nips nips2010 nips2010-102 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Vladimir Kolmogorov
Abstract: ˆ Consider a convex relaxation f of a pseudo-boolean function f . We say that ˆ the relaxation is totally half-integral if f (x) is a polyhedral function with halfintegral extreme points x, and this property is preserved after adding an arbitrary combination of constraints of the form xi = xj , xi = 1 − xj , and xi = γ where 1 γ ∈ {0, 1, 2 } is a constant. A well-known example is the roof duality relaxation for quadratic pseudo-boolean functions f . We argue that total half-integrality is a natural requirement for generalizations of roof duality to arbitrary pseudo-boolean functions. Our contributions are as follows. First, we provide a complete characterization ˆ of totally half-integral relaxations f by establishing a one-to-one correspondence with bisubmodular functions. Second, we give a new characterization of bisubmodular functions. Finally, we show some relationships between general totally half-integral relaxations and relaxations based on the roof duality. 1
Reference: text
sentIndex sentText sentNum sentScore
1 Generalized roof duality and bisubmodular functions Vladimir Kolmogorov Department of Computer Science University College London, UK v. [sent-1, score-1.218]
2 uk Abstract ˆ Consider a convex relaxation f of a pseudo-boolean function f . [sent-5, score-0.167]
3 We say that ˆ the relaxation is totally half-integral if f (x) is a polyhedral function with halfintegral extreme points x, and this property is preserved after adding an arbitrary combination of constraints of the form xi = xj , xi = 1 − xj , and xi = γ where 1 γ ∈ {0, 1, 2 } is a constant. [sent-6, score-0.722]
4 A well-known example is the roof duality relaxation for quadratic pseudo-boolean functions f . [sent-7, score-0.741]
5 We argue that total half-integrality is a natural requirement for generalizations of roof duality to arbitrary pseudo-boolean functions. [sent-8, score-0.552]
6 First, we provide a complete characterization ˆ of totally half-integral relaxations f by establishing a one-to-one correspondence with bisubmodular functions. [sent-10, score-1.099]
7 Second, we give a new characterization of bisubmodular functions. [sent-11, score-0.726]
8 Finally, we show some relationships between general totally half-integral relaxations and relaxations based on the roof duality. [sent-12, score-0.913]
9 In this paper we consider convex relaxations ˆ f : K → R of f which we call totally half-integral: ˆ Definition 1. [sent-14, score-0.392]
10 (b) Function f : K → R is called totally half-integral if ˆ the form (x, f ˆ restrictions f : P → R are half-integral for all subsets P ⊆ K obtained from K by adding an arbitrary combination of constraints of the form xi = xj , xi = xj , and xi = γ for points x ∈ K. [sent-16, score-0.539]
11 2 A well-known example of a totally half-integral relaxation is the roof duality relaxation for quadratic pseudo-boolean functions f (x) = i ci x i + (i,j) cij xi xj studied by Hammer, Hansen and Simeone [13]. [sent-18, score-1.197]
12 It is known to possess the persistency property: for any half-integral minimizer ˆx x ∈ arg min f (ˆ) there exists minimizer x ∈ arg min f (x) such that xi = xi for all nodes i with ˆ ˆ integral component xi . [sent-19, score-0.535]
13 The goal of this paper is to generalize the roof duality approach to arbitrary pseudo-boolean functions. [sent-22, score-0.514]
14 1 We provide a complete characterization of totally half-integral relaxations. [sent-25, score-0.228]
15 Namely, we prove in secˆ tion 2 that if f : K → R is totally half-integral then its restriction to K1/2 is a bisubmodular function, and conversely any bisubmodular function can be extended to a totally half-integral relaxation. [sent-26, score-1.767]
16 Using this characterization, we then prove several results showing links with the roof duality relaxation (section 4). [sent-29, score-0.684]
17 For some vision applications the roof duality approach [13] has shown a good performance [30, 32, 23, 24, 33, 1, 16, 17]. [sent-38, score-0.536]
18 Therefore, studying generalizations of roof duality to arbitrary pseudo-boolean functions is an important task. [sent-40, score-0.582]
19 Indeed, ˆ ˆ in practice, the relaxation f is obtained as the sum of relaxations fC constructed for each term independently. [sent-42, score-0.345]
