resnet_model.py

# Copyright 2018 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""ResNet50 model for Keras.

Adapted from tf.keras.applications.resnet50.ResNet50().
This is ResNet model version 1.5.

Related papers/blogs:
- https://arxiv.org/abs/1512.03385
- https://arxiv.org/pdf/1603.05027v2.pdf
- http://torch.ch/blog/2016/02/04/resnets.html

"""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

from tensorflow.python.keras import backend
from tensorflow.python.keras import initializers
from tensorflow.python.keras import layers
from tensorflow.python.keras import models
from tensorflow.python.keras import regularizers


L2_WEIGHT_DECAY = 1e-4
BATCH_NORM_DECAY = 0.9
BATCH_NORM_EPSILON = 1e-5


def _gen_l2_regularizer(use_l2_regularizer=True):
  return regularizers.l2(L2_WEIGHT_DECAY) if use_l2_regularizer else None


def identity_block(input_tensor,
                   kernel_size,
                   filters,
                   stage,
                   block,
                   use_l2_regularizer=True):
  """The identity block is the block that has no conv layer at shortcut.

  Args:
    input_tensor: input tensor
    kernel_size: default 3, the kernel size of middle conv layer at main path
    filters: list of integers, the filters of 3 conv layer at main path
    stage: integer, current stage label, used for generating layer names
    block: 'a','b'..., current block label, used for generating layer names
    use_l2_regularizer: whether to use L2 regularizer on Conv layer.

  Returns:
    Output tensor for the block.
  """
  filters1, filters2, filters3 = filters
  if backend.image_data_format() == 'channels_last':
    bn_axis = 3
  else:
    bn_axis = 1
  conv_name_base = 'res' + str(stage) + block + '_branch'
  bn_name_base = 'bn' + str(stage) + block + '_branch'

  x = layers.Conv2D(
      filters1, (1, 1),
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2a')(
          input_tensor)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2a')(
          x)
  x = layers.Activation('relu')(x)

  x = layers.Conv2D(
      filters2,
      kernel_size,
      padding='same',
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2b')(
          x)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2b')(
          x)
  x = layers.Activation('relu')(x)

  x = layers.Conv2D(
      filters3, (1, 1),
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2c')(
          x)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2c')(
          x)

  x = layers.add([x, input_tensor])
  x = layers.Activation('relu')(x)
  return x


def conv_block(input_tensor,
               kernel_size,
               filters,
               stage,
               block,
               strides=(2, 2),
               use_l2_regularizer=True):
  """A block that has a conv layer at shortcut.

  Note that from stage 3,
  the second conv layer at main path is with strides=(2, 2)
  And the shortcut should have strides=(2, 2) as well

  Args:
    input_tensor: input tensor
    kernel_size: default 3, the kernel size of middle conv layer at main path
    filters: list of integers, the filters of 3 conv layer at main path
    stage: integer, current stage label, used for generating layer names
    block: 'a','b'..., current block label, used for generating layer names
    strides: Strides for the second conv layer in the block.
    use_l2_regularizer: whether to use L2 regularizer on Conv layer.

  Returns:
    Output tensor for the block.
  """
  filters1, filters2, filters3 = filters
  if backend.image_data_format() == 'channels_last':
    bn_axis = 3
  else:
    bn_axis = 1
  conv_name_base = 'res' + str(stage) + block + '_branch'
  bn_name_base = 'bn' + str(stage) + block + '_branch'

  x = layers.Conv2D(
      filters1, (1, 1),
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2a')(
          input_tensor)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2a')(
          x)
  x = layers.Activation('relu')(x)

  x = layers.Conv2D(
      filters2,
      kernel_size,
      strides=strides,
      padding='same',
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2b')(
          x)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2b')(
          x)
  x = layers.Activation('relu')(x)

  x = layers.Conv2D(
      filters3, (1, 1),
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '2c')(
          x)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '2c')(
          x)

  shortcut = layers.Conv2D(
      filters3, (1, 1),
      strides=strides,
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name=conv_name_base + '1')(
          input_tensor)
  shortcut = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name=bn_name_base + '1')(
          shortcut)

  x = layers.add([x, shortcut])
  x = layers.Activation('relu')(x)
  return x


def resnet50(num_classes,
             dtype='float32',
             batch_size=None,
             use_l2_regularizer=True):
  """Instantiates the ResNet50 architecture.

  Args:
    num_classes: `int` number of classes for image classification.
    dtype: dtype to use float32 or float16 are most common.
    batch_size: Size of the batches for each step.
    use_l2_regularizer: whether to use L2 regularizer on Conv/Dense layer.

  Returns:
      A Keras model instance.
  """
  input_shape = (224, 224, 3)
  img_input = layers.Input(
      shape=input_shape, dtype=dtype, batch_size=batch_size)

  if backend.image_data_format() == 'channels_first':
    x = layers.Lambda(
        lambda x: backend.permute_dimensions(x, (0, 3, 1, 2)),
        name='transpose')(
            img_input)
    bn_axis = 1
  else:  # channels_last
    x = img_input
    bn_axis = 3

  x = layers.ZeroPadding2D(padding=(3, 3), name='conv1_pad')(x)
  x = layers.Conv2D(
      64, (7, 7),
      strides=(2, 2),
      padding='valid',
      use_bias=False,
      kernel_initializer='he_normal',
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name='conv1')(
          x)
  x = layers.BatchNormalization(
      axis=bn_axis,
      momentum=BATCH_NORM_DECAY,
      epsilon=BATCH_NORM_EPSILON,
      name='bn_conv1')(
          x)
  x = layers.Activation('relu')(x)
  x = layers.MaxPooling2D((3, 3), strides=(2, 2), padding='same')(x)

  x = conv_block(
      x,
      3, [64, 64, 256],
      stage=2,
      block='a',
      strides=(1, 1),
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [64, 64, 256],
      stage=2,
      block='b',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [64, 64, 256],
      stage=2,
      block='c',
      use_l2_regularizer=use_l2_regularizer)

  x = conv_block(
      x,
      3, [128, 128, 512],
      stage=3,
      block='a',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [128, 128, 512],
      stage=3,
      block='b',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [128, 128, 512],
      stage=3,
      block='c',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [128, 128, 512],
      stage=3,
      block='d',
      use_l2_regularizer=use_l2_regularizer)

  x = conv_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='a',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='b',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='c',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='d',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='e',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [256, 256, 1024],
      stage=4,
      block='f',
      use_l2_regularizer=use_l2_regularizer)

  x = conv_block(
      x,
      3, [512, 512, 2048],
      stage=5,
      block='a',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [512, 512, 2048],
      stage=5,
      block='b',
      use_l2_regularizer=use_l2_regularizer)
  x = identity_block(
      x,
      3, [512, 512, 2048],
      stage=5,
      block='c',
      use_l2_regularizer=use_l2_regularizer)

  rm_axes = [1, 2] if backend.image_data_format() == 'channels_last' else [2, 3]
  x = layers.Lambda(lambda x: backend.mean(x, rm_axes), name='reduce_mean')(x)
  x = layers.Dense(
      num_classes,
      kernel_initializer=initializers.RandomNormal(stddev=0.01),
      kernel_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      bias_regularizer=_gen_l2_regularizer(use_l2_regularizer),
      name='fc1000')(
          x)

  # TODO(reedwm): Remove manual casts once mixed precision can be enabled with a
  # single line of code.
  x = backend.cast(x, 'float32')
  x = layers.Activation('softmax')(x)

  # Create model.
  return models.Model(img_input, x, name='resnet50')