mmdet/models/utils/positional_encoding.py

import math

import torch
import torch.nn as nn
from mmcv.cnn import uniform_init

from .builder import POSITIONAL_ENCODING


@POSITIONAL_ENCODING.register_module()
class SinePositionalEncoding(nn.Module):
    """Position encoding with sine and cosine functions.

    See `End-to-End Object Detection with Transformers
    <https://arxiv.org/pdf/2005.12872>`_ for details.

    Args:
        num_feats (int): The feature dimension for each position
            along x-axis or y-axis. Note the final returned dimension
            for each position is 2 times of this value.
        temperature (int, optional): The temperature used for scaling
            the position embedding. Default 10000.
        normalize (bool, optional): Whether to normalize the position
            embedding. Default False.
        scale (float, optional): A scale factor that scales the position
            embedding. The scale will be used only when `normalize` is True.
            Default 2*pi.
        eps (float, optional): A value added to the denominator for
            numerical stability. Default 1e-6.
    """

    def __init__(self,
                 num_feats,
                 temperature=10000,
                 normalize=False,
                 scale=2 * math.pi,
                 eps=1e-6):
        super(SinePositionalEncoding, self).__init__()
        if normalize:
            assert isinstance(scale, (float, int)), 'when normalize is set,' \
                'scale should be provided and in float or int type, ' \
                f'found {type(scale)}'
        self.num_feats = num_feats
        self.temperature = temperature
        self.normalize = normalize
        self.scale = scale
        self.eps = eps

    def forward(self, mask):
        """Forward function for `SinePositionalEncoding`.

        Args:
            mask (Tensor): ByteTensor mask. Non-zero values representing
                ignored positions, while zero values means valid positions
                for this image. Shape [bs, h, w].

        Returns:
            pos (Tensor): Returned position embedding with shape
                [bs, num_feats*2, h, w].
        """
        not_mask = ~mask
        y_embed = not_mask.cumsum(1, dtype=torch.float32)
        x_embed = not_mask.cumsum(2, dtype=torch.float32)
        if self.normalize:
            y_embed = y_embed / (y_embed[:, -1:, :] + self.eps) * self.scale
            x_embed = x_embed / (x_embed[:, :, -1:] + self.eps) * self.scale
        dim_t = torch.arange(
            self.num_feats, dtype=torch.float32, device=mask.device)
        dim_t = self.temperature**(2 * (dim_t // 2) / self.num_feats)
        pos_x = x_embed[:, :, :, None] / dim_t
        pos_y = y_embed[:, :, :, None] / dim_t
        pos_x = torch.stack(
            (pos_x[:, :, :, 0::2].sin(), pos_x[:, :, :, 1::2].cos()),
            dim=4).flatten(3)
        pos_y = torch.stack(
            (pos_y[:, :, :, 0::2].sin(), pos_y[:, :, :, 1::2].cos()),
            dim=4).flatten(3)
        pos = torch.cat((pos_y, pos_x), dim=3).permute(0, 3, 1, 2)
        return pos

    def __repr__(self):
        """str: a string that describes the module"""
        repr_str = self.__class__.__name__
        repr_str += f'(num_feats={self.num_feats}, '
        repr_str += f'temperature={self.temperature}, '
        repr_str += f'normalize={self.normalize}, '
        repr_str += f'scale={self.scale}, '
        repr_str += f'eps={self.eps})'
        return repr_str


@POSITIONAL_ENCODING.register_module()
class LearnedPositionalEncoding(nn.Module):
    """Position embedding with learnable embedding weights.

    Args:
        num_feats (int): The feature dimension for each position
            along x-axis or y-axis. The final returned dimension for
            each position is 2 times of this value.
        row_num_embed (int, optional): The dictionary size of row embeddings.
            Default 50.
        col_num_embed (int, optional): The dictionary size of col embeddings.
            Default 50.
    """

    def __init__(self, num_feats, row_num_embed=50, col_num_embed=50):
        super(LearnedPositionalEncoding, self).__init__()
        self.row_embed = nn.Embedding(row_num_embed, num_feats)
        self.col_embed = nn.Embedding(col_num_embed, num_feats)
        self.num_feats = num_feats
        self.row_num_embed = row_num_embed
        self.col_num_embed = col_num_embed
        self.init_weights()

    def init_weights(self):
        """Initialize the learnable weights."""
        uniform_init(self.row_embed)
        uniform_init(self.col_embed)

    def forward(self, mask):
        """Forward function for `LearnedPositionalEncoding`.

        Args:
            mask (Tensor): ByteTensor mask. Non-zero values representing
                ignored positions, while zero values means valid positions
                for this image. Shape [bs, h, w].

        Returns:
            pos (Tensor): Returned position embedding with shape
                [bs, num_feats*2, h, w].
        """
        h, w = mask.shape[-2:]
        x = torch.arange(w, device=mask.device)
        y = torch.arange(h, device=mask.device)
        x_embed = self.col_embed(x)
        y_embed = self.row_embed(y)
        pos = torch.cat(
            (x_embed.unsqueeze(0).repeat(h, 1, 1), y_embed.unsqueeze(1).repeat(
                1, w, 1)),
            dim=-1).permute(2, 0,
                            1).unsqueeze(0).repeat(mask.shape[0], 1, 1, 1)
        return pos

    def __repr__(self):
        """str: a string that describes the module"""
        repr_str = self.__class__.__name__
        repr_str += f'(num_feats={self.num_feats}, '
        repr_str += f'row_num_embed={self.row_num_embed}, '
        repr_str += f'col_num_embed={self.col_num_embed})'
        return repr_str