sparse_unet.py

import torch
from torch import nn as nn

from mmdet3d.ops import SparseBasicBlock, make_sparse_convmodule
from mmdet3d.ops import spconv as spconv
from ..registry import MIDDLE_ENCODERS


@MIDDLE_ENCODERS.register_module()
class SparseUNet(nn.Module):
    r"""SparseUNet for PartA^2.

    See the `paper <https://arxiv.org/abs/1907.03670>`_ for more detials.

    Args:
        in_channels (int): The number of input channels.
        sparse_shape (list[int]): The sparse shape of input tensor.
        norm_cfg (dict): Config of normalization layer.
        base_channels (int): Out channels for conv_input layer.
        output_channels (int): Out channels for conv_out layer.
        encoder_channels (tuple[tuple[int]]):
            Convolutional channels of each encode block.
        encoder_paddings (tuple[tuple[int]]): Paddings of each encode block.
        decoder_channels (tuple[tuple[int]]):
            Convolutional channels of each decode block.
        decoder_paddings (tuple[tuple[int]]): Paddings of each decode block.
    """

    def __init__(self,
                 in_channels,
                 sparse_shape,
                 order=('conv', 'norm', 'act'),
                 norm_cfg=dict(type='BN1d', eps=1e-3, momentum=0.01),
                 base_channels=16,
                 output_channels=128,
                 encoder_channels=((16, ), (32, 32, 32), (64, 64, 64), (64, 64,
                                                                        64)),
                 encoder_paddings=((1, ), (1, 1, 1), (1, 1, 1), ((0, 1, 1), 1,
                                                                 1)),
                 decoder_channels=((64, 64, 64), (64, 64, 32), (32, 32, 16),
                                   (16, 16, 16)),
                 decoder_paddings=((1, 0), (1, 0), (0, 0), (0, 1))):
        super().__init__()
        self.sparse_shape = sparse_shape
        self.in_channels = in_channels
        self.order = order
        self.base_channels = base_channels
        self.output_channels = output_channels
        self.encoder_channels = encoder_channels
        self.encoder_paddings = encoder_paddings
        self.decoder_channels = decoder_channels
        self.decoder_paddings = decoder_paddings
        self.stage_num = len(self.encoder_channels)
        # Spconv init all weight on its own

        assert isinstance(order, tuple) and len(order) == 3
        assert set(order) == {'conv', 'norm', 'act'}

        if self.order[0] != 'conv':  # pre activate
            self.conv_input = make_sparse_convmodule(
                in_channels,
                self.base_channels,
                3,
                norm_cfg=norm_cfg,
                padding=1,
                indice_key='subm1',
                conv_type='SubMConv3d',
                order=('conv', ))
        else:  # post activate
            self.conv_input = make_sparse_convmodule(
                in_channels,
                self.base_channels,
                3,
                norm_cfg=norm_cfg,
                padding=1,
                indice_key='subm1',
                conv_type='SubMConv3d')

        encoder_out_channels = self.make_encoder_layers(
            make_sparse_convmodule, norm_cfg, self.base_channels)
        self.make_decoder_layers(make_sparse_convmodule, norm_cfg,
                                 encoder_out_channels)

        self.conv_out = make_sparse_convmodule(
            encoder_out_channels,
            self.output_channels,
            kernel_size=(3, 1, 1),
            stride=(2, 1, 1),
            norm_cfg=norm_cfg,
            padding=0,
            indice_key='spconv_down2',
            conv_type='SparseConv3d')

    def forward(self, voxel_features, coors, batch_size):
        """Forward of SparseUNet.

        Args:
            voxel_features (torch.float32): Voxel features in shape [N, C].
            coors (torch.int32): Coordinates in shape [N, 4],
                the columns in the order of (batch_idx, z_idx, y_idx, x_idx).
            batch_size (int): Batch size.

        Returns:
            dict[str, torch.Tensor]: Backbone features.
        """
        coors = coors.int()
        input_sp_tensor = spconv.SparseConvTensor(voxel_features, coors,
                                                  self.sparse_shape,
                                                  batch_size)
        x = self.conv_input(input_sp_tensor)

        encode_features = []
        for encoder_layer in self.encoder_layers:
            x = encoder_layer(x)
            encode_features.append(x)

        # for detection head
        # [200, 176, 5] -> [200, 176, 2]
        out = self.conv_out(encode_features[-1])
        spatial_features = out.dense()

        N, C, D, H, W = spatial_features.shape
        spatial_features = spatial_features.view(N, C * D, H, W)

        # for segmentation head, with output shape:
        # [400, 352, 11] <- [200, 176, 5]
        # [800, 704, 21] <- [400, 352, 11]
        # [1600, 1408, 41] <- [800, 704, 21]
        # [1600, 1408, 41] <- [1600, 1408, 41]
        decode_features = []
        x = encode_features[-1]
        for i in range(self.stage_num, 0, -1):
            x = self.decoder_layer_forward(encode_features[i - 1], x,
                                           getattr(self, f'lateral_layer{i}'),
                                           getattr(self, f'merge_layer{i}'),
                                           getattr(self, f'upsample_layer{i}'))
            decode_features.append(x)

        seg_features = decode_features[-1].features

        ret = dict(
            spatial_features=spatial_features, seg_features=seg_features)

        return ret

    def decoder_layer_forward(self, x_lateral, x_bottom, lateral_layer,
                              merge_layer, upsample_layer):
        """Forward of upsample and residual block.

