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# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
import math
import torch
import torch.nn.functional as F
from torch import nn
from maskrcnn_benchmark.layers import ROIAlign
from .utils import cat
class LevelMapper(object):
"""Determine which FPN level each RoI in a set of RoIs should map to based
on the heuristic in the FPN paper.
"""
def __init__(self, k_min, k_max, canonical_scale=224, canonical_level=4, eps=1e-6):
"""
Arguments:
k_min (int)
k_max (int)
canonical_scale (int)
canonical_level (int)
eps (float)
"""
self.k_min = k_min
self.k_max = k_max
self.s0 = canonical_scale
self.lvl0 = canonical_level
self.eps = eps
def __call__(self, boxlists):
"""
Arguments:
boxlists (list[BoxList])
"""
# Compute level ids
s = torch.sqrt(cat([boxlist.area() for boxlist in boxlists]))
# Eqn.(1) in FPN paper
target_lvls = torch.floor(self.lvl0 + torch.log2(s / self.s0 + self.eps))
target_lvls = torch.clamp(target_lvls, min=self.k_min, max=self.k_max)
return target_lvls.to(torch.int64) - self.k_min
class Pooler(nn.Module):
"""
Pooler for Detection with or without FPN.
It currently hard-code ROIAlign in the implementation,
but that can be made more generic later on.
Also, the requirement of passing the scales is not strictly necessary, as they
can be inferred from the size of the feature map / size of original image,
which is available thanks to the BoxList.
"""
def __init__(self, output_size, scales, sampling_ratio):
"""
Arguments:
output_size (list[tuple[int]] or list[int]): output size for the pooled region
scales (list[flaot]): scales for each Pooler
sampling_ratio (int): sampling ratio for ROIAlign
"""
super(Pooler, self).__init__()
poolers = []
for scale in scales:
poolers.append(
ROIAlign(
output_size, spatial_scale=scale, sampling_ratio=sampling_ratio
)
)
self.poolers = nn.ModuleList(poolers)
self.output_size = output_size
# get the levels in the feature map by leveraging the fact that the network always
# downsamples by a factor of 2 at each level.
lvl_min = -math.log2(scales[0])
lvl_max = -math.log2(scales[-1])
self.map_levels = LevelMapper(lvl_min, lvl_max)
def convert_to_roi_format(self, boxes):
concat_boxes = cat([b.bbox for b in boxes], dim=0)
device, dtype = concat_boxes.device, concat_boxes.dtype
ids = cat(
[
torch.full((len(b), 1), i, dtype=dtype, device=device)
for i, b in enumerate(boxes)
],
dim=0,
)
rois = torch.cat([ids, concat_boxes], dim=1)
return rois
def forward(self, x, boxes):
"""
Arguments:
x (list[Tensor]): feature maps for each level
boxes (list[BoxList]): boxes to be used to perform the pooling operation.
Returns:
result (Tensor)
"""
num_levels = len(self.poolers)
rois = self.convert_to_roi_format(boxes)
if num_levels == 1:
return self.poolers[0](x[0], rois)
levels = self.map_levels(boxes)
num_rois = len(rois)
num_channels = x[0].shape[1]
output_size_h = self.output_size[0]
output_size_w = self.output_size[1]
dtype, device = x[0].dtype, x[0].device
result = torch.zeros(
(num_rois, num_channels, output_size_h, output_size_w),
dtype=dtype,
device=device,
)
for level, (per_level_feature, pooler) in enumerate(zip(x, self.poolers)):
idx_in_level = torch.nonzero(levels == level).squeeze(1)
rois_per_level = rois[idx_in_level]
result[idx_in_level] = pooler(per_level_feature, rois_per_level)
return result
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