20 If c is sufficiently large, then applying the roof duality relaxation to these terms would yield constraints xi = xj and 1 x = xj present in the definition of total half-integrality. [sent-44, score-0.845]
21 Constraints xi = γ ∈ {0, 1, 2 } can also be simulated via the roof duality, e. [sent-45, score-0.424]
22 xi = xj , xi = xj for the same pair of nodes i, j implies xi = xj = 1 . [sent-47, score-0.44]
23 2 Related work Half-integrality There is a vast literature on using half-integral relaxations for various combinatorial optimization problems. [sent-49, score-0.217]
24 In many cases these relaxations lead to 2-approximation algorithms. [sent-50, score-0.197]
25 Hammer, Hansen and Simeone [13] established that these properties hold for the roof duality relaxation for quadratic pseudo-boolean functions. [sent-53, score-0.711]
26 (The relaxation in [25] relied on converting function f to a multinomial representation; see section 4 for more details. [sent-55, score-0.148]
27 Very recently, Iwata and Nagano [18] formulated a half-integral relaxation for the problem of minimizing submodular function f (x) under constraints of the form xi + xj ≥ 1. [sent-57, score-0.602]
28 2 In computer vision, several researchers considered the following scheme: given a function f (x) = fC (x), convert terms fC (x) to quadratic pseudo-boolean functions by introducing auxiliary binary variables, and then apply the roof duality relaxation to the latter. [sent-62, score-0.784]
29 To the best of our knowledge, all examples of totally half-integral relaxations proposed so far belong to the class of submodular relaxations, which is defined in section 4. [sent-66, score-0.677]
30 They form a subclass of more general bisubmodular relaxations. [sent-67, score-0.674]
31 The notion of the Lov´ sz extension of a bisubmodular function introduced by Qi [29] a will be of particular importance for our work (see next section). [sent-71, score-0.757]
32 It has been shown that some submodular minimization algorithms can be generalized to bisubmodular functions. [sent-72, score-0.997]
33 A weakly polynomial combinatorial algorithm for minimizing bisubmodular functions was given by Fujishige and Iwata [12], and a strongly polynomial version was given by McCormick and Fujishige [26]. [sent-74, score-0.742]
34 Recently, we introduced strongly and weakly tree-submodular functions [22] that generalize bisubmodular functions. [sent-75, score-0.704]
35 If f : K → R is a totally half-integral relaxation then its restriction to K1/2 is bisubmodular. [sent-78, score-0.346]
36 Conversely, if function f : K1/2 → R is bisubmodular then it has a unique totally halfˆ integral extension f : K → R. [sent-79, score-0.926]
37 Under this change totally half-integral relaxations are transformed to totally integral relaxations: ˆ ˆ Definition 4. [sent-83, score-0.605]
38 (a) h is called integral if it is a convex ˆ polyhedral function such that all extreme points of the epigraph {(x, z) | x ∈ L, z ≥ h(x)} have the 1/2 ˆ ˆ form (x, h(x)) where x ∈ L . [sent-85, score-0.175]
39 (b) h is called totally integral if it is integral and for an arbitrary ordering of nodes the following functions of n − 1 variables (if n > 1) are totally integral: ˆ h (x1 , . [sent-86, score-0.617]
40 , xn−1 , γ) for any constant γ ∈ {−1, 0, 1} The definition of a bisubmodular function is adapted as follows: function h : L1/2 → R is bisubmodular if inequality (1) holds for all x, y ∈ L1/2 where operations , are defined by tables (2) 1 after replacements 0 → −1, 2 → 0, 1 → 1. [sent-104, score-1.37]
41 To prove theorem 3, it suffices to establish a link ˆ between totally integral relaxations h : L → R and bisubmodular functions h : L1/2 → R. [sent-105, score-1.155]
42 To each signed ordering ω we associate labelings x0 , x1 , . [sent-112, score-0.221]
43 Given a vector x ∈ R , select a signed ordering ω = (π, σ) as follows: (i) choose π so that values |xi |, i ∈ V are non-increasing, and rename nodes accordingly so that |x1 | ≥ . [sent-130, score-0.179]
44 It is not difficult to check that n λi xi x= (4a) i=1 where labelings xi are defined in (3) (with respect to the selected signed ordering) and λi = |xi | − |xi+1 | for i = 1, . [sent-134, score-0.376]
45 Function h is bisubmodular if and only if its Lov´ sz extension h is convex on a L. [sent-139, score-0.776]
