Mesh Quilting For Geomet-ric Texture Synthesis

Kun Zhou et al.In SIGGRAPH 2006

발표 이성호2009 년 4 월 15 일

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Abstract

• Mesh quilting– Geometric texture synthesis algorithm– 3D texture sample given in the form of a triangle

• inside a thin shell around an arbitrary surface

• Allow interactive and versatile editing and animation

• Based on stitching together 3D geometry elements

• On curved surfaces– Reduce distortion of geometry elements

• inside the 3D space of the thin shell

– Low-distortion parameterization

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Introduction

• Today’s commodity video cards– Exquisite details can be purely geometrically

modeled

• Modeling such complex geometric de-tails– A tedious process

• Creating mesh-based 3D geometric tex-tures– Remains challenging

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Mesh quilting

• To synthesize geometric details by stitching together small patches – of an input geometric texture sample

• Tools to further edit and animate – these geometric details

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Related work

• Modeling of Geomet-ric Detail on Surfaces– Fur• [Kajiya and Kay 1989]

– Rendering fur with three dimensional textures.

– In Proceedings of SIG-GRAPH 89

– Volume textures• [Neyret 1998]

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• More versatile repre-sentations– Geometric textures• [Elber 2005]

– Shell map• [Porumbescu et al.

2005]

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• Limitations– Periodic textures

• Mesh-based creation – Of geometric textures on arbitrary

meshes– [Fleischer et al. 1995]–Mostly restricted to the dissemination • Of simple texture elements over the surface

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Example-based Texture Syn-thesis

• Synthesis based on per-pixel non-parametric sampling– [Turk 2001;Wei and Levoy 2001; Ying et

al. 2001; Tong et al. 2002; Zelinka and Garland 2003]

• Based on the L2-norm– a relatively poor measure

• of perceptual similarity,

– such algorithms are not applicable • to a large spectrum of textures.

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• Textures by directly copying small parts – of an input texture sample– alpha-blending [Praun et al. 2000]– quilting

• [Efros and Freeman 2001; Liang et al. 2001; Soler et al. 2002; Magda and riegman 2003; Kwatra et al. 2003; Wu and Yu 2004; Zhou et al. 2005]

• Searching for the “min-cut” seams– further enhance the smoothness across the

seams• [Efros and Freeman 2001; Kwatra et al. 2003; Zhou et

al. 2005]

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• Feature matching– [Wu and Yu 2004]– Human visual system is so sensitive

• to edges, corners and other high-level features in tex-tures

• Parallel controllable texture synthesis on GPU– [Lefebvre and Hoppe 2005]

• Texture synthesis using Expectation Maximiza-tion optimization– [Kwatra et al. 2005]

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Graphcut textures

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Shell map

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Challenges

• Little work to provide tool – for 3D geometric texture synthesis.• [Bhat et al. 2004; Lagae et al. 2005]

[Lagae et al. 2005]

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• [Bhat et al. 2004] – voxel-based approach

• [Lagae et al. 2005] – used distance fields

[Bhat et al. 2004]

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Challenges

• Irregular mesh– The input texture sample

• is not a regular array of pixel values

• Geometry elements– Each being truly a small 3D object identified

• As a connected component in 3D

• Quilting is performed on curved surfaces– Severe distortion in the 3D space

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Contributions

• Mesh-based geometric texture synthesis– Synthesized over the base mesh.

• Triangle meshes– Both the input geometry and output geometry

• Integrity– Maintains the integrity of geometry elements

• In the synthesized texture• Texture editing and texture animation

– can be easily performed

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• Aligns elements – through local deformation– And merges elements to connect texture

patches

• Mesh quilting on curved surfaces– Low-distortion parameterization • of the shell space

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Mesh Quilting Synthesis

• Setup & Nomenclature

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Algorithm Overview

• Seed Finding– Find a seed region R from which to grow the output mesh texture

further out

• Geometry Matching– Find the best patch placement around region R using geometry

matching to minimize mismatch between the new and the old patch

• Element Correspondences– Find correspondences between elements in the new patch and

those in the old patch

• Element Deformation– Align the corresponding elements through local deformation

• Element Merging– Expand the output texture by merging the new patch into the

output texture space

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Seed Finding

• Grid-based approach– The bounding boxes of both Mout and Min are subdi-

vided in finer regular grids

• These grids are only two-dimensional• Initially, the cells of Mout are tagged unprocessed

• Each time we wish to grow out the current mesh Mout, – we look for an unprocessed cell with the largest

number of adjacent cells that are already processed• this will be the seed cell that we will try to process next.

