Introduction
Laughter is a special, higher-order function that only humans possess. In the analysis of laughter, as Wikipedia says, “Laughter is thought to be a shift of composition (schema)", and laughter frequently occurs when there is a change from a composition of receiver. However, the viewpoint of laughter differs greatly depending on the position of the receiver. Therefore, the quantitative measurement of laughter is very difficult. Image Ogiri on web services such as "Bokete" BIBREF0 have recently appeared, where users post funny captions for thematic images and the captions are evaluated in an SNS-like environment. Users compete to obtain the greatest number of “stars”. Although quantification of laughter is considered to be a very difficult task, the correspondence between evaluations and images on Bokete allows us to treat laughter quantitatively. Image captioning is an active topic in computer vision, and we believe that humorous image captioning can be realized. The main contributions of the present paper are as follows:
BoketeDB
In the experimental section, we compare the proposed method based on Funny Score and BoketeDB pre-trained parameters with a baseline provided by MS COCO Pre-trained CNN+LSTM. We also compare the results of the NJM with funny captions provided by humans. In an evaluation by humans, the results provided by the proposed method were ranked lower than those provided by humans (22.59% vs. 67.99%) but were ranked higher than the baseline (9.41%). Finally, we show the generated funny captions for several images.
Related Research
Through the great research progress with deep neural networks (DNNs), the combination of a convolutional neural network and a recurrent neural network (CNN+RNN) is a successful model for both feature extraction and sequential processing BIBREF1 . Although there is no clear division, a CNN is often used for image processing, whereas an RNN is used for text processing. Moreover, these two domains are integrated. One successful application is image caption generation with CNN+LSTM (CNN+Long-Short Term Memory) BIBREF2 . This technique enables text to be automatically generated from an image input. However, we believe that image captions require human intuition and emotion. In the present paper, we help to guide an image caption has funny expression. In the following, we introduce related research on humorous image caption generation.
Wang et al. proposed an automatic “meme" generation technique BIBREF3 . A meme is a funny image that often includes humorous text. Wang et al. statistically analyzed the correlation between memes and comments in order to automatically generate a meme by modeling probabilistic dependencies, such as those of images and text.
Chandrasekaran et al. conducted a humor enhancement of an image BIBREF4 by constructing an analyzer to quantify “visual humor” in an image input. They also constructed datasets including interesting (3,200) and non-interesting (3,200) human-labeled images to evaluate visual humor. The “funniness” of an image can be trained by defining five stages.
Proposed Method
We effectively train a funny caption generator by using the proposed Funny Score by weight evaluation. We adopt CNN+LSTM as a baseline, but we have been exploring an effective scoring function and database construction. We refer to the proposed method as the Neural Joking Machine (NJM), which is combined with the BoketeDB pre-trained model, as described in Section SECREF4 .
CNN+LSTM
The flow of the proposed method is shown in Figure FIGREF2 . Basically, we adopted the CNN+LSTM model used in Show and Tell, but the CNN is replaced by ResNet-152 as an image feature extraction method. In the next subsection, we describe in detail how to calculate a loss function with a Funny Score. The function appropriately evaluates the number of stars and its “funniness”.
Funny Score
The Bokete Ogiri website uses the number of stars to evaluate the degree of funniness of a caption. The user evaluates the “funniness” of a posted caption and assigns one to three stars to the caption. Therefore, funnier captions tend to be assigned a lot of stars. We focus on the number of stars in order to propose an effective training method, in which the Funny Score enables us to evaluate the funniness of a caption. Based on the results of our pre-experiment, a Funny Score of 100 stars is treated as a threshold. In other words, the Funny Score outputs a loss value INLINEFORM0 when #star is less than 100. In contrast, the Funny Score returns INLINEFORM1 when #star is over 100. The loss value INLINEFORM2 is calculated with the LSTM as an average of each mini-batch.
BoketeDB
We have downloaded pairs of images and funny captions in order to construct a Bokete Database (BoketeDB). As of March 2018, 60M funny captions and 3.4M images have been posted on the Bokete Ogiri website. In the present study, we consider 999,571 funny captions for 70,981 images. A number of pair between image and funny caption is posted in temporal order on the Ogiri website Bokete. We collected images and funny captions to make corresponding image and caption pairs. Thus, we obtained a database for generating funny captions like an image caption one.
