Class wise Subset of MNIST-M Dataset

I am trying to subset particular class (in particular, samples from labels 0, 4, 8) samples from the MNIST-M Dataset (source). Since torchvision does not have a predefined library function to load the MNIST-M Dataset, I am using the following custom dataset class function:

class MNIST_M(
    def __init__(self, root, train, transform=None):
        self.train = train
        self.transform = transform
        if train:
            self.image_dir = os.path.join(root, 'mnist_m_train')
            self.labels_file = os.path.join(root, "mnist_m_train_labels.txt")
            self.image_dir = os.path.join(root, 'mnist_m_test')
            self.labels_file = os.path.join(root, "mnist_m_test_labels.txt")

        with open(self.labels_file, "r") as fp:
        	content = fp.readlines()
        self.mapping = list(map(lambda x: (x[0], int(x[1])), [c.strip().split() for c in content]))

    def __len__(self):
        return len(self.mapping)

    def __getitem__(self, idx):
        image, labels = self.mapping[idx]
        image = os.path.join(self.image_dir, image)
        image = self.transform('RGB'))
        return image, labels

    def _load_data(self):
        data = read_image_file(self.image_dir)
        targets = read_label_file(self.labels_file)

        return data, targets

To create a subset of the dataset, I am using the following code which I have ported from the similar case of subsetting MNIST Dataset (subsetting MNIST reference):

mnist_train_ds_modded = datasets.MNIST(root_dir, download=True, train=True, transform=source_transform)

mnistm_train_ds_modded = MNIST_M(root=root_dir, train=True,
                            transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5)),

mnistm_train_ds_modded.labels = torch.tensor(mnistm_train_ds_modded.labels)

mnistm_train_indexes_0 = 1*(mnistm_train_ds_modded.labels == 0).nonzero().flatten().tolist()
mnistm_train_indexes_4 = 1*(mnistm_train_ds_modded.labels == 4).nonzero().flatten().tolist()
mnistm_train_indexes_8 = 1*(mnistm_train_ds_modded.labels == 8).nonzero().flatten().tolist()

mnistm_train_modded_idx = mnistm_train_indexes_0 + mnistm_train_indexes_4 + mnistm_train_indexes_8
mnistm_train_ds_modded.labels = mnistm_train_ds_modded.labels[mnistm_train_modded_idx] =[mnistm_train_modded_idx]

Clearly, the MNIST-M dataset class written above does not have any attributes called data and labels. So the above code will not work as it does in the case of MNIST.

I went through the source code of MNIST to define the class attributes data and labels but I am unable to do the same for png files (MNIST-M has png files). Kindly help me define the class attributes so that I can subset the dataset.

I don’t know how MNIST-M is stored, but in case the images are stored in folders for each class, you could use ImageFolder to lazily load (and process) each image and target.

Here is the drive location of the MNIST-M Dataset: MNIST-M folder. So the images are not stored in folders for each class, rather all the images have been stored in the same folder.

Kindly note that the MNIST-M class defined above also loads the samples lazily like ImageFolder but since I’m only interested in a subset of the data, this will amount to reading the entire dataset again and again and only filtering the classes I’m interested in, which is computationally expensive as compared to having a preprocessed dataset containing only the classes of interest.