Source code for torchvision.ops.feature_pyramid_network

from collections import OrderedDict

import torch
import torch.nn.functional as F
from torch import nn, Tensor

from torch.jit.annotations import Tuple, List, Dict, Optional

class ExtraFPNBlock(nn.Module):
    Base class for the extra block in the FPN.

        results (List[Tensor]): the result of the FPN
        x (List[Tensor]): the original feature maps
        names (List[str]): the names for each one of the
            original feature maps

        results (List[Tensor]): the extended set of results
            of the FPN
        names (List[str]): the extended set of names for the results
    def forward(
        results: List[Tensor],
        x: List[Tensor],
        names: List[str],
    ) -> Tuple[List[Tensor], List[str]]:

[docs]class FeaturePyramidNetwork(nn.Module): """ Module that adds a FPN from on top of a set of feature maps. This is based on `"Feature Pyramid Network for Object Detection" <>`_. The feature maps are currently supposed to be in increasing depth order. The input to the model is expected to be an OrderedDict[Tensor], containing the feature maps on top of which the FPN will be added. Arguments: in_channels_list (list[int]): number of channels for each feature map that is passed to the module out_channels (int): number of channels of the FPN representation extra_blocks (ExtraFPNBlock or None): if provided, extra operations will be performed. It is expected to take the fpn features, the original features and the names of the original features as input, and returns a new list of feature maps and their corresponding names Examples:: >>> m = torchvision.ops.FeaturePyramidNetwork([10, 20, 30], 5) >>> # get some dummy data >>> x = OrderedDict() >>> x['feat0'] = torch.rand(1, 10, 64, 64) >>> x['feat2'] = torch.rand(1, 20, 16, 16) >>> x['feat3'] = torch.rand(1, 30, 8, 8) >>> # compute the FPN on top of x >>> output = m(x) >>> print([(k, v.shape) for k, v in output.items()]) >>> # returns >>> [('feat0', torch.Size([1, 5, 64, 64])), >>> ('feat2', torch.Size([1, 5, 16, 16])), >>> ('feat3', torch.Size([1, 5, 8, 8]))] """ def __init__( self, in_channels_list: List[int], out_channels: int, extra_blocks: Optional[ExtraFPNBlock] = None, ): super(FeaturePyramidNetwork, self).__init__() self.inner_blocks = nn.ModuleList() self.layer_blocks = nn.ModuleList() for in_channels in in_channels_list: if in_channels == 0: raise ValueError("in_channels=0 is currently not supported") inner_block_module = nn.Conv2d(in_channels, out_channels, 1) layer_block_module = nn.Conv2d(out_channels, out_channels, 3, padding=1) self.inner_blocks.append(inner_block_module) self.layer_blocks.append(layer_block_module) # initialize parameters now to avoid modifying the initialization of top_blocks for m in self.children(): if isinstance(m, nn.Conv2d): nn.init.kaiming_uniform_(m.weight, a=1) nn.init.constant_(m.bias, 0) if extra_blocks is not None: assert isinstance(extra_blocks, ExtraFPNBlock) self.extra_blocks = extra_blocks def get_result_from_inner_blocks(self, x: Tensor, idx: int) -> Tensor: """ This is equivalent to self.inner_blocks[idx](x), but torchscript doesn't support this yet """ num_blocks = 0 for m in self.inner_blocks: num_blocks += 1 if idx < 0: idx += num_blocks i = 0 out = x for module in self.inner_blocks: if i == idx: out = module(x) i += 1 return out def get_result_from_layer_blocks(self, x: Tensor, idx: int) -> Tensor: """ This is equivalent to self.layer_blocks[idx](x), but torchscript doesn't support this yet """ num_blocks = 0 for m in self.layer_blocks: num_blocks += 1 if idx < 0: idx += num_blocks i = 0 out = x for module in self.layer_blocks: if i == idx: out = module(x) i += 1 return out def forward(self, x: Dict[str, Tensor]) -> Dict[str, Tensor]: """ Computes the FPN for a set of feature maps. Arguments: x (OrderedDict[Tensor]): feature maps for each feature level. Returns: results (OrderedDict[Tensor]): feature maps after FPN layers. They are ordered from highest resolution first. """ # unpack OrderedDict into two lists for easier handling names = list(x.keys()) x = list(x.values()) last_inner = self.get_result_from_inner_blocks(x[-1], -1) results = [] results.append(self.get_result_from_layer_blocks(last_inner, -1)) for idx in range(len(x) - 2, -1, -1): inner_lateral = self.get_result_from_inner_blocks(x[idx], idx) feat_shape = inner_lateral.shape[-2:] inner_top_down = F.interpolate(last_inner, size=feat_shape, mode="nearest") last_inner = inner_lateral + inner_top_down results.insert(0, self.get_result_from_layer_blocks(last_inner, idx)) if self.extra_blocks is not None: results, names = self.extra_blocks(results, x, names) # make it back an OrderedDict out = OrderedDict([(k, v) for k, v in zip(names, results)]) return out
class LastLevelMaxPool(ExtraFPNBlock): """ Applies a max_pool2d on top of the last feature map """ def forward( self, x: List[Tensor], y: List[Tensor], names: List[str], ) -> Tuple[List[Tensor], List[str]]: names.append("pool") x.append(F.max_pool2d(x[-1], 1, 2, 0)) return x, names class LastLevelP6P7(ExtraFPNBlock): """ This module is used in RetinaNet to generate extra layers, P6 and P7. """ def __init__(self, in_channels: int, out_channels: int): super(LastLevelP6P7, self).__init__() self.p6 = nn.Conv2d(in_channels, out_channels, 3, 2, 1) self.p7 = nn.Conv2d(out_channels, out_channels, 3, 2, 1) for module in [self.p6, self.p7]: nn.init.kaiming_uniform_(module.weight, a=1) nn.init.constant_(module.bias, 0) self.use_P5 = in_channels == out_channels def forward( self, p: List[Tensor], c: List[Tensor], names: List[str], ) -> Tuple[List[Tensor], List[str]]: p5, c5 = p[-1], c[-1] x = p5 if self.use_P5 else c5 p6 = self.p6(x) p7 = self.p7(F.relu(p6)) p.extend([p6, p7]) names.extend(["p6", "p7"]) return p, names


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