Maximum-A-Posteriori (MAP) inference, that is minimizing an energy function over a discrete set of labelingsand labeling space , see  for an overview of solvers and applications for the prominent special case of Markov Random Fields (MRF). Solvers can roughly be categorized into three categories: (i) Exact solvers that use search techniques (e.g. branch-and-bound) and possibly rely on solving lower bounds with LP-solvers to speed up search, (ii)
primal heuristicsthat find a suboptimal solution with an ad-hoc search strategy adapted to the specific problem and (iii) Lagrangean decomposition (a.k.a. dual decomposition) based algorithms that decompose the original problem into smaller efficiently optimizable subproblems and exchange Lagrangean multipliers between subproblems until consensus between subproblems is achieved.
Except when the energy fulfills special assumptions, exact solvers are usually not applicable, since problem sizes in computer vision are too large. On the other hand, primal heuristics can be fast but solution quality need not be good and no information is given to judge it. Moreover, algorithms from the first two paradigms are usually developed ad-hoc for specific optimization problems and cannot be easily extended to other ones. Lagrangean decomposition based algorithms are a good middle ground, since they optimize a dual lower bound, hence can output a gap that shows the distance to optimum, yet use techniques that scale to large problem sizes. Generalization to new problems is also usually much easier, since subproblems can be readily combined.
A large number of algorithmic techniques have been proposed for optimizing a Lagrangean decomposition for MRFs, including (i) Message passing [16, 41, 37, 6] (a.k.a. block coordinate ascent, belief propagation), (ii) first order proximal splitting methods [26, 31] and (iii) Smoothing based methods [9, 29], (iv) Nesterov schemes [28, 10], (v) mirror descent , (vi) subgradient based algorithms [30, 36, 18]. In the case of MAP inference in MRFs, the study  has shown that message passing techniques outperform competing Lagrangean decomposition based methods by a large margin. However, there are two main practical shortcomings of message passing algorithms: (i) they need not converge to the optimum of the relaxation corresponding to the Lagrangean decomposition: while well-designed algorithms monotonically improve a dual lower bound, they may get stuck in suboptimal fixed points. (ii) So called min-marginals must be computable fast for all the subproblems in the given Lagrangean decomposition. While the above problems do not seem to be an issue for most MAP inference tasks in MRFs, for other problems they are. In such cases, alternative techniques must be used. Subgradient based methods can help here, since they do not possess the above shortcomings: They converge to the optimum of the Lagrangean relaxation and only require finding solutions to the subproblems of the decomposition, which is easier than their min-marginals (as needed for (i)), proximal steps (as needed for (ii)) or smoothed solutions (as needed for (iii) and (iv)).
The simplest subgradient based algorithm is subgradient ascent. However, its convergence is typically slow. Bundle methods, which store a series of subgradients to build a local approximation of the function to be optimized, empirically converge faster.
1.1 Contribution & Organization
We propose a multi plane block-coordinate version of the Frank-Wolfe method to find minimizing directions in a proximal bundle framework, see Section 2. Our method exploits the structure of the problem’s Lagrangean decomposition and is inspired by . Applications of our approach to MRFs, discrete tomography and graph matching are presented in Section 3. An experimental evaluation on these problems is given in Section 4 and suggests that our method is superior to comparable established methods.
1.2 Related work
To our knowledge, the Frank-Wolfe method has not yet been used in our general setting, i.e. underlying a proximal bundle solver for a general class of structured energy minimization problems. Hence, we subdivide related work into (i) subgradient/bundle methods for energy minimization, (ii) ad-hoc approaches that use Frank-Wolfe for specific tasks and (iii) proximal bundle methods.
Subgradient based solvers have first been proposed by  for MAP-inference in MRFs and were later popularized by . These works rely on a decomposition of MRFs into trees. New decompositions for certain MRFs were introduced and optimized in  for max-flow subproblems and in  for perfect matching subproblems. The work  used a covering of the graph by a tree and optimized additional equality constraints on duplicated nodes via subgradient ascent. Usage of bundle methods that store a series of subgradients to build a local approximation of the objective function was proposed by [12, 32] for MAP inference in MRFs.