46 , n − 1 there holds Li = {x ∈ Lω | xi = σi−1 σi xi−1 }, and for i = n we have Ln = {x ∈ Lω | xn = 0}. [sent-180, score-0.159]
47 Suppose function h : L → R with h(0) = 0 is totally integral. [sent-182, score-0.176]
48 If h : L1/2 → R with h(0) = 0 is bisubmodular then its Lov´ sz extension h : L → R is a totally integral. [sent-187, score-0.933]
49 It remains to show that functions h considered in definition 4 are V \{n} totally integral. [sent-199, score-0.206]
50 , xn−1 , γ) , γ ∈ {−1, 0, 1} It can be checked that these functions are bisubmodular, and their Lov´ sz extensions coincide with a ˆ respective functions h used in definition 4. [sent-218, score-0.163]
51 3 A new characterization of bisubmodularity In this section we give an alternative definition of bisubmodularity; it will be helpful later for describing a relationship to the roof duality. [sent-220, score-0.454]
52 The node i for i ∈ V is called the “mate” of i; intuitively, variable ui corresponds to the complement of ui . [sent-222, score-0.392]
53 Function f : X − → R is called bisubmodular if f (u v) + f (u v) ≤ f (u) + f (v) ∀ u, v ∈ X − (6) where u v = u ∧ v, u v = REDUCE(u ∨ v) and REDUCE(w) is the labeling obtained from w by changing labels (wi , wi ) from (1, 1) to (0, 0) for all i ∈ V . [sent-233, score-0.73]
54 For a labeling u ∈ X , define labeling u by (u )i = ui . [sent-236, score-0.263]
55 Labels (ui , ui ) are transformed according to the rules (0, 1) → (0, 1) (1, 0) → (1, 0) (0, 0) → (1, 1) (1, 1) → (0, 0) (7) Equivalently, this mapping can be written as (x, y) = (y, x). [sent-237, score-0.187]
56 Next, we define sets X − = {u ∈ X | u ≤ u } = {u ∈ X | (ui , ui ) = (1, 1) ∀i ∈ V } X + = {u ∈ X | u ≥ u } = {u ∈ X | (ui , ui ) = (0, 0) ∀i ∈ V } X◦ X = {u ∈ X | u = u } = {u ∈ X | (ui , ui ) ∈ {(0, 1), (1, 0)} = X− ∪ X+ ∀i ∈ V } = X − ∩ X + Clearly, u ∈ X − if and only if u ∈ X + . [sent-239, score-0.561]
57 4 Submodular relaxations and roof duality Consider a submodular function g : X → R satisfying the following “symmetry” condition: ∀u ∈ X g(u ) = g(u) (10) We call such function g a submodular relaxation of function f (x) = g(x, x). [sent-257, score-1.467]
58 Clearly, it satisfies conditions of proposition 10, so g is also a bisubmodular relaxation of f . [sent-258, score-0.822]
59 Review of roof duality Consider a quadratic pseudo-boolean function f : B → R: f (x) = fi (xi ) + i∈V fij (xi , xj ) (11) (i,j)∈E where (V, E) is an undirected graph and xi ∈ {0, 1} for i ∈ V are binary variables. [sent-261, score-0.873]
60 Hammer, Hansen and Simeone [13] formulated several linear programming relaxations of this function and 3 Denote u = 1 0 1 0 0 0 0 0 and v = 0 1 0 0 0 0 1 0 where the top and bottom rows correspond to the labelings of V and V \V respectively, with |V | = 4. [sent-262, score-0.332]
61 One of these formulations was called a roof dual. [sent-266, score-0.361]
62 An efficient maxflowbased method for solving the roof duality relaxation was given by Hammer, Boros and Sun [5, 4]. [sent-267, score-0.662]
63 We will rely on this algorithmic description of the roof duality approach [4]. [sent-268, score-0.514]
64 Each variable xi is replaced with two binary variables ui and ui corresponding to xi and 1 − xi respectively. [sent-270, score-0.658]
65 If u ∈ X is a minimizer of g then the roof duality relaxation has a minimizer x with ˆ xi = 1 (ui + ui ) [4]. [sent-273, score-1.018]
66 ˆ 2 It is easy to check that g(u) = g(u ) for all u ∈ X , therefore g is a submodular relaxation. [sent-274, score-0.34]
67 Also, f and g are equivalent when ui = ui for all i ∈ V , i. [sent-275, score-0.374]
68 ∀x ∈ B g(x, x) = f (x) (13) Invariance to variable flipping Suppose that g is a (bi-)submodular relaxation of function f : B → R. [sent-277, score-0.148]
69 Let i be a fixed node in V , and consider function f (x) obtained from f (x) by a change of coordinates xi → xi and function g (u) obtained from g(u) by swapping variables ui and ui . [sent-278, score-0.554]
70 It is easy to check that g is a (bi-)submodular relaxation of f . [sent-279, score-0.184]
71 Furthermore, if f is a quadratic pseudoboolean function and g is its submodular relaxation constructed by the roof duality approach, then applying the roof duality approach to f yields function g . [sent-280, score-1.568]
72 Conversion to roof duality Let us now consider a non-quadratic pseudo-boolean function f : B → R. [sent-282, score-0.514]
