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# Copyright (c) Facebook, Inc. and its affiliates.
from __future__ import division
from typing import Any, Dict, List, Optional, Tuple
import torch
from torch import device
from torch.nn import functional as F
from detectron2.layers.wrappers import move_device_like, shapes_to_tensor
class ImageList:
"""
Structure that holds a list of images (of possibly
varying sizes) as a single tensor.
This works by padding the images to the same size.
The original sizes of each image is stored in `image_sizes`.
Attributes:
image_sizes (list[tuple[int, int]]): each tuple is (h, w).
During tracing, it becomes list[Tensor] instead.
"""
def __init__(self, tensor: torch.Tensor, image_sizes: List[Tuple[int, int]]):
"""
Arguments:
tensor (Tensor): of shape (N, H, W) or (N, C_1, ..., C_K, H, W) where K >= 1
image_sizes (list[tuple[int, int]]): Each tuple is (h, w). It can
be smaller than (H, W) due to padding.
"""
self.tensor = tensor
self.image_sizes = image_sizes
def __len__(self) -> int:
return len(self.image_sizes)
def __getitem__(self, idx) -> torch.Tensor:
"""
Access the individual image in its original size.
Args:
idx: int or slice
Returns:
Tensor: an image of shape (H, W) or (C_1, ..., C_K, H, W) where K >= 1
"""
size = self.image_sizes[idx]
return self.tensor[idx, ..., : size[0], : size[1]]
@torch.jit.unused
def to(self, *args: Any, **kwargs: Any) -> "ImageList":
cast_tensor = self.tensor.to(*args, **kwargs)
return ImageList(cast_tensor, self.image_sizes)
@property
def device(self) -> device:
return self.tensor.device
@staticmethod
def from_tensors(
tensors: List[torch.Tensor],
size_divisibility: int = 0,
pad_value: float = 0.0,
padding_constraints: Optional[Dict[str, int]] = None,
) -> "ImageList":
"""
Args:
tensors: a tuple or list of `torch.Tensor`, each of shape (Hi, Wi) or
(C_1, ..., C_K, Hi, Wi) where K >= 1. The Tensors will be padded
to the same shape with `pad_value`.
size_divisibility (int): If `size_divisibility > 0`, add padding to ensure
the common height and width is divisible by `size_divisibility`.
This depends on the model and many models need a divisibility of 32.
pad_value (float): value to pad.
padding_constraints (optional[Dict]): If given, it would follow the format as
{"size_divisibility": int, "square_size": int}, where `size_divisibility` will
overwrite the above one if presented and `square_size` indicates the
square padding size if `square_size` > 0.
Returns:
an `ImageList`.
"""
assert len(tensors) > 0
assert isinstance(tensors, (tuple, list))
for t in tensors:
assert isinstance(t, torch.Tensor), type(t)
assert t.shape[:-2] == tensors[0].shape[:-2], t.shape
image_sizes = [(im.shape[-2], im.shape[-1]) for im in tensors]
image_sizes_tensor = [shapes_to_tensor(x) for x in image_sizes]
max_size = torch.stack(image_sizes_tensor).max(0).values
if padding_constraints is not None:
square_size = padding_constraints.get("square_size", 0)
if square_size > 0:
# pad to square.
max_size[0] = max_size[1] = square_size
if "size_divisibility" in padding_constraints:
size_divisibility = padding_constraints["size_divisibility"]
if size_divisibility > 1:
stride = size_divisibility
# the last two dims are H,W, both subject to divisibility requirement
max_size = (max_size + (stride - 1)).div(stride, rounding_mode="floor") * stride
# handle weirdness of scripting and tracing ...
if torch.jit.is_scripting():
max_size: List[int] = max_size.to(dtype=torch.long).tolist()
else:
if torch.jit.is_tracing():
image_sizes = image_sizes_tensor
if len(tensors) == 1:
# This seems slightly (2%) faster.
# TODO: check whether it's faster for multiple images as well
image_size = image_sizes[0]
padding_size = [0, max_size[-1] - image_size[1], 0, max_size[-2] - image_size[0]]
batched_imgs = F.pad(tensors[0], padding_size, value=pad_value).unsqueeze_(0)
else:
# max_size can be a tensor in tracing mode, therefore convert to list
batch_shape = [len(tensors)] + list(tensors[0].shape[:-2]) + list(max_size)
device = (
None if torch.jit.is_scripting() else ("cpu" if torch.jit.is_tracing() else None)
)
batched_imgs = tensors[0].new_full(batch_shape, pad_value, device=device)
batched_imgs = move_device_like(batched_imgs, tensors[0])
for i, img in enumerate(tensors):
# Use `batched_imgs` directly instead of `img, pad_img = zip(tensors, batched_imgs)`
# Tracing mode cannot capture `copy_()` of temporary locals
batched_imgs[i, ..., : img.shape[-2], : img.shape[-1]].copy_(img)
return ImageList(batched_imgs.contiguous(), image_sizes)
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