automatic generation of grounded visual questions · 2017-08-06 · automatic generation of...

Automatic Generation of Grounded Visual QuestionsShijie Zhang #1, Lizhen Qu∗ §¶2, Shaodi You §¶3, Zhenglu Yang †4, Jiawan Zhang ‡5

#School of Computer Science and Technology, Tianjin University, Tianjin, China§Data61-CSIRO, Canberra, Australia

¶Australian National University, Canberra, Australia†College of Computer and Control Engineering, Nankai University, Tianjin, China

‡The School of Computer Software, Tianjin University, Tianjin, China{shijiezhang,jwzhang}@tju.edu.cn, {lizhen.qu,shaodi.you}@data61.csiro.au, [email protected]

Abstract

In this paper, we propose the first model to be ableto generate visually grounded questions with di-verse types for a single image. Visual questiongeneration is an emerging topic which aims to askquestions in natural language based on visual input.To the best of our knowledge, it lacks automaticmethods to generate meaningful questions with var-ious types for the same visual input.To circumvent the problem, we propose a modelthat automatically generates visually groundedquestions with varying types. Our model takesas input both images and the captions generatedby a dense caption model, samples the most prob-able question types, and generates the questionsin sequel. The experimental results on two realworld datasets show that our model outperforms thestrongest baseline in terms of both correctness anddiversity with a wide margin.

1 IntroductionMulti-modal learning of vision and language is an impor-tant task in artificial intelligence because it is the basis ofmany applications such as education, user query predic-tion, interactive navigation, and so forth. Apart from de-scribing visual scenes by using declarative sentences [Chenand Zitnick, 2014; Gupta and Mannem, 2012; Karpathy andFei-Fei, 2015; Hodosh et al., 2013; Kulkarni et al., 2011;Kuznetsova et al., 2012; Li et al., 2009; Vinyals et al., 2015;Xu et al., 2015], recently, automatic answering of visuallyrelated questions (VQA) has also attracted a lot of attentionin computer vision communities [Antol et al., 2015; Mali-nowski and Fritz, 2014; Gao et al., 2015; Ren et al., 2015;Yu et al., 2015; Zhu et al., 2015]. However, there is littlework on automatic generation of questions for images.

”The art of proposing a question must be held higher valuethan solving it. -Georg Canton”. An intelligent systemshould be able to ask meaningful questions given the environ-ment. Beyond demonstrating a high-level of AI, in practice,multi-modal question-asking modules find their use in a wide

∗Corresponding author.

What is the food in the picture?

What color is the tablecloth?

What number on the cake?

Where is the coffee?

Figure 1: Automatically generated grounded visual questions.

range of AI systems such as child education and dialogue sys-tems.

To the best of our knowledge, almost all existing VQAsystems rely on manually constructed questions [Antol etal., 2015; Malinowski and Fritz, 2014; Gao et al., 2015;Ren et al., 2015; Yu et al., 2015; Zhu et al., 2015]. Ancommon assumption of the existing VQA systems is that an-swers are visually grounded thus all relevant information canbe found in the visual input. However, the construction ofsuch data sets are labor-intensive and time consuming, thuslimits the diversity and coverage of questions being asked.As a consequence, the data incompleteness imposes a specialchallenge for supervised-learning based VQA systems.

In light of the above analysis, we focus on automatic gener-ation of visually grounded questions, coined VQG. The gen-erated questions should be grammatically well-formed, rea-sonable for given images, and as diverse as possible. How-ever, the existing systems are either rule-based such that theygenerate questions with few limited textual patterns [Ren etal., 2015; Zhu et al., 2015], or they are able to ask only onequestion per image and the generated questions are frequentlynot visually grounded [Simoncelli and Olshausen, 2001].

To tackle this task, we propose the first model capable ofasking questions of various types for the same image. As il-lustrated in Fig. 2, we first apply DenseCap [Johnson et al.,2015] to construct dense captions that provides a almost com-plete coverage of information for questions. Then we feedthese captions into the question type selector to sample themost probable question types. Taking as input the questionstypes, the dense captions, as well as visual features generatedby VGG-16 [Simonyan and Zisserman, 2014], the question

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generator decodes all these information into questions. Weconduct extensive experiments to evaluate our model as wellas the most competitive baseline with three kinds of measuresadapted from the ones commonly used in the tasks of imagecaption generation and machine translation.