The Frank-Wolfe algorithm was developed in the 50s  and was popularized recently by . In  a block coordinate version of Frank-Wolfe was proposed and applied to training structural SVMs. Further improvements were given in [34, 24] where, among other things, caching of planes was proposed for the Frank-Wolfe method. Several works have applied Frank-Wolfe to the MAP-MRF inference problem: (1)  used Frank-Wolfe to compute an approximated steepest-descent direction in the local polytope relaxation for MRFs. (2)  used Frank-Wolfe to solve a modified problem obtained by adding a strictly convex quadratic function to the original objective (either primal or dual). In contrast to these works, we use Frank-Wolfe inside a proximal method. Furthermore, the papers above use a fine decomposition into many small subproblems (corresponding to pairwise terms of the energy function), while we decompose the problem into much larger subproblems. In general, our decomposition results in fewer dual variables to optimize over, and each Frank-Wolfe update can be expected to give a much larger gain. Frank-Wolfe was also used in  for MAP inference in dense MRFs with Potts interactions and Gaussian weights. As we do, they use Frank-Wolfe to optimize proximal steps for MAP-inference. In constrast to our work, they do not optimize Lagrangean multipliers, but the original variables directly. In other words, they work in the primal, while we work in the dual. We remark that our formulation is applicable to more general integer optimization problems than either [33, 23, 1] and it does not seem straightforward to apply these approaches to our more general setting while only requiring access to MAP-oracles of subproblems.
Proximal bundle methods were introduced in [13, 21] to accelerate subgradient descent algorithms. They work by locally building an approximation (bundle) to the function to be optimized and use this bundle to find a descent direction. For stability, a quadratic (proximal) term is added . While not theoretically guaranteed, proximal bundle methods are often faster than subgradient methods.
Original problem We consider the problem of minimizing a function of Boolean variables represented as a sum of individual terms:
Here term is specified by a subset of variables and a function of
variables. Vectoris the restriction of vector to . The arity of function can be arbitrarily large, however we assume the existence of an efficient min-oracle that for a given vector computes together with the cost , where is the effective domain of . It will be convenient to denote
where subsets are defined as follows:
The assumption means that we can efficiently compute a supergradient of concave function at a given .
Since (1) is in general an NP-hard problem, our goal will be to solve a certain convex relaxation of (1), which will turn out to be equivalent to the Basic LP relaxation (BLP) of (1) . This relaxation has been widely studied in the literature, especially for the MAP-MRF problem (in which case it is usually called the local polytope relaxation [40, 41]). We emphasize, however, that our methodology is different from most previous works: before applying the BLP relaxation, we represent the objective as a function of Boolean indicator variables. This allows expressing complicated combinatorial constraints such as those in multi-label discrete tomography and graph matching problems (see Section 3).
Lagrangean relaxation For a vector let us denote the first components as and the last one as (so that ). We also denote and . The -th component of vector will be denoted as . Problem (1) can now be equivalently written as
We form the relaxation of (2) by removing the non-convex constraint :
We will not directly optimize this relaxation, but its Lagrangean dual . For each equality constraint we introduce Lagrange multipliers . The collection of these multipliers will be denoted as . The dual will be optimized over the set
where we denoted
Next, we describe how we maximize function .
Proximal term We have a non-smooth concave maximization problem in , hence algorithms that require a differentiable objective functions will not work. In proximal bundle methods  an additional proximal term is added. This results in the new objective
for a center point . The proximal quadratic terms act as a trust-region term in the vicinity of and make the function strongly concave, hence smoothing the dual. A successively refined polyhedral approximation  is typically used for solving (6). We develop a proximal method that will alternate between minimizing (6) with the help of a multi-plane block coordinate Frank-Wolfe method and updating the proximal center .
2.1 Maximizing : BCFW algorithm
Objectives similar to (6) (without the summation constraint on the
-variables) are used for training structural Support Vector Machines (SSVMs). Following[20, 34, 24], we use a block-coordinate Frank-Wolfe algorithm (BCFW) applied to the dual of (6), more specifically its multi-plane version MP-BCFW . The dual of (6) is formulated below.
The dual problem to is
Define by for . Then the optimal in (7) is and
where denotes the derivative w.r.t. variables .
Next, we review and adapt to our setting BCFW and MP-BCFW algorithms for minimizing function over
. We will also describe a practical improvement to the implementation, namely compact representation of planes, and discuss the problem of estimating the duality gap.
BCFW  The algorithm maintains feasible vectors . At each step BCFW tries to decrease the objective by updating component for a chosen term (while maintaining feasibility). To achieve this, it first linearizes the objective by using the Taylor expansion around the current point:
The optimal solution of the linearized objective is computed by calling the -th oracle: . The new vector
is obtained as the best interpolation ofand with all other components fixed to , i.e. . The step size is chosen to minimize the objective. The optimal can be easily computed in closed form (see Lemma 1 in [swoboda:2018arxiv]). One pass of BCFW is summarized in Algorithm 1. To avoid the expensive recomputation of the sum in Prop. 2 needed for computing the gradient and the step size, it is held and updated explicitly in Algorithm 1.
