group_by_aspect_ratio.py

import bisect
from collections import defaultdict
import copy
import numpy as np

import torch
import torch.utils.data
from torch.utils.data.sampler import BatchSampler, Sampler
from torch.utils.model_zoo import tqdm
import torchvision

from PIL import Image


class GroupedBatchSampler(BatchSampler):
    """
    Wraps another sampler to yield a mini-batch of indices.
    It enforces that the batch only contain elements from the same group.
    It also tries to provide mini-batches which follows an ordering which is
    as close as possible to the ordering from the original sampler.
    Arguments:
        sampler (Sampler): Base sampler.
        group_ids (list[int]): If the sampler produces indices in range [0, N),
            `group_ids` must be a list of `N` ints which contains the group id of each sample.
            The group ids must be a continuous set of integers starting from
            0, i.e. they must be in the range [0, num_groups).
        batch_size (int): Size of mini-batch.
    """
    def __init__(self, sampler, group_ids, batch_size):
        if not isinstance(sampler, Sampler):
            raise ValueError(
                "sampler should be an instance of "
                "torch.utils.data.Sampler, but got sampler={}".format(sampler)
            )
        self.sampler = sampler
        self.group_ids = group_ids
        self.batch_size = batch_size

    def __iter__(self):
        buffer_per_group = defaultdict(list)
        samples_per_group = defaultdict(list)

        num_batches = 0
        for idx in self.sampler:
            group_id = self.group_ids[idx]
            buffer_per_group[group_id].append(idx)
            samples_per_group[group_id].append(idx)
            if len(buffer_per_group[group_id]) == self.batch_size:
                yield buffer_per_group[group_id]
                num_batches += 1
                del buffer_per_group[group_id]
            assert len(buffer_per_group[group_id]) < self.batch_size

        # now we have run out of elements that satisfy
        # the group criteria, let's return the remaining
        # elements so that the size of the sampler is
        # deterministic
        expected_num_batches = len(self)
        num_remaining = expected_num_batches - num_batches
        if num_remaining > 0:
            # for the remaining batches, take first the buffers with largest number
            # of elements
            for group_id, _ in sorted(buffer_per_group.items(),
                                      key=lambda x: len(x[1]), reverse=True):
                remaining = self.batch_size - len(buffer_per_group[group_id])
                buffer_per_group[group_id].extend(
                    samples_per_group[group_id][:remaining])
                assert len(buffer_per_group[group_id]) == self.batch_size
                yield buffer_per_group[group_id]
                num_remaining -= 1
                if num_remaining == 0:
                    break
        assert num_remaining == 0

    def __len__(self):
        return len(self.sampler) // self.batch_size


def _compute_aspect_ratios_slow(dataset, indices=None):
    print("Your dataset doesn't support the fast path for "
          "computing the aspect ratios, so will iterate over "
          "the full dataset and load every image instead. "
          "This might take some time...")
    if indices is None:
        indices = range(len(dataset))

    class SubsetSampler(Sampler):
        def __init__(self, indices):
            self.indices = indices

        def __iter__(self):
            return iter(self.indices)

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

    sampler = SubsetSampler(indices)
    data_loader = torch.utils.data.DataLoader(
        dataset, batch_size=1, sampler=sampler,
        num_workers=14,  # you might want to increase it for faster processing
        collate_fn=lambda x: x[0])
    aspect_ratios = []
    with tqdm(total=len(dataset)) as pbar:
        for _i, (img, _) in enumerate(data_loader):
            pbar.update(1)
            height, width = img.shape[-2:]
            aspect_ratio = float(width) / float(height)
            aspect_ratios.append(aspect_ratio)
    return aspect_ratios


def _compute_aspect_ratios_custom_dataset(dataset, indices=None):
    if indices is None:
        indices = range(len(dataset))
    aspect_ratios = []
    for i in indices:
        height, width = dataset.get_height_and_width(i)
        aspect_ratio = float(width) / float(height)
        aspect_ratios.append(aspect_ratio)
    return aspect_ratios


def _compute_aspect_ratios_coco_dataset(dataset, indices=None):
    if indices is None:
        indices = range(len(dataset))
    aspect_ratios = []
    for i in indices:
        img_info = dataset.coco.imgs[dataset.ids[i]]
        aspect_ratio = float(img_info["width"]) / float(img_info["height"])
        aspect_ratios.append(aspect_ratio)
    return aspect_ratios


def _compute_aspect_ratios_voc_dataset(dataset, indices=None):
    if indices is None:
        indices = range(len(dataset))
    aspect_ratios = []
    for i in indices:
        # this doesn't load the data into memory, because PIL loads it lazily
        width, height = Image.open(dataset.images[i]).size
        aspect_ratio = float(width) / float(height)
        aspect_ratios.append(aspect_ratio)
    return aspect_ratios


def _compute_aspect_ratios_subset_dataset(dataset, indices=None):
    if indices is None:
        indices = range(len(dataset))

    ds_indices = [dataset.indices[i] for i in indices]
    return compute_aspect_ratios(dataset.dataset, ds_indices)


def compute_aspect_ratios(dataset, indices=None):
    if hasattr(dataset, "get_height_and_width"):
        return _compute_aspect_ratios_custom_dataset(dataset, indices)

    if isinstance(dataset, torchvision.datasets.CocoDetection):
        return _compute_aspect_ratios_coco_dataset(dataset, indices)

    if isinstance(dataset, torchvision.datasets.VOCDetection):
        return _compute_aspect_ratios_voc_dataset(dataset, indices)

    if isinstance(dataset, torch.utils.data.Subset):
        return _compute_aspect_ratios_subset_dataset(dataset, indices)

    # slow path
    return _compute_aspect_ratios_slow(dataset, indices)


def _quantize(x, bins):
    bins = copy.deepcopy(bins)
    bins = sorted(bins)
    quantized = list(map(lambda y: bisect.bisect_right(bins, y), x))
    return quantized


def create_aspect_ratio_groups(dataset, k=0):
    aspect_ratios = compute_aspect_ratios(dataset)
    bins = (2 ** np.linspace(-1, 1, 2 * k + 1)).tolist() if k > 0 else [1.0]
    groups = _quantize(aspect_ratios, bins)
    # count number of elements per group
    counts = np.unique(groups, return_counts=True)[1]
    fbins = [0] + bins + [np.inf]
    print("Using {} as bins for aspect ratio quantization".format(fbins))
    print("Count of instances per bin: {}".format(counts))
    return groups