        Args:
            x_lateral (:obj:`SparseConvTensor`): Lateral tensor.
            x_bottom (:obj:`SparseConvTensor`): Feature from bottom layer.
            lateral_layer (SparseBasicBlock): Convolution for lateral tensor.
            merge_layer (SparseSequential): Convolution for merging features.
            upsample_layer (SparseSequential): Convolution for upsampling.

        Returns:
            :obj:`SparseConvTensor`: Upsampled feature.
        """
        x = lateral_layer(x_lateral)
        x.features = torch.cat((x_bottom.features, x.features), dim=1)
        x_merge = merge_layer(x)
        x = self.reduce_channel(x, x_merge.features.shape[1])
        x.features = x_merge.features + x.features
        x = upsample_layer(x)
        return x

    @staticmethod
    def reduce_channel(x, out_channels):
        """reduce channel for element-wise addition.

        Args:
            x (:obj:`SparseConvTensor`): Sparse tensor, ``x.features``
                are in shape (N, C1).
            out_channels (int): The number of channel after reduction.

        Returns:
            :obj:`SparseConvTensor`: Channel reduced feature.
        """
        features = x.features
        n, in_channels = features.shape
        assert (in_channels % out_channels
                == 0) and (in_channels >= out_channels)

        x.features = features.view(n, out_channels, -1).sum(dim=2)
        return x

    def make_encoder_layers(self, make_block, norm_cfg, in_channels):
        """make encoder layers using sparse convs.

        Args:
            make_block (method): A bounded function to build blocks.
            norm_cfg (dict[str]): Config of normalization layer.
            in_channels (int): The number of encoder input channels.

        Returns:
            int: The number of encoder output channels.
        """
        self.encoder_layers = spconv.SparseSequential()

        for i, blocks in enumerate(self.encoder_channels):
            blocks_list = []
            for j, out_channels in enumerate(tuple(blocks)):
                padding = tuple(self.encoder_paddings[i])[j]
                # each stage started with a spconv layer
                # except the first stage
                if i != 0 and j == 0:
                    blocks_list.append(
                        make_block(
                            in_channels,
                            out_channels,
                            3,
                            norm_cfg=norm_cfg,
                            stride=2,
                            padding=padding,
                            indice_key=f'spconv{i + 1}',
                            conv_type='SparseConv3d'))
                else:
                    blocks_list.append(
                        make_block(
                            in_channels,
                            out_channels,
                            3,
                            norm_cfg=norm_cfg,
                            padding=padding,
                            indice_key=f'subm{i + 1}',
                            conv_type='SubMConv3d'))
                in_channels = out_channels
            stage_name = f'encoder_layer{i + 1}'
            stage_layers = spconv.SparseSequential(*blocks_list)
            self.encoder_layers.add_module(stage_name, stage_layers)
        return out_channels

    def make_decoder_layers(self, make_block, norm_cfg, in_channels):
        """make decoder layers using sparse convs.

        Args:
            make_block (method): A bounded function to build blocks.
            norm_cfg (dict[str]): Config of normalization layer.
            in_channels (int): The number of encoder input channels.

        Returns:
            int: The number of encoder output channels.
        """
        block_num = len(self.decoder_channels)
        for i, block_channels in enumerate(self.decoder_channels):
            paddings = self.decoder_paddings[i]
            setattr(
                self, f'lateral_layer{block_num - i}',
                SparseBasicBlock(
                    in_channels,
                    block_channels[0],
                    conv_cfg=dict(
                        type='SubMConv3d', indice_key=f'subm{block_num - i}'),
                    norm_cfg=norm_cfg))
            setattr(
                self, f'merge_layer{block_num - i}',
                make_block(
                    in_channels * 2,
                    block_channels[1],
                    3,
                    norm_cfg=norm_cfg,
                    padding=paddings[0],
                    indice_key=f'subm{block_num - i}',
                    conv_type='SubMConv3d'))
            if block_num - i != 1:
                setattr(
                    self, f'upsample_layer{block_num - i}',
                    make_block(
                        in_channels,
                        block_channels[2],
                        3,
                        norm_cfg=norm_cfg,
                        indice_key=f'spconv{block_num - i}',
                        conv_type='SparseInverseConv3d'))
            else:
                # use submanifold conv instead of inverse conv
                # in the last block
                setattr(
                    self, f'upsample_layer{block_num - i}',
                    make_block(
                        in_channels,
                        block_channels[2],
                        3,
                        norm_cfg=norm_cfg,
                        padding=paddings[1],
                        indice_key='subm1',
                        conv_type='SubMConv3d'))
            in_channels = block_channels[2]