How upload sequence of image on video-classification

9

Assuming your folder structure looks like this:

root/
    - boxing/
        -person0/
            -image00.png
            -image01.png
            - ...
        -person1
            - image00.png
            - image01.png
            - ...
    - jogging
        -person0/
            -image00.png
            -image01.png
            - ...
        -person1
            - image00.png
            - image01.png
            - ...

You could first get all image paths and the corresponding target.
Then we would have to take care of the invalid indices, i.e. images from different persons, as this might be problematic for the training.
Using a sampler, we can get all valid indices for the current sequence length.
Here is a code sample I adapted to your use case:


import os
import glob

import torch
from torch.utils.data import Dataset, DataLoader
import torchvision.transforms as transforms

from PIL import Image


class MySampler(torch.utils.data.Sampler):
    def __init__(self, end_idx, seq_length):        
        indices = []
        for i in range(len(end_idx)-1):
            start = end_idx[i]
            end = end_idx[i+1] - seq_length
            indices.append(torch.arange(start, end))
        indices = torch.cat(indices)
        self.indices = indices
        
    def __iter__(self):
        indices = self.indices[torch.randperm(len(self.indices))]
        return iter(indices.tolist())
    
    def __len__(self):
        return len(self.indices)


class MyDataset(Dataset):
    def __init__(self, image_paths, seq_length, transform, length):
        self.image_paths = image_paths
        self.seq_length = seq_length
        self.transform = transform
        self.length = length
        
    def __getitem__(self, index):
        start = index
        end = index + self.seq_length
        print('Getting images from {} to {}'.format(start, end))
        indices = list(range(start, end))
        images = []
        for i in indices:
            image_path = self.image_paths[i][0]
            image = Image.open(image_path)
            if self.transform:
                image = self.transform(image)
            images.append(image)
        x = torch.stack(images)
        y = torch.tensor([self.image_paths[start][1]], dtype=torch.long)
        
        return x, y
    
    def __len__(self):
        return self.length


root_dir = './video_data_test/'
class_paths = [d.path for d in os.scandir(root_dir) if d.is_dir]

class_image_paths = []
end_idx = []
for c, class_path in enumerate(class_paths):
    for d in os.scandir(class_path):
        if d.is_dir:
            paths = sorted(glob.glob(os.path.join(d.path, '*.png')))
            # Add class idx to paths
            paths = [(p, c) for p in paths]
            class_image_paths.extend(paths)
            end_idx.extend([len(paths)])

end_idx = [0, *end_idx]
end_idx = torch.cumsum(torch.tensor(end_idx), 0)
seq_length = 10

sampler = MySampler(end_idx, seq_length)
transform = transforms.Compose([
    transforms.Resize((32, 32)),
    transforms.ToTensor()
])

dataset = MyDataset(
    image_paths=class_image_paths,
    seq_length=seq_length,
    transform=transform,
    length=len(sampler))

loader = DataLoader(
    dataset,
    batch_size=1,
    sampler=sampler
)

for data, target in loader:
    print(data.shape)

If you use the Dataset without the provided sampler, you will get invalid sequences, e.g. one part might come from person0 while the other from person1.

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