The contributions of our paper are three-fold:

• We propose the first model capable of asking visuallygrounded questions with diverse types for a single im-age.

• Our model outperforms the strongest baseline up to216% in terms of the coverage of asked questions.

• The grammaticality of the questions generated by ourmodel as well as their relatedness to visual input alsooutperform the strongest baseline with a wide margin.

The rest of the paper is organized as follows: we cover therelated work in Section 2, followed by presenting our modelin Section 3. After introducing the experimental setup in Sec-tion 4, we discuss the results in Section 5, and draw the con-clusion in Section 6.

2 Related WorkThe generation of textual description for visual informationhas gained popularity in recent years. The key challengeis to learn the alignment between text and visual informa-tion [Barnard et al., 2003; Kong et al., 2014; Zitnick et al.,2013]. Herein, a popular task is to describe images with afew declarative sentences, which are often referred to as im-age captions [Barnard et al., 2003; Chen and Zitnick, 2014;Gupta and Mannem, 2012; Karpathy and Fei-Fei, 2015;Hodosh et al., 2013; Kulkarni et al., 2011; Kuznetsova et al.,2012; Li et al., 2009; Vinyals et al., 2015; Xu et al., 2015].

Visual Question and Answering Automatic answering ofquestions based on visual input is one of the most pop-ular tasks in computer vision [Geman et al., 2015; Ma-linowski and Fritz, 2014; Malinowski et al., 2015; Pirsi-avash et al., 2014; Ren et al., 2015; Weston et al., 2015;Yu et al., 2015]. Most VQA models are evaluated on a fewbenchmark datasets [Antol et al., 2015; Malinowski and Fritz,2014; Gao et al., 2015; Ren et al., 2015; Yu et al., 2015;Zhu et al., 2015]. The images in those datasets are sampledfrom the MS-COCO dataset [Lin et al., 2014], the questions-answer pairs are manually constructed [Antol et al., 2015;Gao et al., 2015; Yu et al., 2015; Zhu et al., 2015].

Visual Question Generation Automatic question genera-tion from text is explored in-depth in NLP, however, it israrely studied for visual questions, despite of the fact thatsuch questions are highly desired for many applications. Inorder to generate multiple questions per image, the mostcommon approach is to ask human to manually build thequestion-answer pairs, which is labor-intensive and time-consuming [Antol et al., 2015; Gao et al., 2015; Malinowskiand Fritz, 2014]. As one of the most recent examples, Zhuet al.[Zhu et al., 2015] manually create questions of seven

wh-question types such as what, where, when and etc. Peo-ple also explored automatic generation of visual questions byusing rules. Yu et al.[Yu et al., 2015] consider question gen-eration as a task of selectively removing content words thatserve as answers from a caption and reformulate the resultedsentences as questions. In a similar manner, Ren et al.[Ren etal., 2015] carefully designed a handful of rules to transformimage captions into questions with limited types. However,those rule-based methods are limited by the types of questionsthey can generate. Apart from that, model-based methods arealso studied to overcome the diversity issue, the most closedwork is [Simoncelli and Olshausen, 2001], which trains animage caption model on a dataset of visual questions. How-ever, their model cannot generate more than one question perimage.

Knowledge Base (KB) based Question Answering (KB-QA) KB-QA has attracted considerable attention due to theubiquity of the World Wide Web and the rapid development ofthe artificial intelligence (AI) technology. Large-scale struc-tured KBs, such as DBpedia [Auer et al., 2007], Freebase[Bollacker et al., 2008], and YAGO [Suchanek et al., 2007],provide abundant resources and rich general human knowl-edge, which can be used to respond to users’ queries in open-domain question answering (QA). However, how to bridgethe gap between visual questions and structured data in KBsremains a huge challenge.

