Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
1. The main problem for me is that the paper promises a very real scenario (Fig. 1) of how a user can refine search by using a sequence of refined queries. However, majority of the model design and evaluation (except section 4.2) is performed with dense region captions that have almost no sequential nature. While this is partially a strength as no additional labels are required, the method seems suited especially towards such disconnected queries -- there is space for M disconnected queries and only then updates are required. 2. It would be good to have simple baselines that are modified suitably for multi-query retrieval. This would provide a deeper understanding of when the proposed method works better. For example, (a) scoring each query separately and performing late ranking fusion (either scores or ranks); (b) concatenating the query and performing retrieval through a typical joint text-image embedding. 3. Results on real user queries are most critical. In Fig. 1, the user queries seem very natural, but the simulated queries in Fig. 4 are not. It is not clear from the provided information, whether the users learn to create queries that just list the objects in the image, or have a more natural pattern of searching for the correct scene, followed by bigger objects/elements, and then their details. Overall: I'm not convinced of the paper in it's current state, although it is quite borderline. Personally, when doing image search, I noticed that I'm mostly searching for images with a single big object. It is unclear whether users need to find images like those shown in Fig. 4 (middle) and would not rather search using terms like "London skyline", and refine with "Big-Ben tower" instead of vague comments like "water with shadows cast by building". Additionally, Google image search provides a few options based on attributes such as clipart vs. drawing / black-and-white vs. specific color tones / etc. This crucial aspect seems to be missing in the current paper. ----------------------------- Post-rebuttal: The rebuttal does clarify some aspects about difference between user queries and dense captions (used during training). However, more examples and user queries would be required to fully ascertain that this is a viable option. There are a lot of todo experiments as well for the final version. I update my rating to reflect the score assuming these are incorporated into the final version.
The approach is an reasonable approach that allows a user to interactively "drill down" to find a desired image. The evaluation is fairly strong, including both simulated queries based on Visual Genome annotations and a user study that asks users to iteratively construct queries to focus in on a desired image, and compares to reasonable baselines representing the state of the art on dialog-based image retrieval The approach is new, but relatively simple and straightforward, using multiple user query encodings and a learned similarity that allows it to retrieve images that match all of these queries The language encoding uses uni-directional GRU. Why not bidirectional? Why not use a more recent transformer-based language encoding such as BERT? These have been shown to produce better language embeddings for most NL problems. The user experiments give users an image and then have them iteratively search for it in the corpus. Does this accurately model the real world problem where people are searching for an image that they are not actually looking at while they are constructing queries? It is a reasonable experimental methodology that allows effectively quantitative evaluation but I wonder if it truly models a realistic scenario.
*Originality* This paper introduces a new approach for language-based image retrieval in a multi-turn setting. The method is built on Visual Genome, using image region descriptions as placeholders for sentences in a multi-turn setting. The concept of updating an external memory has been explored extensively in the past (another relevant paper is Kiddon et al. 2016, neural checklist models). Dialogue in image tasks has been explored previously, so my understanding is the main contribution of this paper is a new way of encoding dialogue turns. Distinguishing the paper’s contribution with memory networks (with respect to the sentence encoding) would be important to add to the paper. Another related paper is Visual Dialog (Das et al. 2017). *Quality* Most experiments were conducted on Visual Genome, which was not built for this task. While some conclusions (such as learning capacity with different encoders) could perhaps be drawn using this data, conclusions about the general task or how humans would interact with a retrieval system are less supported by the experiments on Visual Genome. Claims about HCI should be cited (e.g., “users naturally underspecify in their queries by referring to local regions of the target image”, L104). Critical experimental details are missing (or I couldn’t find them). For example, (1) How is the order of region descriptions chosen during training or testing? (2) How is the subset of 10 region descriptions chosen? (3) Why are images with fewer than 10 region captions removed for training, but during human evaluation, dialogues can reach only up to 5 turns? (4) From what set of images are images retrieved -- the set of all image in the dataset, or just the testing set? Is it different during human evaluation? (5) How are images scored for retrieval -- is inference run over all images in the candidate set and the highest returned? Is the set of candidate images narrowed as more sentences are provided? (6) Are result in Figure 3 shown on the development or testing set? There seems to be a correlation between the number of placeholder vectors and performance (as shown in Figure 3). Accordingly, there are a few experiments that I would like to see: . Using the same number of placeholder vectors as sentences. (Based on how placeholders are sampled, this would be equivalent to applying a transformation on each sentence encoding). . Instead of using placeholder vectors, just directly computing attention with the sentence encodings and the image regions. Is learning the extra parameters for encoding into a placeholder necessary? I suppose it makes the attention computation more cost-effective for very long dialogues, but if dialogues range around 5-10 turns, then it doesn’t seem justified. . Similarly, the logic that always samples an empty placeholder while one exists seems unnecessary -- why not just allow the model to sample whichever placeholder it wants? It seems like sampling empty ones first could cause weird ordering effects (because the first N may not use the same placeholder). Some comments/questions about the human evaluation: . Why limit up to five turns? If evaluating with a 5 x 256 model, this just means each of their utterances is encoded in a different placeholder. . Did you evaluate the language used by the humans and how it differed from the Visual Genome data? I imagine it would be quite different because it’s highly conditioned on the distractors that appear in the first few queries. . How many queries are required (on average) before the target image is in the top N? . Human evaluation shouldn’t be done on the test set of the data -- this is a form of peeking. *Clarity* The paper was easy to read. Some details are missing (see above section), but the contribution is clear. Some small points: . L20: “We focus in” → “We focus on” . L45: “vector .” → “vector.” . L48--L50: This sentence should be split into two. . Is the FasterCNN updated during training? . The notation in Section 3.4 is confusing. \phi is commonly used for word embeddings, not RNNs. . L166: “computational” → “computationally” . Using two different notations for max in Eq 4 is confusing. . HRED stands for hierarchical recurrent encoder-decoder. This method doesn’t have a decoder. . Table 1 is very difficult to read with so many slashes. *Significance* The proposed method is easy to understand and could be applied to other tasks using sequential language understanding for vision (e.g., Visual Dialogue). However, using Visual Genome as a placeholder for the real task of image retrieval is not convincing (it was not annotated with this task in mind, and sampling a subset of region descriptions in no specific order means this method was not evaluated with ordering information in mind, as would be present in an image retrieval system). Also, important questions remain about the evaluation (missing baselines and the candidate set for retrieval). *Update after review* Thanks for including details about the inference and evaluation details in the rebuttal. These should be added to the main paper. After reading the rebuttal, my main concerns are that (1) Because the data wasn't created originally for a sequential language understanding task, it's hard to draw conclusions about the proposed method for such a task, though using human evaluation is a great way to do it. (2) Some choices about the architecture (e.g., learning placeholders rather than doing simple dot-product attention with encoded captions) should be motivated through more detailed experiments as described earlier in my review.