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| # Copyright (c) Meta Platforms, Inc. and affiliates. | |
| # All rights reserved. | |
| # This source code is licensed under the license found in the | |
| # LICENSE file in the root directory of this source tree. | |
| from typing import List, Optional | |
| import torch | |
| import torch.nn as nn | |
| import torch.nn.functional as F | |
| class ImageEncoder(nn.Module): | |
| def __init__( | |
| self, | |
| trunk: nn.Module, | |
| neck: nn.Module, | |
| scalp: int = 0, | |
| ): | |
| super().__init__() | |
| self.trunk = trunk | |
| self.neck = neck | |
| self.scalp = scalp | |
| assert ( | |
| self.trunk.channel_list == self.neck.backbone_channel_list | |
| ), f"Channel dims of trunk and neck do not match. Trunk: {self.trunk.channel_list}, neck: {self.neck.backbone_channel_list}" | |
| def forward(self, sample: torch.Tensor): | |
| # Forward through backbone | |
| features, pos = self.neck(self.trunk(sample)) | |
| if self.scalp > 0: | |
| # Discard the lowest resolution features | |
| features, pos = features[: -self.scalp], pos[: -self.scalp] | |
| src = features[-1] | |
| output = { | |
| "vision_features": src, | |
| "vision_pos_enc": pos, | |
| "backbone_fpn": features, | |
| } | |
| return output | |
| class FpnNeck(nn.Module): | |
| """ | |
| A modified variant of Feature Pyramid Network (FPN) neck | |
| (we remove output conv and also do bicubic interpolation similar to ViT | |
| pos embed interpolation) | |
| """ | |
| def __init__( | |
| self, | |
| position_encoding: nn.Module, | |
| d_model: int, | |
| backbone_channel_list: List[int], | |
| kernel_size: int = 1, | |
| stride: int = 1, | |
| padding: int = 0, | |
| fpn_interp_model: str = "bilinear", | |
| fuse_type: str = "sum", | |
| fpn_top_down_levels: Optional[List[int]] = None, | |
| ): | |
| """Initialize the neck | |
| :param trunk: the backbone | |
| :param position_encoding: the positional encoding to use | |
| :param d_model: the dimension of the model | |
| :param neck_norm: the normalization to use | |
| """ | |
| super().__init__() | |
| self.position_encoding = position_encoding | |
| self.convs = nn.ModuleList() | |
| self.backbone_channel_list = backbone_channel_list | |
| for dim in backbone_channel_list: | |
| current = nn.Sequential() | |
| current.add_module( | |
| "conv", | |
| nn.Conv2d( | |
| in_channels=dim, | |
| out_channels=d_model, | |
| kernel_size=kernel_size, | |
| stride=stride, | |
| padding=padding, | |
| ), | |
| ) | |
| self.convs.append(current) | |
| self.fpn_interp_model = fpn_interp_model | |
| assert fuse_type in ["sum", "avg"] | |
| self.fuse_type = fuse_type | |
| # levels to have top-down features in its outputs | |
| # e.g. if fpn_top_down_levels is [2, 3], then only outputs of level 2 and 3 | |
| # have top-down propagation, while outputs of level 0 and level 1 have only | |
| # lateral features from the same backbone level. | |
| if fpn_top_down_levels is None: | |
| # default is to have top-down features on all levels | |
| fpn_top_down_levels = range(len(self.convs)) | |
| self.fpn_top_down_levels = list(fpn_top_down_levels) | |
| def forward(self, xs: List[torch.Tensor]): | |
| out = [None] * len(self.convs) | |
| pos = [None] * len(self.convs) | |
| assert len(xs) == len(self.convs) | |
| # fpn forward pass | |
| # see https://github.com/facebookresearch/detectron2/blob/main/detectron2/modeling/backbone/fpn.py | |
| prev_features = None | |
| # forward in top-down order (from low to high resolution) | |
| n = len(self.convs) - 1 | |
| for i in range(n, -1, -1): | |
| x = xs[i] | |
| lateral_features = self.convs[n - i](x) | |
| if i in self.fpn_top_down_levels and prev_features is not None: | |
| top_down_features = F.interpolate( | |
| prev_features.to(dtype=torch.float32), | |
| scale_factor=2.0, | |
| mode=self.fpn_interp_model, | |
| align_corners=( | |
| None if self.fpn_interp_model == "nearest" else False | |
| ), | |
| antialias=False, | |
| ) | |
| prev_features = lateral_features + top_down_features | |
| if self.fuse_type == "avg": | |
| prev_features /= 2 | |
| else: | |
| prev_features = lateral_features | |
| x_out = prev_features | |
| out[i] = x_out | |
| pos[i] = self.position_encoding(x_out).to(x_out.dtype) | |
| return out, pos | |