Bottom-Up and Top-Down Attention for Visual Question Answering

An efficient PyTorch implementation of the winning entry of the 2017 VQA Challenge.

The implementation follows the VQA system described in "Bottom-Up and Top-Down Attention for Image Captioning and Visual Question Answering" (https://arxiv.org/abs/1707.07998) and "Tips and Tricks for Visual Question Answering: Learnings from the 2017 Challenge" (https://arxiv.org/abs/1708.02711).

Results

Model	Validation Accuracy	Training Time
Reported Model	63.15	12 - 18 hours (Tesla K40)
Implemented Model	63.58	40 - 50 minutes (Titan Xp)

The accuracy was calculated using the VQA evaluation metric.

About

This is part of a project done at CMU for the course 11-777 Advanced Multimodal Machine Learning and a joint work between Hengyuan Hu, Alex Xiao, and Henry Huang.

As part of our project, we implemented bottom up attention as a strong VQA baseline. We were planning to integrate object detection with VQA and were very glad to see that Peter Anderson and Damien Teney et al. had already done that beautifully. We hope this clean and efficient implementation can serve as a useful baseline for future VQA explorations.

Implementation Details

Our implementation follows the overall structure of the papers but with the following simplifications:

1. We don't use extra data from Visual Genome.
2. We use only a fixed number of objects per image (K=36).
3. We use a simple, single stream classifier without pre-training.
4. We use the simple ReLU activation instead of gated tanh.

The first two points greatly reduce the training time. Our implementation takes around 200 seconds per epoch on a single Titan Xp while the one described in the paper takes 1 hour per epoch.

The third point is simply because we feel the two stream classifier and pre-training in the original paper is over-complicated and not necessary.

For the non-linear activation unit, we tried gated tanh but couldn't make it work. We also tried gated linear unit (GLU) and it works better than ReLU. Eventually we choose ReLU due to its simplicity and since the gain from using GLU is too small to justify the fact that GLU doubles the number of parameters.

With these simplifications we would expect the performance to drop. For reference, the best result on validation set reported in the paper is 63.15. The reported result without extra data from visual genome is 62.48, the result using only 36 objects per image is 62.82, the result using two steam classifier but not pre-trained is 62.28 and the result using ReLU is 61.63. These numbers are cited from the Table 1 of the paper: "Tips and Tricks for Visual Question Answering: Learnings from the 2017 Challenge". With all the above simplification aggregated, our first implementation got around 59-60 on validation set.

To shrink the gap, we added some simple but powerful modifications. Including:

1. Add dropout to alleviate overfitting
2. Double the number of neurons
3. Add weight normalization (BN seems not work well here)
4. Switch to Adamax optimizer
5. Gradient clipping

These small modifications bring the number back to ~62.80. We further change the concatenation based attention module in the original paper to a projection based module. This new attention module is inspired by the paper "Modeling Relationships in Referential Expressions with Compositional Modular Networks" (https://arxiv.org/pdf/1611.09978.pdf), but with some modifications (implemented in attention.NewAttention). With the help of this new attention, we boost the performance to ~63.58, surpassing the reported best result with no extra data and less computation cost.

Usage

Prerequisites

Make sure you are on a machine with a NVIDIA GPU and Python 2 with about 70 GB disk space.

1. Install PyTorch v0.3 with CUDA and Python 2.7.
2. Install h5py.

Data Setup

All data should be downloaded to a 'data/' directory in the root directory of this repository.

The easiest way to download the data is to run the provided script tools/download.sh from the repository root. The features are provided by and downloaded from the original authors' repo. If the script does not work, it should be easy to examine the script and modify the steps outlined in it according to your needs. Then run tools/process.sh from the repository root to process the data to the correct format.

Training

Simply run python main.py to start training. The training and validation scores will be printed every epoch, and the best model will be saved under the directory "saved_models". The default flags should give you the result provided in the table above.