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My presentation on the "Geometric and Visual Computing Seminar" at the Universita della Svizzera italiana. The topic covered is generalized barycentric coordinates for convex polygons. At the beginning I do some short introduction into what is barycentric coordinates and then consider two types of generalization of these coordinates to convex polygons namely Wachspress and Mean Value Coordinates. Date of presentation: April 2012 For preparing my slides I take pictures and some other information from the internet and I try to use only legal one. But if I did not notice something and you have Rights for any kind of this information and do not want to see it in the presentation please let me know and I will remove it from the slides as fast as possible or remove the slides themselves. Thanks for your collaboration.
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Generalized Barycentric Coordinates
Generalized Barycentric Coordinates
Anisimov Dmitry
Simple
Generalized Barycentric Coordinates
Anisimov Dmitry
Simplex
Generalized Barycentric Coordinates
Anisimov Dmitry
Simplex
Generalized Barycentric Coordinates
Anisimov Dmitry
V1
V3 V2
P A2
A1
A3
A=A1+A2+A3
b1=A1/A b2=A2/A b3=A3/A
Generalized Barycentric Coordinates
Anisimov Dmitry
1790-1868
1827
V1
V3 V2
P A2
A1
A3
A=A1+A2+A3
b1=A1/A b2=A2/A b3=A3/A
Generalized Barycentric Coordinates
Anisimov Dmitry
V1
V3 V2
P
b1=A1/A b2=A2/A b3=A3/A
Proper:es:
• P is inside the triangle if and only if 0 < b1, b2, b3 < 1.
Ø If b1, b2, b3 > 0 hence P -‐ within the interior of the triangle.
Ø If one of bi = 0 hence P -‐ on some edge of the triangle.
Ø If two of bi = 0 hence P -‐ in some vertex of the triangle.
Ø b1 + b2 + b3 = 1.
Generalized Barycentric Coordinates
Anisimov Dmitry
Proper:es:
• By changing the values of b1, b2, b3 between 0 and 1, the point P will move smoothly inside the triangle.
V1
V3 V2
P
b1=A1/A b2=A2/A b3=A3/A
Generalized Barycentric Coordinates
Anisimov Dmitry
Proper:es:
• P is the barycenter of the points v1, v2 and v3 with weights A1, A2 and A3 if and only if:
P =
• The center of the triangle is obtained when b1 = b2 = b3 = .
A1v1+A2v2+A3v3
A1+A2+A3
13
V1
V3 V2
P
b1=A1/A b2=A2/A b3=A3/A
Generalized Barycentric Coordinates
Anisimov Dmitry
Proper:es:
• P is inside the triangle if and only if 0 < b1, b2, b3 < 1.
• By changing the values of b1, b2, b3 between 0 and 1, the point P will move smoothly inside the triangle.
• P is the barycenter of the points v1, v2 and v3 with weights A1, A2 and A3 if and only if:
P =
A1v1+A2v2+A3v3
A1+A2+A3
V1
V3 V2
P
b1=A1/A b2=A2/A b3=A3/A
Generalized Barycentric Coordinates
Anisimov Dmitry
Applica:ons: • Since P is inside the triangle if and only if 0 < b1, b2, b3 < 1
we can determine if a point P is inside the triangle. • Since all bi are linear polynomials and by changing the values of b1, b2, b3 between 0
and 1, the point P moves smoothly inside the triangle
we can linearly interpolate data placed in the ver:ces overall triangle:
F = bifi
i =1
3
∑
V1
V3 V2
P
b1=A1/A b2=A2/A b3=A3/A
Outline:
• Introduc:on
• Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
vi
vi+1
vi-1
P
Ai-1
Ai
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Ai-1
Ai vi
vi+1
vi-1
P
Bi
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Ai-1 Ai Bi
bi=
wi
wjj =1
n∑
Normalized Barycentric Coordinates:
Where weights: wi=c
i +1A
i −1−c
iB
i+c
i −1A
i
Ai −1
Ai
with certain real func:ons ci .
