Efficient image patches sampling

I’m training a Convolutional Kernel Network (https://arxiv.org/pdf/1406.3332.pdf https://arxiv.org/pdf/1605.06265.pdf) and I need to sample random image patches after the last convolution layer.

My current approach is the following, where x_in is the image of shape [batch_size, channels, height, width], fs is the length of the side of the square I want to sample, and n is the number of samples I want to extract from the current batch.

def sample_patches(self, x_in, fs, n):
    all_patches = x_in.unfold(2, fs, 1).unfold(3, fs, 1).transpose(1, 3).contiguous().view(-1,
                                                                                           x_in.size(1) * fs * fs)
    # print(all_patches.size())
    n_sampling_patches = min(all_patches.size(0), n)
    indices = torch.randperm(all_patches.size(0))[:n_sampling_patches]
    indices = indices.cuda()
    patches = all_patches[indices]
    return patches

This code works but is quite slow. I think because the “contiguous” does a lot of copying.
Is there a more efficient way to achieve this?

Matteo Ronchetti


what is the transpose for?