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Torch Tutorial for PlantVillage Challenge

Torch Tutorial for PlantVillage Challenge.

Lets know prepare the dataset and set the stage for training.

Firstly download the dataset and extract it into a directory. We’ll divide the images into two directories, train and val for training and validation sets respectively. I used a simple bash script to do this:

    cd directory/containing/c_0c_1...etc directories
    mkdir -p train val
    for i in {0..37}; do mkdir val/c_$i; done
    mv c_* train

    cd train
    find . -iname *.jpg | shuf | head -n 2100| xargs -I{} mv {} ../val/{}

This will move random 2100 images (about 10% of the dataset) in to val directory and rest into train directory. Directory structure should now look like:

.
├── train
│   ├── c_0
│   │   ├── img_name.JPG
│   │   ├── ...
│   │   └── img_name.JPG
│   ├── c_1
│   ├── ...
│   ├── c_36
│   └── c_37
│
└── val
    ├── c_0
    ├── c_1
    ├── ...
    ├── c_36
    └── c_37
        ├── img_name.JPG
        ├── ...
        └── img_name.JPG

Before feeding images into neural networks we’ll resize the images to 224 x 224 and normalize the images with mean and standard deviation of RGB channels computed from a random subset of ImageNet.

In the world of deep learning, dataset of 20,000 images is a relatively small dataset. We’ll therefore augment the data during training with

  • RandomSizedCrop: A randomly sized crop covering anywhere between 8%-100% of the image
  • ColorJitter: Randomly vary brightness, contrast and saturation of the image
  • Lighting: Alexnet style PCA-based noise.
  • HorizontalFlip: Flip the image horizontally

Code to do these transformations is in datasets/transforms.lua. Most of it is borrowed from fb.resnet.torch repo.

We will load the images in batches and do all this processing/augmentation on the fly. This is done by writing a class named DataGen. Essentially, code Understanding how iterators work in Lua can be a little tricky. Read the following documentation for details. can be summarized as :

datasets/plantvillage.lua

    require 'paths'
    t = require 'datasets/transforms.lua'

    local DataGen = torch.class 'DataGen'

    function DataGen:__init(path)
        -- path is path of directory containing 'train' and 'val' folders
        -- find all the images in train and val folders.
        self.rootPath = path
        self.trainImgPaths = self.findImages(paths.concat(self.rootPath, 'train'))
        self.valImgPaths = self.findImages(paths.concat(self.rootPath, 'val'))
        self.nbTrainExamples = #self.trainImgPaths
        self.nbValExamples = #self.valImgPaths
    end

    -- Some utility functions
    function DataGen.findImages(dir)
        -- Returns a table with all the image paths found in dir using 'find'
        ...
    end

    local function getClass(path)
        -- gets class from the name of the parent directory
        local className = paths.basename(paths.dirname(path))
        return tonumber(className:sub(3)) + 1
    end

    --- Iterator
    function DataGen:generator(pathsList, batchSize, preprocess)
        -- pathsList is table with paths of images to be iterated over
        -- batchSize is number of images to be loaded in one iteration
        -- preprocess is function which will be applied to image after it's loaded

        -- Split all the paths into random batches
        local pathIndices = torch.randperm(#pathsList)
        local batches = pathIndices:split(batchSize)
        local i = 1

        return function ()
            if i <= #batches then
                local currentBatch = batches[i]

                local X = torch.Tensor(currentBatch:size(1), 3, 224, 224)
                local Y = torch.Tensor(currentBatch:size(1))

                for j = 1, currentBatch:size(1) do
                    local currentPath = pathsList[currentBatch[j]]
                    X[j] = preprocess(t.loadImage(currentPath))
                    Y[j] = getClass(currentPath)
                end

                i = i + 1
                return X, Y
            end
       end
    end

    function DataGen:trainGenerator(batchSize)
        local trainPreprocess = t.Compose{
            t.RandomSizedCrop(224),
            t.ColorJitter({
                brightness = 0.4,
                contrast = 0.4,
                saturation = 0.4,
            }),
            t.Lighting(0.1, t.pca.eigval, t.pca.eigvec),
            t.ColorNormalize(t.meanstd),
            t.HorizontalFlip(0.5),}

       return self:generator(self.trainImgPaths, batchSize, trainPreprocess)
    end

    function DataGen:valGenerator(batchSize)
        local valPreprocess = t.Compose{
             t.Scale(256),
             t.ColorNormalize(t.meanstd),
             t.CenterCrop(224),}
       return self:generator(self.valImgPaths, batchSize, valPreprocess)
    end

Complete code for this class with some error catching is at datasets/plantvillage.lua. We can now simply use a DataGen object to write a for loop to iterate over all the images:

    for input, target in dataGen:trainGenerator(batchSize) do
        -- code to train your model
    end

Neat, isn’t it?