Multi-Scale CNN Classifier
Abstract
This project uses Google TensorFlow to implement a convolutional neural network architecture created using concepts from the LeNet 5 architecture (LeCun, 1998), the multi-scale network architecture (Sermanet, 2011), the dropout method (Srivastava, 2014), and regularization. The classifier's performance is tested using the German Traffic Sign Dataset which contains over 51000 image patches from automobile dashcams, each annotated with one of 43 traffic sign classes. This implementation achieves 99.1% validation accuracy and 97.2% test accuracy on this dataset. These results are encouraging given that human performance on this dataset is 98.8% (Sermanet & LeCun, 2011). However, overfitting the GTSRB dataset remains a challenge when attempting to generalize to any image of German traffic signs captured by any camera. Future work includes further research into generalization including further preprocessing to make reliable inferences on images from any input source.
Installation
This procedure was tested on Ubuntu 16.04 and Mac OS X 10.11.6 (El Capitan). GPU-accelerated training is supported on Ubuntu only.
Prerequisites: Install Python package dependencies using my instructions. Then, activate the environment:
source activate deep-learning
Optional, but recommended on Ubuntu: Install support for NVIDIA GPU acceleration with CUDA v8.0 and cuDNN v5.1:
wget https://www.dropbox.com/s/08ufs95pw94gu37/cuda-repo-ubuntu1604-8-0-local-ga2_8.0.61-1_amd64.deb
sudo dpkg -i cuda-repo-ubuntu1604-8-0-local-ga2_8.0.61-1_amd64.deb
sudo apt-get update
sudo apt-get install cuda
wget https://www.dropbox.com/s/9uah11bwtsx5fwl/cudnn-8.0-linux-x64-v5.1.tgz
tar -xvzf cudnn-8.0-linux-x64-v5.1.tgz
cd cuda/lib64
export LD_LIBRARY_PATH=`pwd`:$LD_LIBRARY_PATH
cd ..
export CUDA_HOME=`pwd`
sudo apt-get install libcupti-dev
pip install --ignore-installed --upgrade https://storage.googleapis.com/tensorflow/linux/gpu/tensorflow_gpu-1.1.0-cp35-cp35m-linux_x86_64.whl
Clone the image_classifier repo:
git clone https://github.com/alexhagiopol/image_classifier
cd image_classifier
Download the German Traffic Dataset with annotations in a Python Pickle format that is easy to work with:
wget https://www.dropbox.com/s/mwjjb67xbpj6rgp/traffic-signs-data.tar.gz
tar -xvzf traffic-signs-data.tar.gz
rm -rf traffic-signs-data.tar.gz
Execution
Perform preprocessing and data augmentation:
python preproc.py
Run the code to train and validate the model on your machine:
python main.py
Technical Report
The implementation and reluts can be viewed simultaneously in the Traffic_Sign_CLassifier.ipynb iPython notebook
Dataset Summary
The GTSRB dataset contains 51839 total images, each annotated with one of 43 sign classes. Each image is a cropped traffic sign from frames in a vehicle dashcam. The dataset contains images that often blurry, too dark, or captured from challenging view angles. For this project, the dataset is divided into 34799 training examples, 4410 validation examples, and 12630 testing examples. A sample of raw images in the dataset is shown below:
Exploratory Visualization
This graphic shows the class distributions of the training examples and validation examples in the input dataset. It's clear that the training and validation sets have different class distributions i.e. some classes may be overrepresented in one set but underrepresented in another set. This observation motivates data augmentation.
Preprocessing
I followed the guidance of Sermanet and LeCun's 2011 paper linked above and began by grayscaling all input images. The authors state that grayscaling yielded higher accuracy results in their experiments. A possible reason is that sign color may be a misleading indicator of sign class: several sign types share the same color, and the appearance of color may be diminished under poor lighting conditions such as those we frequently observe in the raw data. Next, I used the contrast limited adaptive histogram equalization algorithm (CLAHE) to equalize the histograms in the input images. This has the effect of making underexposed or overexposed images contain pixel values that are numerically closer to a proper exposure. It is intended to mitigate the effects of poor lighting in the input dataset. My final preprocessing step was to normalize the images such that their pixel value range is from -1 to 1 instead of 0 to 255. This is intended to ensure numerical stability during the weight value optimization procedure.
The next preprocessing step was data augmentation. It was clear that the class distribution of the training set was not the same as that of the validation set. I saw this as an opportunity for a model trained on the training set to fail to make correct inferences on the validation set. Furthermore, Sermanet and LeCun encourage data augmentation to push accuracy higher. They recommend perturbing the input images with random rotation, translation, shear, scale, brightness, and blur effects to generate "new" labeled training examples. I implemented these augmentation strategies, and I provide the figures below to show examples of the augmented dataset and the class distribution of the augmented dataset.
