouettes 4 4 l.jpg
Silhouettes 4/4

  • Scaling

    • Scaling magnitude of the offsets

      • Only when zoomed out


Where we are29 l.jpg
Where We Are…

Hatching


Hatching l.jpg
Hatching

  • Structured hatching

    • A group of roughly parallel strokes

    • Automatic LOD control

User specified strokes

Start transition


Hatching31 l.jpg
Hatching

  • Free hatching

    • Building LODs explicitly by the artist


Hatching32 l.jpg
Hatching

  • Mobile hatching

    • Hatching moving on the surface

      ex) the shade and the highlight


Hatching33 l.jpg

a mobile hatching group

Hatching

  • Mobile hatching

    • Inferring light direction

      • Assume “diffuse highlights” or “dark light”

    • How…

      • 1. Projects the strokes into uv-space

      • 2. Computes their convex hull

      • 3. Finds its centroid c and records the normal nc

nc

c

v

u


Hatching34 l.jpg

n(u)

a mobile hatching group

T

Hatching

  • Mobile hatching

    • Creating a mobile hatching group at each local maximum in l(u)

      • Using a threshold T

        • T=1/2

      • Avoiding popping effect

        • Fade out as l(u) approaches T

l(u) = nc.n(u)

c

v

u


Results l.jpg
Results

  • Environment:

    • 1.5GHz Pentium IV CPU / GeForce3 GPU



Conclusion and future work l.jpg
Conclusion and Future Work

  • A system for drawing stroke-based NPR styles directly on 3D models

  • Extending to

    • Styles such as stippling and pointilism

    • Object-to-object interactions


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