MP-BCFW  In this paper we use the multi-plane version of BCFW. This method caches planes returned by min-oracles for terms . Let be the set of planes currently stored in memory for the -th subproblem. It defines an approximation of term . Note that for any .
MP-BCFW uses exact passes (that call the “exact” oracle for in line 3 of Algorithm 1) and approximate passes (that call the “approximate” oracle for ). One MP-BCFW iteration consists of one exact pass followed by several approximate passes. The number of approximate passes is determined automatically by monitoring how fast the objective decreases per unit of time. Namely, the method computes the ratio where is the vector at the beginning of the MP-BCFW iteration, is the current vector and is the time passed since the beginning of the MP-BCFW iteration. If this ratio drops after an approximate pass then the iteration terminates, and a new MP-BCFW iteration is started with an exact pass. The number of approximate passes will thus depend on the relative speed of exact and approximate oracles.
Note that the time for an approximate oracle call is proportional to the size of the working set . The method in  uses a common strategy for controlling this size: planes that are not used during the last iterations are removed from . We use , which is the default parameter in .
Compact representation of planes Recall that planes in the set have the form for some . A naive approach (used, in particular, in previous works [34, 24]) is to store them explicitly as vectors of size . We observe that in some applications a special structure of allows storing and manipulating these planes more efficiently. For example, in MAP-MRF inference problems a variable with possible values can be represented by a single integer, rather than indicator variables.
In our implementation we assume that each vector can be represented by an object in a possibly more compact space . To specify term , the user must provide an array that determines set in a natural way, specify the size of objects , and implement the following functions:
A bijection .
Min-oracle that for a given computes and returns its compact representation (i.e. ) together with the cost .
A function that computes inner product for a given vector and object .
Note, calling the approximate oracle in line 3 involves calling the function in (F3) times; it typically takes time where is the length of the array for storing .
The efficient plane storage mechanism gives roughly a speedup of a single MP-BCFW pass on the protein-folding MRF dataset (see Sections 3 and 4). Moreover, it typically results in a slightly better objective value obtained after each MP-BCFW pass, since more approximate passes can be done before an exact pass is called (since approximate passes are accelerated by the compact plane storage, their objective decrease per unit of time is higher, hence they are called more often).
2.2 Algorithm’s summary
Recall that our goal is to maximize function over ; this gives a lower bound on the original discrete optimization problem . We now summarize our algorithm for solving , and describe our choices of parameters. To initialize, we set , and for each . Then we start the main algorithm. After every 10 iterations of MP-BCFW we update (keeping vectors and sets unchanged), where is the vector with the largest value of objective seen so far. Since evaluating is an expensive operation, we do it for the current vector only after every 5 iterations of MP-BCFW.
Remark 2 (Convergence).
If we evaluated the inner iterations in MP-BCFW exactly, our method would amount to the proximal point algorithm which is convergent . Even with non-exact evaluation, convergence can be proved when the error in the evaluation of the proximal is shrinking fast enough towards zero. However, we have use a more aggressive scheme that updates the proximal point every 10 iterations and does not bound the inexactness of the proximal step evaluation. Experimentally, we have found that it gives good results w.r.t. the objective of the overall problem (5).
2.3 Estimating duality gap
To get a good stopping criterion, it is desirable to have a bound on the gap between the current and optimal objectives. This could be easily done if we had feasible primal and dual solutions. Unfortunately, in our case vector is not a feasible solution of problem (3), since it does not satisfy equality constraints . 111 We say that vector is a feasible (optimal) solution of (3) if there exists a vector so that is a feasible (optimal) solution of (3). Clearly, can be easily computed from feasible , so omit it for brevity. To get a handle on the duality gap, we propose to use the following quantities:
Note, if we knew that an optimal solution belongs to some bounded region then we could use (11) to obtain a bound on the duality gap. Such region can be obtained for some applications, but we did not pursue this direction.
In this section we give a detailed description of the three applications used in the evaluation: Markov Random Fields (MRFs), discrete tomography and the graph matching problem. The latter two are both extensions of the MAP-inference problems for MRFs. Those three problems are reviewed below.
3.1 Markov Random Fields
An MRF consists of a graph and a discrete set of labels for each . The goal in Maximum-A-Posteriori (MAP) inference is to find labeling that is minimal with respect to the potentials:
This problem is NP-hard for general graphs , but can be solved efficiently for trees. Hence, we choose a covering by trees, as done in . Additionally, we seek a small number of trees, such that the number of Lagrangean variables stays small and optimization becomes faster.