73 Several papers [33, 1, 16] proposed the following scheme: (1) Convert f to a quadratic pseudo˜ ˜ boolean function f by introducing k auxiliary binary variables so that f (x) = minα∈{0,1}k f (x, α) ˜ by applying the roof for all labelings x ∈ B. [sent-283, score-0.568]
74 (2) Construct submodular relaxation g (x, α, y, β) of f ˜ ˜ duality relaxation to f ; then ˜ g (x, α, y, β) = g (y, β, x, α) , g (x, α, x, α) = f (x, α) ˜ ˜ ˜ (3) Obtain function g by minα,β ∈{0,1}k g (x, α, y, β). [sent-284, score-0.771]
75 ˜ minimizing out auxiliary ∀x, y ∈ B, α, β ∈ {0, 1}k variables: g(x, y) = One can check that g(x, y) = g(y, x), so g is a submodular relaxation4 . [sent-285, score-0.378]
76 Existence of submodular relaxations It is easy to check that if f : B → R is submodular 1 then function g(x, y) = 2 [f (x) + f (y)] is a submodular relaxation of f . [sent-289, score-1.293]
77 5 Thus, monomials of the form cΠi∈A xi where c ≤ 0 and A ⊆ V have submodular relaxations. [sent-290, score-0.444]
78 Using the “flipping” operation xi → xi , we conclude that submodular relaxations also exist for monomials of the form 4 It is well-known that minimizing variables out preserves submodularity. [sent-291, score-0.758]
79 Indeed, suppose that h(x) = ˜ ˜ minα h(x, α) where h is a submodular function. [sent-292, score-0.324]
80 Then h is also submodular since ˜ ˜ ˜ ˜ h(x) + h(y) = h(x, α) + h(y, β) ≥ h(x ∧ y, α ∧ β) + h(x ∨ y, α ∨ β) ≥ h(x ∧ y) + h(x ∨ y) 5 In fact, it dominates all other bisubmodular relaxations g : X − → R of f . [sent-293, score-1.195]
81 ¯ g ¯ 7 cΠi∈A xi Πi∈B xi where c ≤ 0 and A, B are disjoint subsets of U . [sent-296, score-0.162]
82 This implies that any pseudo-boolean function f has a submodular relaxation. [sent-300, score-0.304]
83 Note that this argument is due to Lu and Williams [25] who converted function f to a sum of monomials of the form cΠi∈A xi and cxk Πi∈A xi , c ≤ 0, k ∈ A. [sent-301, score-0.221]
84 It is possible to show that the / relaxation proposed in [25] is equivalent to the submodular relaxation constructed by the scheme above (we omit the derivation). [sent-302, score-0.6]
85 bisubmodular relaxations An important question is whether bisubmodular relaxations are more “powerful” compared to submodular ones. [sent-304, score-2.046]
86 Let g be the submodular relaxation of a quadratic pseudo-boolean function f defined by (12), and assume that the set E does not have parallel edges. [sent-307, score-0.501]
87 Then g dominates any other bisubmodular relaxation g of f , i. [sent-308, score-0.842]
88 we give an example of a function f of n = 4 variables which has a tight bisubmodular relaxation g (i. [sent-313, score-0.84]
89 g has a minimizer in X ◦ ), but all submodular relaxations are not tight. [sent-315, score-0.545]
90 Persistency Finally, we show that bisubmodular functions possess the autarky property, which implies persistency. [sent-316, score-0.775]
91 Let f : K1/2 → R be a bisubmodular function and x ∈ K1/2 be its minimizer. [sent-318, score-0.674]
92 Then z ∈ B and [Persistency] Function f : B → R has a minimizer x∗ ∈ B such that x∗ = xi for nodes i ∈ V i with integral xi . [sent-322, score-0.306]
93 It can be checked that zi = yi if xi = 2 and zi = xi if xi ∈ {0, 1}. [sent-324, score-0.283]
94 5 Conclusions and future work We showed that bisubmodular functions can be viewed as a natural generalization of the roof duality approach to higher-order cliques. [sent-329, score-1.218]
95 An important ˆ open question is how to construct bisubmodular relaxations fC for individual terms. [sent-331, score-0.871]
96 However, in our case we need to minimize a bisubmodular function which has a special structure: it is represented as a sum of low-order bisubmodular terms. [sent-337, score-1.348]
97 We recently showed [21] that a sum of low-order submodular terms can be optimized more efficiently using maxflow-like techniques. [sent-338, score-0.304]
98 We conjecture that similar techniques can be developed for bisubmodular functions as well. [sent-339, score-0.704]
99 Submodularity on a tree: Unifying L -convex and bisubmodular functions. [sent-452, score-0.674]
100 Strongly polynomial and fully combinatorial algorithms for bisubmodular function minimization. [sent-471, score-0.694]
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