fpn.py

# coding=utf-8
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import copy
from collections import OrderedDict

from paddle import fluid
from paddle.fluid.param_attr import ParamAttr
from paddle.fluid.initializer import Xavier
from paddle.fluid.regularizer import L2Decay

__all__ = ['FPN']


def ConvNorm(input,
             num_filters,
             filter_size,
             stride=1,
             groups=1,
             norm_decay=0.,
             norm_type='affine_channel',
             norm_groups=32,
             dilation=1,
             lr_scale=1,
             freeze_norm=False,
             act=None,
             norm_name=None,
             initializer=None,
             name=None):
    fan = num_filters
    conv = fluid.layers.conv2d(
        input=input,
        num_filters=num_filters,
        filter_size=filter_size,
        stride=stride,
        padding=((filter_size - 1) // 2) * dilation,
        dilation=dilation,
        groups=groups,
        act=None,
        param_attr=ParamAttr(name=name + "_weights", initializer=initializer, learning_rate=lr_scale),
        bias_attr=False,
        name=name + '.conv2d.output.1')
    norm_lr = 0. if freeze_norm else 1.
    pattr = ParamAttr(name=norm_name + '_scale', learning_rate=norm_lr * lr_scale, regularizer=L2Decay(norm_decay))
    battr = ParamAttr(name=norm_name + '_offset', learning_rate=norm_lr * lr_scale, regularizer=L2Decay(norm_decay))
    if norm_type in ['bn', 'sync_bn']:
        global_stats = True if freeze_norm else False
        out = fluid.layers.batch_norm(
            input=conv,
            act=act,
            name=norm_name + '.output.1',
            param_attr=pattr,
            bias_attr=battr,
            moving_mean_name=norm_name + '_mean',
            moving_variance_name=norm_name + '_variance',
            use_global_stats=global_stats)
        scale = fluid.framework._get_var(pattr.name)
        bias = fluid.framework._get_var(battr.name)
    elif norm_type == 'gn':
        out = fluid.layers.group_norm(
            input=conv, act=act, name=norm_name + '.output.1', groups=norm_groups, param_attr=pattr, bias_attr=battr)
        scale = fluid.framework._get_var(pattr.name)
        bias = fluid.framework._get_var(battr.name)
    elif norm_type == 'affine_channel':
        scale = fluid.layers.create_parameter(
            shape=[conv.shape[1]], dtype=conv.dtype, attr=pattr, default_initializer=fluid.initializer.Constant(1.))
        bias = fluid.layers.create_parameter(
            shape=[conv.shape[1]], dtype=conv.dtype, attr=battr, default_initializer=fluid.initializer.Constant(0.))
        out = fluid.layers.affine_channel(x=conv, scale=scale, bias=bias, act=act)
    if freeze_norm:
        scale.stop_gradient = True
        bias.stop_gradient = True
    return out


class FPN(object):
    """
    Feature Pyramid Network, see https://arxiv.org/abs/1612.03144

    Args:
        num_chan (int): number of feature channels
        min_level (int): lowest level of the backbone feature map to use
        max_level (int): highest level of the backbone feature map to use
        spatial_scale (list): feature map scaling factor
        has_extra_convs (bool): whether has extral convolutions in higher levels
        norm_type (str|None): normalization type, 'bn'/'sync_bn'/'affine_channel'
    """
    __shared__ = ['norm_type', 'freeze_norm']

    def __init__(self,
                 num_chan=256,
                 min_level=2,
                 max_level=6,
                 spatial_scale=[1. / 32., 1. / 16., 1. / 8., 1. / 4.],
                 has_extra_convs=False,
                 norm_type=None,
                 freeze_norm=False):
        self.freeze_norm = freeze_norm
        self.num_chan = num_chan
        self.min_level = min_level
        self.max_level = max_level
        self.spatial_scale = spatial_scale
        self.has_extra_convs = has_extra_convs
        self.norm_type = norm_type

    def _add_topdown_lateral(self, body_name, body_input, upper_output):
        lateral_name = 'fpn_inner_' + body_name + '_lateral'
        topdown_name = 'fpn_topdown_' + body_name
        fan = body_input.shape[1]
        if self.norm_type:
            initializer = Xavier(fan_out=fan)
            lateral = ConvNorm(
                body_input,
                self.num_chan,
                1,
                initializer=initializer,
                norm_type=self.norm_type,
                freeze_norm=self.freeze_norm,
                name=lateral_name,
                norm_name=lateral_name)
        else:
            lateral = fluid.layers.conv2d(
                body_input,
                self.num_chan,
                1,
                param_attr=ParamAttr(name=lateral_name + "_w", initializer=Xavier(fan_out=fan)),
                bias_attr=ParamAttr(name=lateral_name + "_b", learning_rate=2., regularizer=L2Decay(0.)),
                name=lateral_name)
        topdown = fluid.layers.resize_nearest(upper_output, scale=2., name=topdown_name)

        return lateral + topdown

    def get_output(self, body_dict):
        """
        Add FPN onto backbone.

