Painterly Rendering with Curved Brush Strokes of Multiple Sizes

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Painterly Rendering with Curved Brush Strokes of
Multiple Sizes

• 2003. 04. 17
• Graphics Media Research Lab
• 3rd S. J. Kim

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• Aaron Hertzmann, Painterly Rendering with Curved
  Brush Strokes of Multiple Sizes, SIGGRAH 98
  Conference proceedings

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Abstract

• Multiple rendering passes
• Larger strokes first
• Add detail with smaller strokes
• Curved brush strokes
• Makes different styles possible
• A style
• An intuitive set of parameters to the painting
  algorithm that a designer can adjust to vary the
  style of painting

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PAINTING TECHNIQUES
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Varying the brush size

• Begin a painting as a rough sketch ? small brush
  (to add detail) yields desirable visual
  effects

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• Algorithm
• Largest to smallest
• The initial canvas is a constant color image

fs some constant factor
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• A jittered grid
• They search each grid points neighborhood to
  find the nearby point with the greatest error,
  and paint at this position
• All strokes for the layer are planned at once
  before rendering

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• The following formula for color difference is
  used to create the difference image
• (r1, g1, b1) (r2, g2, b2) ((r1
  r2)2 (g1 g2)2 (b1 b2)2)1/2
• Z-buffer avoid the overhead of storing and
  randomizing a large list of brush strokes
• makeStroke() in the right code listing is a
  generic procedure that places a stroke on the
  canvas beginning at (x1, y1), given a reference
  image and a brush radius. fg is a constant grid
  size factor.

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Creating curved brush strokes

• A method for painting long, continuous curves
• Focus on painting solid strokes of constant
  thickness to approximate the coloration of the
  reference image
• An element
• Color thickness
• Dragging a circular brush mask along the sweep of
  the spline
• In the system
• Limit brush strokes to constant color
• Use image gradients to guide stroke placement

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• Algorithm
• Begin (x0, y0) with brush radius
• Represented as a list f control points, a color,
  and a brush radius
• (x0, y0) added to the spline
• The color of the reference image at (x0, y0) is
  used as the color of the spline
• Exaggerate or reduce the brush stroke curvature
  by applying an infinite impulse response filter
  to the stroke directions
• Previous stroke direction
• Di-1 (dxi-1, dyi-1)
• A current stroke direction
• Di (dxi, dyi)

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Di fcDi(l-fc)Di-1(fcdxi(1-fc)dxi-1,
fcdyi(1-)dyi-1)
The filter is controlled by a single
predetermined filter constant, fc.
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RENDERING STYLES
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• In order to quantify the notion of painterly
  styles, we propose the use of style parameters to
  control the rendering process
• Parameter
• Provide an intuitive way to vary visual qualities
  of the painting

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• Intuitiveness
• correspond to a visual quality of the painting.
• intuitive to an artist without any technical
  computer knowledge.
• Consistency
• Styles should produce the same visual character
  for different images.
• Robustness
• Each parameter should produce reasonable results
  over a predetermined range, without breaking
  for some values.
• Independence
• Style parameters should be independent of one
  another.

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Some Style Parameters

• Approximation threshold (T)
• Brush sizes
• Smallest brush radius (R1)
• Number of Brushes (n)
• Size Ratio (Ri1/Ri)
• Curvature Filter (fc)
• Blur Factor(fs)
• Minimum and maximum stroke lengths
• Opacity (a)
• Grid size (fg)
• Color Jitter hue (jh), saturation (js), value
  (jv), red (jr), green (jg), blue (jb) color
  components

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Experiments

• The styles are defined as follows.
• Impressionist A normal painting style, with
  no curvature filter, and no random color. T
  100, R(8,4,2), fc1, fs.5, a1, fg1,
  minLength4, maxLength16
• Expressionist Elongated brush strokes.
  Jitter is added to color value. T 50,
  R(8,4,2), fc.25, fs.5, a.7, fg1,
  minLength10, maxLength16, jv.5
• Colorist Wash Loose, semi-transparent brush
  strokes. Random jitter is added to R, G, and B
  color components. T 200, R(8,4,2), fc1,
  fs.5, a.5, fg1, minLength4, maxLength16,
  jrjgjb.3
• Pointillist Densely-placed circles with
  random hue and saturation. T 100, R(4,2),
  fc1, fs.5, a1, fg.5, minLength0,
  maxLength0, jv1, jh.3. (This is similar to the
  Pointillist style provided by 22.)

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Applying different painterly styles. Left column
Impressionist. Middle column Expressionist.
Right column Colorist Wash. Note that the
styles have a consistent visual appearance when
applied to different images.
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Applying different painterly styles. Left column
Impressionist. Middle column Expressionist.
Right column Colorist Wash. Note that the
styles have a consistent visual appearance when
applied to different images.
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Interpolating rendering styles. Images (a) and
(c) are rendered in the Colorist Wash and
Pointillist styles, respectively. The average
of their parameters was used to produce the style
for (b). (The number of layers (n) was rounded up
to 3.)