Convolutional neural network and CIFAR-10, part 3

This is a continuation from the last post. This time I implemented translation + horizontal flipping. The translation works by cropping the 32×32 image into smaller 24×24 sub-images (9 to be exact) to expand the training set and avoid over fitting.

This is the network I used

  • Layer 1 – 5×5 Rectified Linear Unit, 64 output maps
  • Layer 2 – 2×2 Max-pool
  • Layer 3 – 5×5 Rectified Linear Unit, 64 output maps
  • Layer 4 – 2×2 Max-pool
  • Layer 5 – 3×3┬áRectified Linear Unit, 64 output maps
  • Layer 6 – Fully connected Rectified Linear Unit, 64 output neurons
  • Layer 7 – Fully connected linear units, 10 output neurons
  • Layer 8 – Softmax

Below is the validation error during training. I’ve changed the way the training data is loaded to save memory. Each dataset is loaded and trained one at a time, instead of loading it all into memory. After a dataset is trained I output the validation error to file. Since I use 4 datasets for training each data point on the graph represents 1/4 of an epoch (one pass through all the training data).

CNN-24-validation

I used an initial learning rate of 0.01, then changed to 0.001 at x=26 then finally 0.0001 at x=30. The other training parameters are

  • momentum = 0.9
  • mini batch size = 64
  • all the data centred (mean subtracted) prior to training
  • all weights initialised using a Gaussian of u=0 and stdev=0.1 (for some reason it doesn’t work with 0.01 like most people do)

The final results are:

  • training error ~ 17.3%
  • validation error ~ 18.5%
  • testing error ~ 20.1%

My last testing error was 24.4% so there is some slight improvement, though at the cost of much more computation. The classification code has been modified to better suit the 24×24 cropped sub-images. Rather than classify using only the centre sub-image all 9 sub-images are used. The softmax results from each sub-image is accumulated and the highest score picked. This works much better than using the centre image only. This is idea is borrowed from cuda-convnet.

Here are the features learnt for the first layer.

weights

Using cropped sub-images and horizontal flipping the training set has expanded 18 times. The error gap between training error and validation error is now much smaller than before. This suggests I can gain improvements by using a neural network with a larger modeling capacity. This is true for the network used by cuda-convnet to get < 13% training error. Their network is more complex than what I’m using to achieve those results. This seems to be a ‘thing’ with neural networks where to get that extra bit of oomph the complexity of the network can grow monstrously, which is rather off putting.

Based on results collected for the CIFAR-10 dataset by this blog post the current best is using something called a Multi-Column Deep Neural Network, which achieves an error of 11.21%. It uses 8 different convolution neural networks (dubbed ‘column’) and aggregate the results together (like a random forest?). Each column receives the original RGB images plus some pre-processed variations. The individual neural network column themselves are fairly big beasts consisting of 10 layers.

I think there should be a new metric (or maybe there already is) along the lines of “best bangs for bucks”, where the state of the art algorithms are ranked based on something like [accuracy]/[number of model parameters], which is of particular interest in resource limited applications.

Download

ConvNN.tar.gz

To compile and run this code you’ll need

  • CodeBlocks
  • OpenCV 2.x
  • CUDA SDK
  • C++ compiler that supports the newer C++11 stuff, like GCC

Instructions are in the README.txt file.