        Args:
            body_dict(OrderedDict): Dictionary of variables and each element is the
                output of backbone.

        Return:
            fpn_dict(OrderedDict): A dictionary represents the output of FPN with
                their name.
            spatial_scale(list): A list of multiplicative spatial scale factor.
        """
        spatial_scale = copy.deepcopy(self.spatial_scale)
        body_name_list = list(body_dict.keys())[::-1]
        num_backbone_stages = len(body_name_list)
        self.fpn_inner_output = [[] for _ in range(num_backbone_stages)]
        fpn_inner_name = 'fpn_inner_' + body_name_list[0]
        body_input = body_dict[body_name_list[0]]
        fan = body_input.shape[1]
        if self.norm_type:
            initializer = Xavier(fan_out=fan)
            self.fpn_inner_output[0] = ConvNorm(
                body_input,
                self.num_chan,
                1,
                initializer=initializer,
                norm_type=self.norm_type,
                freeze_norm=self.freeze_norm,
                name=fpn_inner_name,
                norm_name=fpn_inner_name)
        else:
            self.fpn_inner_output[0] = fluid.layers.conv2d(
                body_input,
                self.num_chan,
                1,
                param_attr=ParamAttr(name=fpn_inner_name + "_w", initializer=Xavier(fan_out=fan)),
                bias_attr=ParamAttr(name=fpn_inner_name + "_b", learning_rate=2., regularizer=L2Decay(0.)),
                name=fpn_inner_name)
        for i in range(1, num_backbone_stages):
            body_name = body_name_list[i]
            body_input = body_dict[body_name]
            top_output = self.fpn_inner_output[i - 1]
            fpn_inner_single = self._add_topdown_lateral(body_name, body_input, top_output)
            self.fpn_inner_output[i] = fpn_inner_single
        fpn_dict = {}
        fpn_name_list = []
        for i in range(num_backbone_stages):
            fpn_name = 'fpn_' + body_name_list[i]
            fan = self.fpn_inner_output[i].shape[1] * 3 * 3
            if self.norm_type:
                initializer = Xavier(fan_out=fan)
                fpn_output = ConvNorm(
                    self.fpn_inner_output[i],
                    self.num_chan,
                    3,
                    initializer=initializer,
                    norm_type=self.norm_type,
                    freeze_norm=self.freeze_norm,
                    name=fpn_name,
                    norm_name=fpn_name)
            else:
                fpn_output = fluid.layers.conv2d(
                    self.fpn_inner_output[i],
                    self.num_chan,
                    filter_size=3,
                    padding=1,
                    param_attr=ParamAttr(name=fpn_name + "_w", initializer=Xavier(fan_out=fan)),
                    bias_attr=ParamAttr(name=fpn_name + "_b", learning_rate=2., regularizer=L2Decay(0.)),
                    name=fpn_name)
            fpn_dict[fpn_name] = fpn_output
            fpn_name_list.append(fpn_name)
        if not self.has_extra_convs and self.max_level - self.min_level == len(spatial_scale):
            body_top_name = fpn_name_list[0]
            body_top_extension = fluid.layers.pool2d(
                fpn_dict[body_top_name], 1, 'max', pool_stride=2, name=body_top_name + '_subsampled_2x')
            fpn_dict[body_top_name + '_subsampled_2x'] = body_top_extension
            fpn_name_list.insert(0, body_top_name + '_subsampled_2x')
            spatial_scale.insert(0, spatial_scale[0] * 0.5)
        # Coarser FPN levels introduced for RetinaNet
        highest_backbone_level = self.min_level + len(spatial_scale) - 1
        if self.has_extra_convs and self.max_level > highest_backbone_level:
            fpn_blob = body_dict[body_name_list[0]]
            for i in range(highest_backbone_level + 1, self.max_level + 1):
                fpn_blob_in = fpn_blob
                fpn_name = 'fpn_' + str(i)
                if i > highest_backbone_level + 1:
                    fpn_blob_in = fluid.layers.relu(fpn_blob)
                fan = fpn_blob_in.shape[1] * 3 * 3
                fpn_blob = fluid.layers.conv2d(
                    input=fpn_blob_in,
                    num_filters=self.num_chan,
                    filter_size=3,
                    stride=2,
                    padding=1,
                    param_attr=ParamAttr(name=fpn_name + "_w", initializer=Xavier(fan_out=fan)),
                    bias_attr=ParamAttr(name=fpn_name + "_b", learning_rate=2., regularizer=L2Decay(0.)),
                    name=fpn_name)
                fpn_dict[fpn_name] = fpn_blob
                fpn_name_list.insert(0, fpn_name)
                spatial_scale.insert(0, spatial_scale[0] * 0.5)
        res_dict = OrderedDict([(k, fpn_dict[k]) for k in fpn_name_list])
        return res_dict, spatial_scale