def _shortcut(input_feature, residual, conv_name_base=None, bn_name_base=None):
"""Adds a shortcut between input and residual block and merges them with "sum"
"""
# Expand channels of shortcut to match residual.
# Stride appropriately to match residual (width, height)
# Should be int if network architecture is correctly configured.
input_shape = K.int_shape(input_feature)
residual_shape = K.int_shape(residual)
stride_width = int(round(input_shape[ROW_AXIS] / residual_shape[ROW_AXIS]))
stride_height = int(round(input_shape[COL_AXIS] / residual_shape[COL_AXIS]))
equal_channels = input_shape[CHANNEL_AXIS] == residual_shape[CHANNEL_AXIS]
shortcut = input_feature
# 1 X 1 conv if shape is different. Else identity.
if stride_width > 1 or stride_height > 1 or not equal_channels:
print('reshaping via a convolution...')
if conv_name_base is not None:
conv_name_base = conv_name_base + '1'
shortcut = Conv2D(filters=residual_shape[CHANNEL_AXIS],
kernel_size=(1, 1),
strides=(stride_width, stride_height),
padding="valid",
kernel_initializer="he_normal",
kernel_regularizer=l2(0.0001),
name=conv_name_base)(input_feature)
if bn_name_base is not None:
bn_name_base = bn_name_base + '1'
shortcut = BatchNormalization(axis=CHANNEL_AXIS, name=bn_name_base)(shortcut)
return add([shortcut, residual])
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