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Geometry Matching

• Find how to complete the mesh tex-ture in the seed cell– and possibly add to its surroundings too.

Using the nearby existing mesh texture available near the seed cell

– Find a portion of the original swatch Min best matching this surrounding to ex-tend Mout .

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• Restrict the translation t to be in – grid unit

• Element deformation described in Section 2.6 – will compensate for an imperfect ele-

ment alignment

• Octree data structure for the input texture– Significant speed-up

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Element Correspon-dences

• the overlapping region is usually larger than the small sub-patch Pout – since the input mesh texture covers Pout

completely.

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Element Deformation

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Element Merging

• Every element (either from Cout or Cin) without correspondence – directly added to Mout .

• For every established correspondence (Cout ,Cin)– If Cout is entirely within the overlapping re-

gion, Cout is ignored • and Cin is instead added to the final results

– if Cin is entirely within the overlapping region,• Cin is ignored and Cout is added to Mout .

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In all other cases

• stitch parts of Cin and Cout– to get a singly-connected, combined element– seek a cut path in each element– the graph cut algorithm

• [Boykov et al. 2001]

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Mesh Quilting Over Curved Surfaces

• Setup– Let Mbase be the base mesh that we wish

to enhance with added geometric de-tails.

–Min the geometric texture mesh • used as a swatch

– seamlessly tile the base mesh

– S• the scale of the geometric details

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From Planar to Curved

• 2D grid -> base mesh– Quilting process will stop

• Only when there are no more unprocessed triangles

• Define a local surface patch – By starting from the chosen triangle – Growing the region

• Using breadth-first traversal – Until we reach a certain depth– Or when the total area of the patch exceeds a user-

defined threshold

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• Position of vertices located with re-spect to the base mesh

• Location of a vertex v – over a triangle Tbase – is defined by the barycentric coordinates – of its orthogonal projection • on Tbase

– along with the orthogonal distance (i.e., height) » from the triangle to v

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discrete conformal map-ping

• surface patch is flattened over the 2D plane – using a discrete conformal mapping• DCM [Desbrun et al. 2002]

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• Local operations – Described for planar mesh quilting– Can be performed • Over this parameterization plane

• Position of the newly synthesized vertices–Will be reprojected • Onto the local mesh-based coordinate sys-

tem

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in very curved regions

• If the area distortion induced – by the local parameterization is too

large– Reduce the area of the surface patch• This will decrease

– the size of the output-sub-patch Pout

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Final Mesh Embedding

• Convert the vertex positions– Stored in local coordinates for now– Into a stand-alone, common embedding

• Self-intersections can be created• Build a texture atlas for Mbase

– Convert the above local representation of vertex positions to locations • in a geometry texture space

• Then, construct a shell space around Mbase– Mapping the vertices

• from the geometry texture space to the shell space– will fix the location of the vertices in 3D space

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Shell Mapping

• Porumbescu et al. [2005]– Creates large distortion in curved re-

gions

• We alleviate this!– By optimizing a stretch metric on this

tetrahedral mesh– A natural extension of • low-distortion parameterization

– of triangle meshes

• [Sander et al. 2001]

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Minimization Algorithm

• Only update the u and v texture co-ordinates of the vertices on the offset surface

• Optimization of the stretch metric – along a randomly chosen search direc-

tion – in the (u, v) plane – as in [Sander et al. 2001].

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Discussions

• Regions with very high curvature can be badly handled– parametric distortion of small surface

patches may be high

• Cannot always achieve perfect matching – if the swatch is untileable

• even with major element deformation

– Postprocessing step is performed • to remove those visually-displeasing elements• Figure 5(b)

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Conclusions

• Mesh-based 3D texture synthesis al-gorithm