The existing KB-QA methods can be broadly classifiedinto two main categories, namely, semantic parsing basedmethods[Kwiatkowski et al., 2013; Reddy et al., 2016] andinformation retrieval based methods [Yao and Durme, 2014;Bordes et al., 2014] methods. Most semantic parsing basedmethods transform a question into its meaning representa-tion (i.e., logical form), which will be then translated to aKB query to retrieve the correct answer(s). Information re-trieval based methods initially roughly retrieve a set of candi-date answers, and subsequently perform an in-depth analysisto re-rank the candidate answers and select the correct ones.These methods focus on modeling the correlation of question-answer pairs from the perspective of question topic, relationmapping, answer type, and so forth.

3 Question GenerationOur goal is to generate visually grounded questions directlyfrom images with diverse question types. We start with ran-domly picking a caption from a set of automatically generatedcaptions, which describes a certain region of image with nat-ural language. Then we sample a reasonable question typeand varying the caption. In the last step, our question genera-tor learns the correlation between the caption and the image,generates a question of the chosen type.

Formally, for each raw image x, our model generates a setof captions {c1, c2, ..., cM}, samples a set of question types{t1, t2, ..., tM}, followed by yielding a set of grounded ques-tions {q1,q2, ...,qM}. Herein, a caption or a question is asequence of words.

w = {w1, ...,wL} (1)


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VGG-16

DenseCap

Question Type Selector

Question Generator

LSTM

Image and Caption Correlation Layer

DecoderLSTM ClassifierQuestion Type 1*1

Rich visual feature 1*300

Rich textual feature

Grounded Questions

1*300

What is the food in the picture?

What color is the table cloth?

What number on the cake?

Where is the coffee?

Figure 2: The proposed framework.

Where L is the length of the word sequence. Each word wi

employs 1-of-K encoding, where K is the size of the vo-cabulary. A question type is represented by the first word of aquestion, adopting 1-of-T encoding where T is the number ofquestion types. The same as [Zhu et al., 2015], we considersix question types in our experiments: what, when, where,who, why and how.

For each image xi, we apply a dense caption model(DenseCap) [Johnson et al., 2015] trained on the VisualGenome dataset [Krishna et al., 2016] to produce a set ofcaptions Ci. Then the generative process is described as fol-lows:

1. Choose a caption cn from Ci.2. Choose a question type tn given cn.3. Generate a question qn conditioned on cn and tn.Denoted by θ all model parameters, for each image xi, the

joint distribution of cn, tn and qn is factorized as follows:

P (qn, tn, cn|xi, Ci;θ) =P (qn|cn,xi, tn;θq)

P (tn|cn;θt)P (cn|Ci) (2)

where θ = θq ∪ θt, P (qn|cn,xi, tn;θq) is the distributionof generating question, P (tn|cn;θt) and P (cn|Ci) are thedistributions for sampling question type and caption respec-tively. More details are given in the following sections.

Since we do not observe the alignment between captionsand questions, cn is latent. Sum over c, we obtain:

P (qn, tn|xi, Ci;θ) =∑

cn∈Ci

P (qn, tn, cn|xi, Ci;θ)

Let Qi denote the question set of the image xi, the probabil-ity of the training dataset D is given by taking the product ofthe above probabilities over all images and their questions.

P (D|θ) =∏i

∏n∈Qi

P (qn, tn|xi, Ci;θ) (3)

For word representations, we initialize a word embeddingmatrix E ∈ R300×K by using Glove [Pennington et al.,2014], which are trained on 840 billions of words. For theimage representations, we apply a VGG-16 model [Szegedyet al., 2015] trained on ImageNet [Deng et al., 2009] withoutfine-tuning to produce 300-dimensional feature vectors. Thedimension is chosen to match the size of the pre-trained wordembeddings.

Compared to the question generation model [Simoncelliand Olshausen, 2001], which generates only one question per

image, the probabilistic nature of this model allows generat-ing questions of multiple types which refer to different re-gions of interests, because each caption predicted by Dense-Cap is associated with a different region.