Arboricity A tree covering of a graph is called a minimal tree cover, if there is no covering consisting of fewer trees. The associated number of trees is called the graph’s arboricity. We compute the graph’s arboricity together with a minimal tree cover efficiently with the method 
Boolean encoding To phrase the problem as an instance of (1), we encode labelings via indicator variables for while adding constraints (i.e. assigning infinite cost to configurations that do not satisfy this constraint).
A tree cover and a Boolean encoding are also used for the two problems below; we will not explicitly comment on this anymore.
3.2 Discrete tomography
The discrete tomography problem is to reconstruct an image from a set of linear (tomographic) projections taken at different angles, where image intensity values are restricted to be from a discrete set of intensities. See Figure 2 for an illustration. The problem is ill-posed, since the number of linear projections is smaller than the number of pixels to reconstruct. Hence, we use a regularizer to penalize non-regular image structures. Formally, we have an MRF , where the node set corresponds to the pixel grid and the edges connect nodes which correspond to neighboring pixels. The label space is for some . Additionally, the labeling must satisfy linear projections with . Usually, no local information is available, hence the unary potentials are zero: for . The problem reads
where . A typical choice for the pairwise potentials is the truncated -norm . Each row of the projections forms another subproblem. The -th row of hence is of the form . Efficient solvers for this problem were recently considered in . We follow their recursive decomposition approach for the solution of the projection subproblems. Details are given below.
Discrete tomography subproblems
We use a simplified version of the recursive decomposition approach of  for efficiently solving the summation constraints of the discrete tomography problem. Below we give the corresponding details. Let the -th row of be of the form and recall that . Each such tomographic projection will correspond to a single subproblem. Taking care of Lagrangean multipliers , we can rename variables and rewrite the problem as
We will follow a recursive decomposition approach. To this end, we introduce helper summation variables .
We partition the set into and . We recursively partition and analoguously until reaching single variables. This results in a tree with as root.
Constraints on summation variables
Whenever we have partitions , we add the constraint .
We will propose a dynamic programming approach to solving (14). First, for each value of summation variable we store a value . We compute recursively from leaves to the root. For partitions we compute
After having computed , we set . Subsequently, we make a pass from root to leaves to compute the optimal label sum for each variable as follows:
Fast dynamic programming
3.3 Graph matching
The graph matching problem consists of finding a MAP-solution in an MRF where the labels for each node come from a common universe . The additional matching constraint requires that two nodes cannot take the same label: . Hence, any feasible solution defines an injective mapping into the set of labels. For an illustration see Figure 2. The problem is
|synthetic 2 proj.||9||1024||1984||266.12||265.89||239.39|
|synthetic 4 proj.||9||1024||1984||337.88||336.33||316.61|
|synthetic 6 proj.||9||1024||1984||424.36||417.76||391.09|
|6d scene flow||6||48-126||1148-5352||-2864.2||-2865.61||-2867.60||-2877.08|
We have chosen a selection of challenging MRF, discrete tomography and graph matching problems where message passing methods struggle or are not applicable. Detailed descriptions of these problems can be found in Appendix A.
Note that there are a large number of MRF and graph matching problems where message passing is the method of choice due to its greater speed and sufficient solution quality, see [11, 38]. In such cases there is no advantage in using subgradient based solvers, which tend to be slower. However, our chosen evaluation problems contain some of the most challenging MRF and graph matching problems with corresponding Lagrangean decompositions not solved satisfactorily with message passing solvers.
We have excluded primal heuristics that do not solve a relaxation corresponding to our Lagrangean decomposition at all from comparison. First, they do not deliver any lower bounds, hence cannot be directly compared. Second, we see them as complementary solvers, since, when they are applied on the solved dual problem, their solution quality typically improves. We have run every instance for 10 minutes on a Intel i5-5200U CPU. Per-dataset plots showing averaged lower bound over time can be seen in Figure 3. Dataset statistics and final lower bounds averaged over datasets can be seen in Table 1. Detailed results for each instance in each dataset can be found in in Appendix A.
Solvers We focus our evaluation on methods that are able to handle general MAP inference problems in which the access to subproblems is given by min-oracles.
Our solver as described in Section 2.
The state-of-the-art bundle method ConicBundle , which does not treat individual subproblems individually, but performs ascent on all Lagrangean multipliers simultaneously.