Comparison with MS COCO BIBREF5 . MS COCO contains a correspondence for each of 160,000 images to one of five types of captions. In comparison with MS COCO, BoketeDB has approximately half the number of the images and 124% the number of captions.
Experiment
We conducted evaluations to confirm the effectiveness of the proposed method. We describe the experimental method in Section SECREF11 , and the experimental results are presented in Section SECREF12 .
Experimental contents
Here, we describe the experimental method used to validate the effectiveness of the NJM. We compare the proposed method with two other methods of generating funny captions: 1) human generated captions, which are highly ranked on Bokete (indicated by “Human" in Table TABREF10 ), and 2) Japanese image caption generation using CNN+LSTM pre-trained by STAIR caption BIBREF7 . Based on the captions provided by MS COCO, the STAIR caption is translated from English to Japanese (indicated by “STAIR caption” in Table TABREF10 ). We use a questionnaire as the evaluation method. We selected a total of 30 themes from the Bokete Ogiri website that included “people”, “two or more people”, “animals”, “landscape”, “inorganics”, and “illustrations”. The questionnaire asks respondents to rank the captions provided by humans, the NJM, and STAIR caption in order of “funniness”. The questionnaire does not reveal the origins of the captions.
Questionnaire Results
In this subsection, we present the experimental results along with a discussion. Table TABREF10 shows the experimental results of the questionnaire. A total of 16 personal questionnaires were completed. Table TABREF10 shows the percentages of captions of each rank for each method of caption generation considered herein. Captions generated by humans were ranked “funniest” 67.99% of the time, followed by the NJM at 22.59%. The baseline captions, STAIR caption, were ranked “funniest” 9.41% of the time. These results suggest that captions generated by the NJM are less funny than those generated by humans. However, the NJM is ranked much higher than STAIR caption.
Posting to Bokete
We are currently posting funny captions generated by the NJM to the Bokete Ogiri website in order to evaluate the proposed method. Here, we compare the proposed method with STAIR captions. As reported by Bokete users, the funny captions generated by STAIR caption averaged 1.71 stars, whereas the NJM averaged 3.23 stars. Thus, the NJM is funnier than the baseline STAIR caption according to Bokete users. We believe that this difference is the result of using (i) Funny Score to effectively train the generator regarding funny captions and (ii) the self-collected BoketeDB, which is a large-scale database for funny captions.
Visual results
Finally, we present the visual results in Figure FIGREF14 , which includes examples of funny captions obtained using NJM. Although the original caption is in Japanese, we also translated the captions into English. Enjoy!
Conclusion
In the present paper, we proposed a method by which to generate captions that draw laughter. We built the BoketeDB, which contains pairs comprising a theme (image) and a corresponding funny caption (text) posted on the Bokete Ogiri website. We effectively trained a funny caption generator with the proposed Funny Score by weight evaluation. Although we adopted CNN+LSTM as a baseline, we have been exploring an effective scoring function and database construction. The experiments of the present study suggested that the NJM was much funnier than the baseline STAIR caption.

Question: What is the performance of NJM?
Answer: NJM vas selected as the funniest caption among the three options 22.59% of the times, and NJM captions posted to Bokete averaged 3.23 stars

It obtained a score of 22.59%

Captions generated by NJM were ranked "funniest" 22.59% of the time.

Question: How are the results evaluated?
Answer: The captions are ranked by humans in order of "funniness".

a questionnaire

With a questionnaire asking subjects to rank methods according to its "funniness". Also, by posting the captions to Bokete to evaluate them by received stars

Question: How big is the self-collected corpus?
Answer: 999,571 funny captions for 70,981 images

 999,571 funny captions for 70,981 images

999571 captions for 70981 images.

Question: How is the funny score calculated?
Answer: Based on the number of stars users assign funny captions, an LSTM calculates the loss value L as an average of each mini-batch and returns  L when the number of stars is less than 100, otherwise L-1.0

The funny score is L if the caption has fewer than 100 stars and 1-L if the caption has 100 or more stars, where L is the average loss value calculated with the LSTM on the mini-batch.