3.1 Sample Captions and Question TypesThe caption model DenseCap generates a set of captions fora given image. Each caption c is associated with a regionand a confidence oc of the proposed region. Intuitively, weshould give a higher probability to the caption with higherconfidence than the lower one. Thus, given a caption set Ci ofan image xi, we define the prior distribution as:

P (ck|Ci) =exp(ok)∑

j∈Ci exp(oj)

A caption is either a declarative sentence, a word, or aphrase. We are able to ask many different types of questionsbut not all of them for a chosen caption. For example, for acaption ”floor is brown” we can ask ”what color is the floor”but it would be awkward to ask a who question. Thus, ourmodel draws a question type given a caption with the proba-bility P (tn|cn) by assuming it suffices to infer question typesgiven a caption.

Our key idea is to learn the association between ques-tion types and key words/phrases in captions. The modelP (tn|cn) consists of two components. The first one is aLong Short Term Memory (LSTM) [Hochreiter and Schmid-huber, 1997] that maps a caption into a hidden representa-tion. LSTM is a recurrent neural network taking the follow-ing form:

ht,mt = LSTM(xt,ht−1,mt−1) (4)

where xt is the input and the hidden state of LSTM at timestep t, and ht and mt are the hidden states and memory statesof LSTM at time step t, respectively. As the representation ofthe whole sequence, we take the last state hL generated at theend of the sequence. This representation is further fed into asoftmax layer to compute a probability vector pt for all ques-tion types. The probability vector characterizes a multinomialdistribution of all question types.

3.2 Generate QuestionsAt the core of our model is the question generation module,which models P (qn|cn,xi, tn;θq), given a chosen captioncn and a question type tn. It is composed of three modules:i) an LSTM encoder to generate caption embeddings; ii) a


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correlation module to learn the association between imagesand captions; iii) a decoder consisting of an LSTM decoderand an ngram language model.

A grounded question is deeply anchored in both the sam-pled caption and the associated image. In our preliminaryexperiments, we found it useful to let the LSTM encoderLSTM(xt,ht−1,mt−1) to read the image features prior toreading captions. In particular, at time step t = 0, we initial-ize the state vector m0 to zero and feed the image features asx0. At the 1st time step, the encoder reads in a special tokenS0 indicating the start of a sentence, which is a good prac-tice adopted by many caption generation models [Vinyals etal., 2015]. After reading the whole caption of length L, theencoder yields the last state vector mL as the embedding ofcaption.

The correlation module takes as input the caption embed-dings from the encoder and the image features from VGG-16,produces a 300-dimensional joint feature map. We apply alinear layer of size 300× 600 and a PReLU [He et al., 2015]layer in sequel to learn the associations between captions andimages. Since an image gives an overall context and the cho-sen caption provides the focus in the image, the joint rep-resentation provides sufficient context to generate groundedquestions. Although the LSTM encoder incorporates imagefeatures before reading captions, this correlation module en-hances the correlation between images and text by buildingmore abstract representations.

Our decoder extends the LSTM decoder of [Vinyals et al.,2015] with a ngram language model. The LSTM decoderconsists of an LSTM layer and a softmax layer. The LSTMlayer starts with reading the joint feature map and the starttoken S0 in the same fashion as the caption encoder. Fromtime step t = 0, the softmax layer predicts the most likelyword given the state vector at time t yielded by the LSTMlayer. A word sequence ends when the end of sequence tokenis produced.

Joint decoding Although the LSTM decoder alone cangenerate questions, we found that it would frequently producerepeated words and phrases such as ”the the”. The problemdidn’t disappear even the beam search [Koehn et al., 2003]was applied. It is due to the fact that the state vectors pro-duced at adjunct time steps tend to be similar. Since repeatedwords and phrases are rarely observed in text corpora, wediscount such occurrence by joint decoding with a ngram lan-guage model. Given a word sequence w = {w1, ..., wN}, abigram language model is defined as:

P (w) =N∏i=2

P (wi|wi−1)P (w0)

Instead of using neural models, we adopt the word countbased estimation of model parameters. In particular, we applythe KneserNey smoothing [Kneser and Ney, 1995] to estimateP (wi|wi−1), which is given by:

max(count(wi−1, wi)− d, 0)count(wi)

+ λ(wt−1)PKN (wi)

where count(x) denotes the corpus frequency of term x,PKN (wi) is a back-off statistic of unigram wi in case the bi-gram (wi, wi−1) does not appear in the training corpus. Theparameter d is usually fixed to 0.75 to avoid overfitting forlow frequency bigrams. And λ(wt−1) is a normalizing con-stant conditioned on wi.