Subgradient ascent with a Polyak step size rule. This solver was also used in  for MRFs.
Additionally, we tested state-of-the-art versions of message passing (MP) solvers, when applicable. MP is a popular method for MAP-MRF problems, and has recently been applied to the graph matching problem .
All solvers optimize the same underlying linear programming relaxation. Additionally, all subgradient based solvers also use the same decomposition. This ensures that we compare the relevant solver methodology, not differences between relaxations or decompositions.
Choice of proximal weight from (6) The performance of our algorithm depends on the choice of the proximal weight parameter from (6). A too large value will make each proximal step take long, while a too small value will mean too many proximal steps until convergence. This behaviour can be seen in Figure 5, where we have plotted lower bound against time for an MRF problem and FWMAP with different choices of proximal weight . We see that there is an optimal value of , with larger and smaller values having inferior performance. However, we can also observe that performance of FWMAP is good for values an order of magnitude larger or smaller, hence FWMAP is not too sensitive on . It is enough to choose roughly the right order of magnitude for this parameter.
We have observed that the more subproblems there are in a problem decomposition (1), the smaller the proximal weight should be. Since more subproblems usually translate to more complex dependencies in the decomposition, a smaller value of will be beneficial, as it makes the resulting more complicated proximal steps better conditioned. A good formula for will hence be decreasing for increasing numbers of subproblems . We have taken three instances out of the 50 we evaluated on and roughly fitted a curve that takes suitable values of proximal weight for these three instances, resulting in
4.1 Markov Random Fields.
4.2 Discrete tomography.
In  a dual decomposition based solver was proposed for the multi-label discrete tomography problem. The decomposition was optimized with ConicBundle . For our decomposition, message passing solvers are unfortunately not applicable. The main problem seems that due to the unary potentials being zero, min-marginals for all subproblems are also zero. Hence any min-marginal based step used in message passing will result in no progress. In other words, the initially zero Lagrangean multipliers are a local fix-point for message passing algorithms. Therefore, we only compare against SA and CB.
4.3 Graph matching.
As shown in [39, 44], Lagrangean decomposition based solvers are superior to solvers based on primal heuristics also in terms of the quality of obtained primal solutions. In particular,  has proposed a message passing algorithm that is typically the method of choice and is on par/outperforms other message passing and subgradient based techniques on most problems. We denote it by MP.
Our solver FWMAP achieved the highest lower bound on each instance. It was substantially better on the hardest and largest problems, e.g. on sheep logan. While message passing solvers were faster (whenever applicable) in the beginning stages of the optimization, our solver FWMAP was fastest among subgradient based ones and eventually achieved a higher lower bound than the message passing one. We also would like to mention that our solver had a much lower memory usage than the competing bundle solver CB. On the larger problem instances, CB would often use all available memory on our 8 GB machine.
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Appendix A Proofs
The BLP relaxation 
introduces a probability distributionover for each and a probability distribution over for each . It can be written as follows:
Define an extension of function as follows: for a vector set
Note, if then (20) does not have a feasible solution, and so . Observe that the constraints in the last line of (19) for are redundant - they follow from the remaining constraints. Also observe that constraints for can be written as if we denote for . Therefore, problem (19) can be equivalently rewritten as follows:
It can be seen that the last problem is equivalent to (3). Indeed, we just need to observe that for each and we have
Proof of Proposition 1.
Write , then problem (3) can be written as
The Lagrangian w.r.t. the equality constraints is given by
Therefore, the dual function for is
The problem can thus be formulated as , or equivalently as . This coincides with formulation given in Proposition 1.
Since constraint can be expressed as a linear program, the duality between (3) and (5) can be viewed as a special case of linear programming (LP) duality (where the value of function is also written as a resulting of some LP). For LPs it is known that strong duality holds assuming that either the primal or the dual problems have a feasible solution. This holds in our case, since vector is feasible. We can conclude that we have a strong duality between (3) and (5). ∎
Proof of Proposition 2.
First, we derive the dual of :
The function has a closed form expression, since it is a quadratic function subject to linear equalities. Write for . The in the expression defining are
The function value is
The gradient is . ∎
Proof of Proposition 3.
Let be the set of vectors satisfying the equality constraints for all . By construction, for any we have
Consider vector . Then if and only if for some . (This can be seen from the definition of in Section 2.3).
Consider vectors and . They are an optimal primal-dual pair if and only if , which in turn holds if and only if (since ).
It remains to show inequality (11). Denote , then for any . Denoting and , we get
Summing these inequalities over gives
Recalling that , we obtain the desired claim: