Painterly Rendering with Curved Brush Strokes of Multiple Sizes

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Painterly Rendering with Curved Brush Strokes of
Multiple Sizes

• Aaron Hertzman, NYU
• http//mrl.nyu.edu/publications/painterly98/
• (SIGGRAPH 98)
• Presented by Marc Flury

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What is Painterly Rendering?

• An oxymoron?
• Its a method of reproducing the artistic style
  and/or expression of a painting using a source
  image.
• An image space technique
• Sort of a glorified Photoshop filter

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An Example
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An Example
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In This Presentation

• The rationale of painterly rendering using
  curved brush strokes of multiple styles
• Overview of implementation
• Examples of applying different parameters to
  achieve different styles

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Rationale

• Real painting requires
• Time
• Skill
• Artistic talent
• Typical filtering methods
• Use only one size brush stroke
• Cant refine style with multiple passes
• Only support one style
• Image is flattened

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Typical Filtering - Photoshop
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More detail needs smaller strokes
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Improved Quality and Detail
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Advantages

• Much faster than painting
• Can be used for video (or even interactive
  rendering)
• Multiple brush sizes allow for varying detail and
  continuous color regions
• Multi-pass method is somewhat analogous to true
  artistic techniques (better results?)
• The system can be parameterized to create
  different styles

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General Painting Algorithm
function paint (sourceImage, R1 Rn) canvas
a new constant color image // paint the
canvas for each brush radius Ri, from
largest to smallest do // apply Gaussian
blur referenceImage sourceImage G(fsRi)
// paint a layer paintLayer(canvas,
referenceImage, Ri) return canvas
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source image
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General Painting Algorithm
function paint (sourceImage, R1 Rn) canvas
a new constant color image // paint the
canvas for each brush radius Ri, from
largest to smallest do // apply Gaussian
blur referenceImage sourceImage G(fsRi)
// paint a layer paintLayer(canvas,
referenceImage, Ri) return canvas
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Layer Painting Algorithm
function paintLayer(canvas, referenceImage, R)
S a new set of strokes, initially empty D
difference(canvas, referenceImage) grid
fg R for x0 to imageWidth stepsize grid do
for y0 to imageHeight stepsize grid do M
the region(x-grid/2xgrid/2,
y-grid/2ygrid/2) areaError sumOfError(M,
D) / grid2 if (areaError gt T) then
(x1, y1) maxPoint(areaError) stroke
makeStroke(R, x1, y1, referenceImage) add
stroke to S paint all strokes S on
canvas random order
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Curved Brush Strokes

• Anti-aliased cubic B-splines
• Each stroke models the color gradient of the
  reference image
• Stroke Representation
• List of control points
• Color
• Brush Size

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Curved Stroke Algorithm
function makeSplineStroke(x0, y0, R, refImage)
strokeColor refImage.color(x0, y0) K new
stroke, radius R, color strokeColor add point
(x0, y0) to K (x, y) (x0, y0) (lastDx,
lastDy) (0, 0) for i1 to maxStrokeLength do
if (i gt minStrokeLength and
(refImage.color(x,y) canvas.color(x,y) lt
refImage.color(x,y)- strokeColor)) then return
K if (refImage.gradientMag(x,y) 0) then
return K (gx, gy) refImage.gradientDirectio
n(x, y) (dx, dy) (-gy, gx) if (lastDx
dx lastDy dy lt 0) then (dx, dy)
(-dx, -dy) (dx, dy) fc (dx,dy) (1-fc)
(lastDx,lastDy) (dx, dy) (dx,dy)/(dx2
dy2)1/2 (x, y) (x Rdx, y Rdy)
(lastDx, lastDy) (dx, dy) add the point
(x, y) to K return K
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Finding Control Points
D2
D1
D0
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Style Parameters

• Approximation Threshold (T)
• Brush Sizes Smallest (Ri), Number (n),
  Size Ratio (Ri-1/Ri)
• Curvature Filter (fc)
• Blur Factor (fs)
• Min and max stroke lengths (minLength, maxLength)
• Opacity (a)
• Grid size (fg)
• Color Jitter (jh, js, jv, jr, jg, jb)

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Examples Source Image
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Examples Impressionism
T 100 R (8, 4, 2) fc 1 fs .5 a 1 fg
1 minLength 4 maxLength 16
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Examples Expressionism
T 50 R (8, 4, 2) fc .25 fs .5 a .7 fg
1 minLength 10 maxLength 16 js .5
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Examples Water Color
T 200 R (8, 4, 2) fc 1 fs .5 a .5 fg
1 minLength 4 maxLength 16 jr jg jh .3
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Examples Pointillism
T 100 R (4, 2) fc 1 fs .5 a 1 fg
.5 minLength 0 maxLength 0 jv jh .3
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More Examples.
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More Examples
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More Examples