We incorporate bigram statistics with the LSTM decoderfrom the time step t = 1 because the LSTM decoder canwell predict the first words of questions. The LSTM decoderessentially captures the conditional probability Pl(qt|q<t),while the bigram model considers only the previous wordPb(qt|qt−1) by using word counts. By interpolating thesetwo, we obtain the final probability as:

P (qt|q<t) = (1− β)Pl(qt|q<t) + βPb(qt|qt−1)

where β ∈ [0, 1] is an interpolation weight. In addition, wefix the first k words of questions during decoding accordingto the chosen question types.

3.3 TrainingThe key challenge of training is the involvement of the latentvariables indicating the alignment between captions and goldstandard questions for a deep neural network. We estimatethe latent variables in a similar fashion as EM but computa-tionally more efficient.

Suppose we are given the training set{(x1,q1), ..., (xN ,qN )}, the loss is given by:

l(θ) =

N∑i=1

∑n∈Qi

− logP (qn, tn|xi, Ci;θ)

Suppose Q(cn) denote some proposed distribution such that∑nQ(cn) = 1 and Q(cn) ≥ 0. Consider the following:

logP (qn, tn|xi, Ci;θ) = log∑

ckinCi

Q(ck)P (qn, tn, ck|xi, Ci;θ)

Q(ck)

≥∑

ckinCi

Q(ck) logP (qn, tn, ck|xi, Ci;θ)

Q(ck)

(5)

The last step used Jensens inequality. The Equation (5) givesa upper bound of the loss l(θ). When the bound is tight, wehave Q(ck) = P (ck|qn, tn;θ).

To save the EM loop, we propose a non-parametric estima-tion of P (ck|qn, tn;θ). As a result, for each question-imagepair (xn,qn), we maximize the lower bound by optimizing:argmin

θ−P (cn|qn, tn;θc) log[P (qn|cn,xn, tn;θq)P (tn|cn;θt)]

+const(6)

This in fact assigns a weight P (cn|qn, tn;θ) to each in-stance. By using a non-parametric estimation, we are stillable to apply BackProp and the SGD style optimizing algo-rithms by just augmenting each instance with an estimatedweight.

Given a question q and a caption set C from the train set,we estimate P (ck|q, t;θ) by using the kernel density estima-tor [Scott, 2008]:

P (ck|q, t;θ) = P (ck|q; θ) =s(q, ck)∑

cj∈C s(q, cj)(7)


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where s(q, c) is a similarity function between a question anda caption. We assume ck are conditionally independent oft because we can directly extract the question type from thequestion q by looking at the first few words.

For a given question, there are usually very few matchedcaptions generated by DenseCap , hence the distribution ofcaptions given a question is highly skewed. It is sufficient torandomly draw a caption each time to compute the probabilitybased on Equation (7).

We formulate the similarity between a question and a cap-tion by using both string similarity and embedding based sim-ilarity measures.

The surface string of a caption could be an exact or partialmatch of a given question. Thus we employ the Jaccard Indexas string similarity measure between the surface string of acaption and that of a question.

sims(q, c) =q ∩ cq ∪ c

where c and q denote their surface string respectively. Bothstrings are broken down to a set of char-based trigrams duringthe computation so that this measure still gives a high simi-larity if two strings differ only in some small variations suchas singular and plural forms of nouns.

In case of synonyms or words of similar meanings comewith different form such as ”car” and ”automobile”, we adoptthe pre-trained word embeddings to calculate their similarityby using the weighted averaged of word embeddings:

sime(q, c) = cos(∑wi∈q

IDF(wi)∑j IDF(wj)

Ewi,∑wk∈c

IDF(wk)∑j IDF(wj)

Ewk)

where cos denotes the cosine similarity, IDF(x) is the inversedocument frequency of word x defined by |V|

|{d∈D:x∈d}| , andDis the corpus containing all questions, answers, and captions.

