study diffusion curves: a vector representation for smooth-shaded images
DESCRIPTION
studyTRANSCRIPT
Diffusion Curves: A Vector Representation for Smooth-Shaded
Image
Alexandrina OrzanAdrien Bousseau
Holger Winnem¨ollerPascal Barla
Jo¨elle ThollotDavid Salesin3
SIGGRAPH08
Abstract• Diffusion curve
represent smooth shaded image
• Manual, assisted or automatic extraction for the diffusion curve
• GPU-based multi-grid gradient solution
Outline
• Introduction• Relative Work• Diffusion Curves• Creating Diffusion Curves• Results• Discussion & Future Work
Introduction
• Benefits of vector-based primitives – More compact representation, resolution-
independence, geometric editability, easity animated, more readily stylized
• Limited to represent complex color gradients– Only support linear or radial gradients– Fail in soft shadows, defocus blur, diffuse shading,
glossy reflection …
Introduction – cont.
• Gradient mesh (Adobe Illustrator and Corel CorelDraw)
– A lattice with colors at each vertex that are linearly interpolated across the mesh
– Difficult to create meshes• Optimized gradient mesh– [Sun et al.,SIGGRAPH07]– A semi-automatic method for optimizing a
manually initialized mesh
Diffusion Curve
• A curve that diffuses colors on both sides1. Support traditional freehand drawing• Artists sketch lines first as color boundaries
2. Most color variations can be assumed to caused by edges• Edges constitute a near-complete and natural
primitive for encoding and editing images [Carlsson 88; Elder 99; Elder and Goldberg 01]
RELATIVE WORK
Gradient Tools
Optimized Gradient Meshes[Sun el al. SIGGRAPH07]1. A manually initialized mesh2. Sample and estimate color of control
points on Ferguson patches3. Optimize the reconstruct image with
constrains – smooth, vector line guided and boundary constrained
photographic Initial mesh Optimized mesh reconstructed
Gradient ToolsGradient Brush[McCann & Pollard, SIGGRAPH08]
• Interactive edge-focused drawing tools on gradient domain G – Brush, edge copy, clone
• I = G– Solve as Poisson Eq. with GPU-based
multigrid method
I = G‧
Gradient brush
Edge brush
Clone brush
GPU-Based Multigrid Method for Gradient-Domain
Multi-grid method• Use a coarse version of the
domain to efficiently solve for the low frequency components of the solution,
• Use a fine version of the domain to refine the high frequency components.
GPU-based multigrid method• [Kazhdan and Hoppe,
SIGGRAPH08]• [Goodnight et al.,03]• [Briggs et al.,00]
Standard Multigrid V-cycle
Figure 1 of “Streaming Multigrid for Gradient-Domain Operations on Large Images”, SIGGRAPH 08
u= F
DIFFUSION CURVES
Diffusion curve Final image
Rendering
Diffusion curve
Color source (CL, CR)
WyWx
Blur sources
Sharp color image
Blur map (B)Smoothness of the transition between Left and Right
Final image
2.Diffuse3. Reblur
1. Rasterize sources
1. Rasterize sources
P[npo
s]
•(x,y,Tangent)CL[nl
]
•(r,g,b,t)
CR[nr
]
•(r,g,b,t)
Σ[nσ] •(σ,t)
P
CRCL
Σ
Bezier spline
Blur source - interpolation
Diffusion curve(similar to edge-based
representation [Elder 99])
Color source - interpolation
Gradient Field
• Compute the gradient of RGB 3 channels on the edges
wx,y = (CL – CR) N
P
CR
CL
0 0 0 …0 ..….
….0 ..0 0 0 ..
N
2. Diffusion
• Compute I from gradient w with color source C as constrain
• Apply GPU-based multigrid method– Use Jocobi relaxation to solve
each level of multigrid– Limit the number of relaxation
iteration, ex: 512x512 image• 5i Jacobi iteration per multigrid
level• i is the level no. (fine coarse)
Solve Poisson equation with color constrain C
I =‧wI(x,y) = C(x,y)
where pixel (x,y) store color value
C Sharp color image
w = (wx, wy)
I
3. Reblurring
• Diffuse the Blur map [Elder 99] to define blur kernel size with multigrid method
• Blur each pixel with the blur kernel defined in blur map
B = 0 B(x,y) = σ(x,y)
if pixel (x,y) is on a curve
σ B Sharp color image
Final image
Panning and Zooming
• Require to solve a global eq. • How to pan & zoom without a full Poisson solution at a higher resolution ?1. Compute a low-resolution
diffusion on the un-zoomed image domain
2. Use the obtained solution to define Dirichlet boundary conditions around the zooming window
I =‧wI(x,y) = C(x,y)
Curves outside the current viewport still influence the
viewport’s content !
CREATING DIFFUSION CURVES
Creating Diffusion Curves
• Manual– artists can create an image with our tool by
sketching the lines of the drawing and then filling in the color
• Assisted– artists can trace manually over parts of an image
and we recover the colors of the underlying content• Automatic– artists can automatically convert an image into our
representation and possibly post-edit it
Assisted
1. Identify color outliers1. Sample colors along the curve at
distance d in N2. Measure σ of the neighborhood each
sample. Identify outliers where if it deviates too much from mean
2. Fit a polyline to the color points using DouglasPeucker algorithm– Start from the first and the last pt.– Repeatedly subdivide the line into
smaller segments until the max diff < є
– The end points of the final polyline yield the color controls points
N
d
working on L*a*b channels
Extracting color control points along a drawn curve
Original image Stylistic tracing using color sampling (drawing time < 1 min.)
Original image Active contours and color sampling (drawing time 90 min.)
Automatic Extraction
2. Conversion
to diffusion curves
1. Data extractionOriginal bitmap Automatic reconstruction
1. Data Extraction• Structure-preserving manipulation[Orzan et al.,07]– Extract edge locations and blur values for edge pixels after
scale space analysis– Extract colors at both side of edge
2. Conversion to Diffusion Curve
• Open source Portrace s.w. [Selinger 03]– Approximate a pixel chain with a polyline that has
a min. number of segments• Least approximation error
– Transform the polyline into a smooth curve made from end-to-end connected Bezier curve• Least square Bezier fitting based on a max. user-
specified fitting error and degree of smoothness
Result
• Nvidia GeForce 8800• Realtime performance– 512 x 512 grid– Several thousands
curves
• website
DISCUSSION & FUTURE WORK
photograph Manually created gradient mesh. 340 vertices
Our drawing – manually tracing, 38 diffusion curves, 365 geometric, 176 CL, 156 CR control point
Comparison with Gradient Meshes
• Representation efficiency– D seems more compact. But G has more regular mesh
• Usability– D are more natural drawing tool– D requires good understanding of the final combination. The
meshes are often overlapping • Topology
– D is hard to move a part of an image or warp the entire mesh• Relevant edges have to be selected• Hard to make sure how the colors of outer edges should interact
with their new surrounds for D
Future Challenges
• Layered system– Interaction of multi layers (a
global Poisson solution)– Blending layers with gradual
transparency
• Intersections– Curve splitting– Color editing
• Still poor to create texture
The colors attached to intersecting curves compete with each other creating a smooth color gradient after diffusion
Diffusion curves at intersection can be corrected by curve splitting and
color editing
Conclusion
• Introduce Diffusion Curve as s new image representation– Offer most benefit of vector primitives– Allow to create highly complex image
• Compared with gradient mesh– Comparable both in quality and coding efficiency– Simpler to create– Diffusion curves can be capture automatically
END