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Do bi sa:sfy all three proper:es of triangular barycentric coordinates?
I.e.
• Posi:vity: bi ≥ 0 for all i
• Par::on of unity:
• Reproduc:on:
bi=1
i =1
n∑
biv
i= P
i =1
n∑
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Do bi sa:sfy all three proper:es of triangular barycentric coordinates?
• Posi:vity: bi ≥ 0 for all i
• Par::on of unity:
• Reproduc:on:
YES for
biv
i= P
i =1
n∑
bi=1
i =1
n∑
I.e.
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Do bi sa:sfy all three proper:es of triangular barycentric coordinates?
• Posi:vity: bi ≥ 0 for all i
• Par::on of unity:
• Reproduc:on:
YES for
bi=1
i =1
n∑
biv
i= P
i =1
n∑
I.e.
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Do bi sa:sfy all three proper:es of triangular barycentric coordinates?
• Posi:vity: bi ≥ 0 for all i
• Par::on of unity:
• Reproduc:on:
bi=1
i =1
n∑
biv
i= P
i =1
n∑
YES for
To get Posi:vity we have to properly choose func:ons ci .
I.e.
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
We choose func:ons ci to be Euclidean distance between P and vi to the power k :
ci = rik with ri = ||P - vi|| and
vi
P
ri
k ∈ R
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Do bi sa:sfy all three proper:es of triangular barycentric coordinates?
• Posi:vity: bi ≥ 0 for all i
• Par::on of unity:
• Reproduc:on:
bi=1
i =1
n∑
biv
i= P
i =1
n∑
YES for I.e.
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
With such a choice of ci we get a whole family of three-‐point coordinates bi :
bi=
wi
wjj =1
n∑
with wi=ri +1k A
i −1−r
ikB
i+r
i −1k A
i
Ai −1
Ai
Bi Ai-1
Ai ri-1 ri
ri+1
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Three-‐point coordinates:
• Wachspress Coordinates for k = 0 and ci = 1.
• Mean Value Coordinates for k = 1 and ci = ri .
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Wachspress coordinates: For the first :me they were introduced by E. L. Wachspress in the work: “A Ra:onal Finite Element Basis” in 1975.
Weight func:ons: wi=
Di
Ai −1
Ai
and bi=
wi
wjj =1
n∑
Di Ai-1 Ai
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Proper:es of Wachspress coordinates:
• Affine precision: • Lagrange property: • Smoothness: bi are C∞ inside arbitrary polygons*
• Par::on of unity:
• Behavior: bi are well-‐defined inside convex polygons
• Posi:vity: bi are posi:ve inside convex polygons
biϕ(v
i)
i =1
n∑ = ϕ for any affine func:on ϕ : R2 →Rd
bi(v
j) = δ
i , j=
1,i = j0,i ≠ j
#$%
&%
bi=1
i =1
n∑
*Except poles
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Proper:es of Wachspress coordinates:
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Mean Value coordinates: For the first :me they were introduced by M. Floater in the work: “Mean Value Coordinates” in 2003.
Weight func:ons: wi=ri −1
Ai−r
iB
i+r
i +1A
i −1
Ai −1
Ai
and bi=
wi
wjj =1
n∑
Bi Ai-1
Ai ri-1 ri
ri+1
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Proper:es of Mean Value coordinates:
• Affine precision:
• Lagrange property:
• Smoothness: bi are C∞ inside arbitrary polygons except at the ver:ces vj where they are only C0
• Par::on of unity:
• Behavior: bi are well-‐defined inside arbitrary polygons • Posi:vity: bi are posi:ve inside convex polygons
biϕ(v
i)
i =1
n∑ = ϕ for any affine func:on ϕ : R2 →Rd
bi=1
i =1
n∑
bi(v
j) = δ
i , j=
1,i = j0,i ≠ j
#$%
&%
Barycentric Coordinates for Planar Convex Polygons
Anisimov Dmitry Generalized Barycentric Coordinates
Proper:es of Mean Value coordinates:
Generalized Barycentric Coordinates