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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

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
Outline
  • Related Work
  • Introduction
  • Objective
  • An Example
  • Paper
  • Toon Shaders
  • Stroke
  • Silleuette
  • Hatching
  • Results
related work 1 2
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
Markosian et al. 2000

Praun et al. 2001

Related Work 2/2
  • NPR approaches
    • Producing still images
    • Interactive rendering (with temporal coherency)
introduction
Introduction
  • Direct WYSIWYG painting

[Hanrahan & Haeberli 1990]

    • Directly painting texture maps onto 3D models
introduction6
billboard

bridge

ground strokes

terrain editing

Introduction
  • Providing direct drawing interfaces for creating stylized scenes

[Cohen et al. 2000]

objective
Objective
  • A system that Provides direct drawing interfaces for creating stylized scenes
    • Starting with a 3D model, and
    • Drawing directly on it
an example
Loading a model.

Displaying the model in a silhouette style.

An Example
an example9
Choosing a toon shader.

Creating background color and texture.

Adjusting the lighting.

An Example
an example10
Choosing a brush. (black pencil)

Selecting silhouette style.

Drawing creases.

An Example
where we are
Choosing a toon shader.

Creating background color and texture.

Adjusting the lighting.

Where we are…
paper 1 2
Paper 1/2
  • Background color or image
paper 2 2
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
Generating texture coordinates from L . nToon Shaders
  • Drawing each patch of the object
    • Using custom 1D texture maps [Lake et al. 2000]
toon shaders with media simulation
Toon Shaders with Media Simulation
  • Rendered over a coarse paper texture
where we are18
Where We Are…

Crease strokes

Decal strokes

Silhouettes

strokes 1 2
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
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
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
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
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
Silhouettes 1/4
  • Applying a sketched prototype stroke to all silhouettes
  • Issues:
    • Silhouette detection
    • Temporal coherency
silhouettes 2 4
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
Silhouettes 3/4
  • Temporal coherency
    • Silhouettes are view-dependent
silhouettes 3 427
Silhouettes 3/4
  • Temporal coherency
    • Using arc-length parameter of the previous frame
silhouettes 4 4
Silhouettes 4/4
  • Scaling
    • Scaling magnitude of the offsets
      • Only when zoomed out
hatching
Hatching
  • Structured hatching
    • A group of roughly parallel strokes
    • Automatic LOD control

User specified strokes

Start transition

hatching31
Hatching
  • Free hatching
    • Building LODs explicitly by the artist
hatching32
Hatching
  • Mobile hatching
    • Hatching moving on the surface

ex) the shade and the highlight

hatching33
a mobile hatching groupHatching
  • 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
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
Results
  • Environment:
    • 1.5GHz Pentium IV CPU / GeForce3 GPU
conclusion and future work
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