ssd_layers.py

"""
Layers for SSD
"""

import tensorflow as tf

# Conv2d: for stride = 1
def conv2d(x, filters, kernel_size, stride=1, padding="same",
           dilation_rate=1, activation=tf.nn.relu, scope="conv2d"):
    kernel_sizes = [kernel_size] * 2
    strides = [stride] * 2
    dilation_rate = [dilation_rate] * 2
    return tf.layers.conv2d(x, filters, kernel_sizes, strides=strides,
                            dilation_rate=dilation_rate, padding=padding,
                            name=scope, activation=activation)

# max pool2d: default pool_size = stride
def max_pool2d(x, pool_size, stride=None, scope="max_pool2d"):
    pool_sizes = [pool_size] * 2
    strides = [pool_size] * 2 if stride is None else [stride] * 2
    return tf.layers.max_pooling2d(x, pool_sizes, strides, name=scope, padding="same")

# pad2d: for conv2d with stride > 1
def pad2d(x, pad):
    return tf.pad(x, paddings=[[0, 0], [pad, pad], [pad, pad], [0, 0]])

# dropout
def dropout(x, rate=0.5, is_training=True):
    return tf.layers.dropout(x, rate=rate, training=is_training)

# l2norm (not bacth norm, spatial normalization)
def l2norm(x, scale, trainable=True, scope="L2Normalization"):
    n_channels = x.get_shape().as_list()[-1]
    l2_norm = tf.nn.l2_normalize(x, [3], epsilon=1e-12)
    with tf.variable_scope(scope):
        gamma = tf.get_variable("gamma", shape=[n_channels, ], dtype=tf.float32,
                                initializer=tf.constant_initializer(scale),
                                trainable=trainable)
        return l2_norm * gamma


# multibox layer: get class and location predicitions from detection layer
def ssd_multibox_layer(x, num_classes, sizes, ratios, normalization=-1, scope="multibox"):
    pre_shape = x.get_shape().as_list()[1:-1]
    pre_shape = [-1] + pre_shape
    with tf.variable_scope(scope):
        # l2 norm
        if normalization > 0:
            x = l2norm(x, normalization)
            print(x)
        # numbers of anchors
        n_anchors = len(sizes) + len(ratios)
        # location predictions
        loc_pred = conv2d(x, n_anchors*4, 3, activation=None, scope="conv_loc")
        loc_pred = tf.reshape(loc_pred, pre_shape + [n_anchors, 4])
        # class prediction
        cls_pred = conv2d(x, n_anchors*num_classes, 3, activation=None, scope="conv_cls")
        cls_pred = tf.reshape(cls_pred, pre_shape + [n_anchors, num_classes])
        return cls_pred, loc_pred