Model Architecture
I implemented the multiscale architecture described in Sermanet and LeCun's paper with added regularization and dropout as described in Vivek Yadav's blog post. The architecture contains three "stacks" consisting of two convolutional layers, one ReLU layer, one max pooling layer, and one dropout layer. Stack one feeds into stack two which feeds into stack three. As described by Sermanet and LeCun, the output of stacks 1, 2, and 3 are combined into a single, flattened vector which is then connected to a fully connected layer, a dropout layer, a second fully connected layer, and a second dropout layer in that order. Finally, reglarization is performed. The model architecture is summarized below:
| Layer | Description |
|---|---|
| Input | 32x32x1 grayscale image |
| Convolution 5x5 | 1x1 stride, same padding, outputs 32x32x32 |
| Convolution 5x5 | 1x1 stride, same padding, outputs 32x32x32 |
| ReLU | |
| Max Pooling | 2x2 stride, outputs 16x16x32 |
| Dropout | 50% likelihood |
| Convolution 5x5 | 1x1 stride, same padding, outputs 16x16x64 |
| Convolution 5x5 | 1x1 stride, same padding, outputs 16x16x64 |
| ReLU | |
| Max Pooling | 2x2 stride, outputs 8x8x64 |
| Dropout | 50% likelihood |
| Convolution 5x5 | 1x1 stride, same padding, outputs 8x8x128 |
| Convolution 5x5 | 1x1 stride, same padding, outputs 8x8x128 |
| ReLU | |
| Max Pooling | 2x2 stride, outputs 4x4x128 |
| Dropout | 50% Likelihood outputs 4x4x128 |
| Flatten + Concatenate | outputs 1x14336 |
| Fully Connected | outputs 1x1024 |
| Dropout | outputs 1x1024 |
| Fully Connected | outputs 1x1024 |
| Dropout | outputs 1x1024 |
| Fully Connected | outputs 1x43 |
| Regularization | |
Training
To train the model architecture above, I set up CUDA and cuDNN on my Ubuntu machine as described and trained using an NVIDIA GTX 980Ti. I used a batch size of 128, 0.0002 learning rate, 50% dropout likelihood, and 10 epochs. After each epoch, I check if the accuracy achieved is the highest ever, and I save the model if so. This aloows me to keep the best weights configuration after each epoch. This training configuration takes about 1 hour on my GPU.
Approach
My highest validation accuracy is 99.1% and my test accuracy is 97.2%. These results are encouraging given that human performance is 98.8%. My solution approach was to first implement the unmodified LeNet architecture with which I was not able to achieve above 85% accuracy. Next, I implemented Sermanet & LeCun's 2011 paper on my own. I improved that architecture by adding dropoout which was developed 3 years after Sermanet and LeCun published their paper. I then searched the Internet for more optimized implementations to push my accuracy higher. I saw Vivek Yadav's blog post where he suggests doubling the number of convolutional layers in addition to adding regularization to the network. I implemented these changes and achieved 99.1% validation accuracy. The key insight from the literature on this topic is the multi-scale convolutional approach. The idea is to create a network that learns high-abstraction, mid-abstraction, and low-abstraction image features to perform classification. This is why the outputs of the first, second, and third convolutional groups are flattened and concatenated before fully-connected layers. The final result will be based on an evaluation of all levels of feature abstraction.
Acquiring New Images from Outside the Dataset
I found five images of German traffic sings from outside the dataset on the Internet. I then resized these images to 32x32x3 and applied the same preprocessing I applied to the GTSRB dataset. Results of this procedure are below:
Performance on New Images
Unfortunately, the model was only able to correctly predict the identity of 3 of 5 signs, giving it a 60% accuracy rate. This accuracy is much lower than the accuracy on the test set of 97%. This indicates that the model has overfit the GTSRB dataset and does not properly generalize to German traffic signs in general. Future work includes investigating this overfitting and attempting to alleviate the issue with perhaps better data augmentation. One issue I see with the differences between the new images and the dataset images is that the new images represent the traffic signs with higher clarity and less blur than the GTSRB dataset. Perhaps additional preprocessing on new images could help achieve better results.
Model Certainty
Below we show the softmax probabilities for each new image. The figure shows that the model is extremely overconfident: it is 100% certain of its wrong inferences. Future work includes researching how to overcome such issues with overconfidence and not generalizing to images from outside the source dataset.