The final similarity measure is computed as the interpola-tion of the two measures:

s(q, c) = αsims(q, c) + (1− α)sime(q, c)

where the hyperparameter α ∈ (0, 1).

4 Experimental Setup4.1 DatasetsWe conduct our experiments on two datasets: VQA-Dataset[Antol et al., 2015] and Visual7W [Zhu et al., 2015]. Theformer is the most popular benchmark for VQA and the lat-ter is a recently created dataset with more visually groundedquestions per image than VQA.VQA: a sample from the MS-COCO dataset [Lin et al.,2014], which contains 254,721 images and 764,163 manu-ally compiled questions respectively. Each image is associ-ated with three questions on average.Visual7W: a dataset composed of 327,939 QA pairs on47,300 COCO images, collected from the MS-COCOdataset [Lin et al., 2014] as well. In addition, it includes1,311,756 human-generated answers in form of multiple-choice and 561,459 object groundings from 36,579 cate-gories. Each image is associated with five questions on av-erage.

4.2 BaselineIn this paper, we consider a baseline by training the imagecaption generation model NeuralTalk2 [Vinyals et al., 2015]on image-question pairs. The baseline is almost the same as[Simoncelli and Olshausen, 2001], which is the only workgenerating questions from visual input. The model of neu-raltalk2 differs from [Simoncelli and Olshausen, 2001] onlyin the RNNs used in the decoder. NeuralTalk2 adopts LSTMwhile [Simoncelli and Olshausen, 2001] chooses GRU [Choet al., 2014]. The two RNN models achieve almost identicalperformance in language modeling [Chung et al., 2015].

4.3 Evaluation MeasuresAs a common practice for evaluating generated word se-quences we employ three different evaluation metrics: BLEU[Papineni et al., 2002], METEOR [Banerjee and Lavie, 2005]and ROUGE-L [Lin, 2004].

BLEU is a modified n-gram precision. We varied the sizeof ngram from one to four, computed the corresponding mea-sures respectively for each image and averaged the resultsacross all images. Both METEOR and ROUGE-L1 are F-Measures favoring precision, computed against the referencequestion with the highest score among all reference questionsin the same image. The measures are averaged in sequelacross all images. Therefore, all three of them are precision-oriented measures.

To measure the diversity of our generated questions, wealso compute the the same set of evaluation measures by com-paring each reference sentence with the best matching gener-ated sentence of the same images. This provides an estimateof coverage in analogy of recall.

4.4 Implementation DetailsWe optimize all models with Adam [Kingma and Ba, 2014].

We fix the batch size to 64. We set the maximal epochs to64 for Visual7W and the maximal epochs to 128 for VQA.The corresponding model hyperparameters were tuned on thevalidation sets. Herein, we set α = 0.75.

5 Results and DiscussionsFigure 3 illustrates all three precision-oriented measures eval-uated on Visual7W and VQA datasets respectively. Our base-line is able to generate only one question per image. Whenwe compare its results with the highest scored question perimage generated by our model, our model outperforms thebaseline with a wide margin. On the VQA test set, in the caseof BLEU measures, the improvement over the baseline growsfrom 24% with unigram to 97% with four-gram. It is evidentthat our model is capable to generate many more higher-ordern-grams co-occurred in reference questions. This improve-ment is also consistent with ROUGE-L because it is based onthe longest common subsequence between generated ques-tions and reference questions. Our model performs betterthan the baseline also not just because it generates more ex-act higher order n-grams than reference questions. METEOR

1We take the same β of F-Measure as the implementation inhttps://github.com/tylin/coco-caption.


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Figure 3: Precision and Recall of our method and baseline method(Neural Talk 2) on VQA Dataset and Visual7W Dataset. Results of ourmethod are depicted with line with circles, we show the results by varying number of questions from one to six. By generating more questions,the Recall increase without a drop on precision. The baseline method (diamonds), however, can generate only one question per image.

Ours(Automatic) Ours(Automatic)Visual7W (Manual) Visual7W (Manual)

What is the name of the truck?

What is the weather day?Where is the bus?

How many trains are there?

