Paper ID: | 1192 |
---|---|

Title: | Interpretable and Globally Optimal Prediction for Textual Grounding using Image Concepts |

The paper proposes a unified framework to solve the
textual grounding problem as an energy minimization problem, and uses
an efficient branch and bound scheme to find the global optimum.
This differs from approaches that evaluate on generated region
proposals.
The energy function used in this framework is a linear combination of
feature functions as score maps, each of either the form of word
priors, geometric information cues, or image based segmentations and
detections. It's easy to add more feature functions of the same form
to the framework. The top-level weights combining the features
functions are formulated and learned in the form of structured SVMs.
There are many parts of the proposed system, including the many
feature functions as score maps being combined in the energy function,
weight learning as a structured SVM problem, global inference using
the efficient sub-window search, etc., but the paper seems to have
given enough details and references to describe all parts of the
system.
One thing I would be interested to know, not clearly given in the
paper, is how efficient the inference is, exactly, in terms of
computation resource and time.

The authors propose a method for textual grounding, that is identifying the
correct object bounding box described by words that are in general ambiguous
but specific enough withing the given image. The main advantage of the method
is that unlike other methods it does not rely on having access to a good set of
proposal bounding boxes.
The authors show how to setup an energy function which can be global minized
using efficient subwindow search (a branch-and-bound-based method). The energy
function is a linear combination of "image concepts" weighted by the words
contained in the query. These weights specify how much a given word relates to
a given concept. The image concepts are features encoding priors of the most
frequent words, probability maps for selected nouns obtained from a
convolutional neural network, and two geometric cues on the size of the
bounding box. Finally, the authors formulate the parameter learning as a
structured support vector machine.
The paper builds upon the efficient subwindow search method applying it to
textual grounding in an interesting non-trivial way. The paper is in general
well written, reasonably easy to understand, the experiments well done and the
results seem convincing.
There are two things that might be limiting:
1. The image concepts are combined linearly in the energy function which brings
the problems with some queries, such as "dirt bike" discussed by the
authors. However, it also brings efficient evaluation of the bound.
2. The runtime of the method. The authors should add a comparison of runtimes
(or at least the runtime of their method) to the tables comparing the
accuracy.
Comments:
L6: we able -> we are able
Fig.2 vs. Eq.1: please use either argmax or argmin consistently
L100+1: "to depends" -> depends
L102: sever -> severe
L113 and Eq.2: the indicator \iota_s seems superfluous, removing it and using iteration over s \in Q would make reading simpler
L135: accumulate -> accumulates
L235: us -> use

Summary
The approach proposes a simple method to find the globally optimal (under the formulation) box in an image which represents the grounding of the textual concept. Unlike previous approaches which adopt a two stage pipeline where region proposals are first extracted and then combined into grounded image regions (see [A] for example), this approach proposes a formulation which finds the globally optimal box for a concept. The approach assumes the presence of spatial heat maps depicting various concepts, and uses priors and geometry related information to construct an energy function, with learnable parameters for combining different cues. A restriction of the approach is that known concepts can only be combined linearly with each other (for instance the score map of “dirt bike” is “dirt” + “bike” with the learn weighting ofcourse), but this also allows for optimal inference for the given model class. More concretely the paper proposes an efficient technique based on [22] to use branch and bound for efficient sub-window search. Training is straightforward and clean through cutting-plane training of structured SVM. The paper also shows how to do efficient loss augmented inference during SVM training which makes the same branch and bound approach applicable to cutting-plane training as well. Finally, results are shown against competitive (near- state of the art approaches) on two datasets where the proposed approach is shown to outperform the state of the art.
Strengths
- Approach alleviates the need for a blackbox stage which generates region proposals.
- The interpretation of the weights of the model and the concepts as word embeddings is a neat little tidbit.
- The paper does a good job of commenting on cases where the approach fails, specifically pointing out some interesting examples such as “dirt bike” where the additive nature of the feature maps is a limitation.
- The paper has some very interesting ideas such as the use of the classic integral images technique to do efficient inference using branch and bound, principled training of the model via. a clean application of Structural SVM training with the cutting plane algorithm etc.
Weakness
1. Paper misses citing a few relevant recent related works [A], [B], which could also benefit from the proposed technique and use region proposals.
2. Another highly relevant work is [C] which does efficient search for object proposals in a similar manner to this approach building on top of the work of Lampert et.al.[22]
3. It is unclear what SPAT means in Table. 2.
4. How was Fig. 6 b) created? Was it by random sub-sampling of concepts?
5. It would be interesting to consider a baseline which just uses the feature maps (used in the work, say shown in Fig. 2) and the phrases and simply regresses to the target coordinates using an MLP. Is it clear that the proposed approach would outperform it? (*)
6. L130: It was unclear to me how the geometry constraints are exactly implemented in the algorithm, i.e. the exposition of how the term k2 is computed was uncler. It would be great to provide details. Clear explanation of this seems especially important since the performance of the system seems highly dependent on this term (as it is trivial to maximize the sum of scores of say detection heat maps by considering the entire image as the set).
Preliminary Evaluation
The paper has a neat idea which is implemented in a very clean manner, and is easy to read. Concerns important for the rebuttal are marked with (*) above.
[A] Hu, Ronghang, Marcus Rohrbach, Jacob Andreas, Trevor Darrell, and Kate Saenko. 2016. “Modeling Relationships in Referential Expressions with Compositional Modular Networks.” arXiv [cs.CV]. arXiv. http://arxiv.org/abs/1611.09978.
[B] Nagaraja, Varun K., Vlad I. Morariu, and Larry S. Davis. 2016. “Modeling Context Between Objects for Referring Expression Understanding.” arXiv [cs.CV]. arXiv. http://arxiv.org/abs/1608.00525.
[C] Sun, Qing, and Dhruv Batra. 2015. “SubmodBoxes: Near-Optimal Search for a Set of Diverse Object Proposals.” In Advances in Neural Information Processing Systems 28, edited by C. Cortes, N. D. Lawrence, D. D. Lee, M. Sugiyama, and R. Garnett, 1378–86. Curran Associates, Inc.