What is the sign saying?What color is the sign?What are the red lines

on the sign?What is the name of

the street?What are the building’s

in the background?

What is the man looking at?What color is the photo?

What are the people holding?What is the man looking at?

How many people arein the picture?

What is written on the train's window?What time of day was this taken at?Where is the train?How many headlights does it have?

What color are the lights painted?How many traffic signalsare shown?What street names are shown?What is above the fence behind the traffic lights?What type of buildings?

Who is wearing a red shirt?What are the people sitting under?What color are the tableand chairs?

Who is wearing a blue tank top?How many people do you see?

What is the color of the grass?Where is the man?

Who is in the picture?What is this a picture of?

What is in the background?Where is this picture taken?

Why are the people standing?Who is in the picture?

How color is the man’s shirt?What is in the background?

Where is the man?What is in the picture?

What is on the surfboard?What is the man wearing?

How are the people standing?Who is wearing a blue shirt?

Who is wearing a helmet?Where is the train?

Who is in the picture?Where is he?What is he doing?What kind of animals are there?What kind of shoes is her wearing?Where was this picturetaken?

What is green?Where are windows?

What are people doing?Who is sitting?What color are the skis?

Figure 4: Comparison between manually composed questions and automatically generated questions by our method. Our method generatesreasonable grounded questions and more versatile the manually provided ones.

considers unigram alignment by allowing multiple matchingmodules to consider synonymy and alternating word forms.With this measure, our model is still 65% higher than thebaseline on the VQA test set. We also observe similar levelof improvement over baseline on Visual7W dataset.

On both datasets, when the number of generated ques-tions per image grows, the precison-oriented measures of ourmodel are either similar or slightly declining because ourmodel often generates meaningful questions that are not in-cluded in the ground-truth. The more questions we generatethe more likely that the questions are not covered by manuallyconstructed ones.

To measure the coverage of generated questions, we com-puted each reference question against all generated questionsper image with all evaluation measures. As shown by Figure3, all measures improves as the number of questions grows.Herein, both ROUGE-L and METEOR are way better thanthe baseline regardless of the number of generated questionson both datasets. When all six questions are generated, ourmodel is 130% better than the baseline across all measures.In particular, with METEOR, our model shows an improve-ment of 216% and 179% over the baseline on VQA and Vi-sual7W respectively. When the number of manually con-structed questions is small, our model provides even more

types questions than manual ones, as shown with the exam-ples in Figure 4.

The distribution of question types generated by our modelis more balanced than that of the ground-truth, while almost55% of questions in Visual7W and 89% in VQA start with”what”, as illustrated by Figure 6. Our model has also notendency of generating too long or too short questions be-cause the length distribution of the generated questions arevery similar to that of the manually constructed datasets.

We also evaluate the effectiveness of the integration of bi-gram language model on both datasets. Herein, we comparetwo variants of our model, with and without the bigram modelduring decoding. As shown in Figure 5, regardless of preci-sion or recall, decoding with the bigram model consistentlyoutperforms the one without it. The inclusion of the bigrammodel effectively eliminates almost all repeated terms such as”the the” because the statistics collected by the bigram modelfavors grammatically well-formed sentences. This observa-tion is also reflected in BLEU with higher-order ngrams byshowing larger gaps.

6 ConclusionIn this paper, we propose the first model to automatically gen-erate visually grounded questions with varying types. Our


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Vis7W Precision Vis7W Recall

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Figure 5: Comparing decoding between with the bigram model (dark blue) and without the bigram model (light blue).

model is capable of automatically selecting most likely ques-tion types and generating corresponding questions based onimages and captions constructed by DenseCap. Experimentson VQA and Visual7W dataset demonstrates that the pro-posed model is able to generate reasonable and grammat-ically well-formed questions with high diversity. For fu-ture work, we consider automatically generation of visualquestion-answer pairs, which will likely enhance training ofVQA systems.

AcknowledgmentsThis work is supported by National Key Technology R&DProgram of China: 2014BAK09B04, National Natural Sci-ence Foundation of China: U1636116, 11431006, ResearchFund for International Young Scientists: 61650110510,Ministry of Education of Humanities and Social Science:16YJC790123.

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