yolo_v3.py

# -*- coding: utf-8 -*-

import numpy as np
import tensorflow as tf

slim = tf.contrib.slim

_BATCH_NORM_DECAY = 0.9
_BATCH_NORM_EPSILON = 1e-05
_LEAKY_RELU = 0.1

_ANCHORS = [(10, 13), (16, 30), (33, 23),
            (30, 61), (62, 45), (59, 119),
            (116, 90), (156, 198), (373, 326)]


def darknet53(inputs):
    """
    Builds Darknet-53 model.
    """
    inputs = _conv2d_fixed_padding(inputs, 32, 3)
    inputs = _conv2d_fixed_padding(inputs, 64, 3, strides=2)
    inputs = _darknet53_block(inputs, 32)
    inputs = _conv2d_fixed_padding(inputs, 128, 3, strides=2)

    for i in range(2):
        inputs = _darknet53_block(inputs, 64)

    inputs = _conv2d_fixed_padding(inputs, 256, 3, strides=2)

    for i in range(8):
        inputs = _darknet53_block(inputs, 128)

    route_1 = inputs
    inputs = _conv2d_fixed_padding(inputs, 512, 3, strides=2)

    for i in range(8):
        inputs = _darknet53_block(inputs, 256)

    route_2 = inputs
    inputs = _conv2d_fixed_padding(inputs, 1024, 3, strides=2)

    for i in range(4):
        inputs = _darknet53_block(inputs, 512)

    return route_1, route_2, inputs


def _conv2d_fixed_padding(inputs, filters, kernel_size, strides=1):
    if strides > 1:
        inputs = _fixed_padding(inputs, kernel_size)
    inputs = slim.conv2d(inputs, filters, kernel_size, stride=strides,
                         padding=('SAME' if strides == 1 else 'VALID'))
    return inputs


def _darknet53_block(inputs, filters):
    shortcut = inputs
    inputs = _conv2d_fixed_padding(inputs, filters, 1)
    inputs = _conv2d_fixed_padding(inputs, filters * 2, 3)

    inputs = inputs + shortcut
    return inputs


@tf.contrib.framework.add_arg_scope
def _fixed_padding(inputs, kernel_size, *args, mode='CONSTANT', **kwargs):
    """
    Pads the input along the spatial dimensions independently of input size.

    Args:
      inputs: A tensor of size [batch, channels, height_in, width_in] or
        [batch, height_in, width_in, channels] depending on data_format.
      kernel_size: The kernel to be used in the conv2d or max_pool2d operation.
                   Should be a positive integer.
      data_format: The input format ('NHWC' or 'NCHW').
      mode: The mode for tf.pad.

    Returns:
      A tensor with the same format as the input with the data either intact
      (if kernel_size == 1) or padded (if kernel_size > 1).
    """
    pad_total = kernel_size - 1
    pad_beg = pad_total // 2
    pad_end = pad_total - pad_beg

    if kwargs['data_format'] == 'NCHW':
        padded_inputs = tf.pad(inputs, [[0, 0], [0, 0],
                                        [pad_beg, pad_end],
                                        [pad_beg, pad_end]],
                               mode=mode)
    else:
        padded_inputs = tf.pad(inputs, [[0, 0], [pad_beg, pad_end],
                                        [pad_beg, pad_end], [0, 0]], mode=mode)
    return padded_inputs


def _yolo_block(inputs, filters):
    inputs = _conv2d_fixed_padding(inputs, filters, 1)
    inputs = _conv2d_fixed_padding(inputs, filters * 2, 3)
    inputs = _conv2d_fixed_padding(inputs, filters, 1)
    inputs = _conv2d_fixed_padding(inputs, filters * 2, 3)
    inputs = _conv2d_fixed_padding(inputs, filters, 1)
    route = inputs
    inputs = _conv2d_fixed_padding(inputs, filters * 2, 3)
    return route, inputs


def _get_size(shape, data_format):
    if len(shape) == 4:
        shape = shape[1:]
    return shape[1:3] if data_format == 'NCHW' else shape[0:2]


def _detection_layer(inputs, num_classes, anchors, img_size, data_format):
    num_anchors = len(anchors)
    predictions = slim.conv2d(inputs, num_anchors * (5 + num_classes), 1,
                              stride=1, normalizer_fn=None,
                              activation_fn=None,
                              biases_initializer=tf.zeros_initializer())

    shape = predictions.get_shape().as_list()
    grid_size = _get_size(shape, data_format)
    dim = grid_size[0] * grid_size[1]
    bbox_attrs = 5 + num_classes

    if data_format == 'NCHW':
        predictions = tf.reshape(
            predictions, [-1, num_anchors * bbox_attrs, dim])
        predictions = tf.transpose(predictions, [0, 2, 1])

    predictions = tf.reshape(predictions, [-1, num_anchors * dim, bbox_attrs])

    stride = (img_size[0] // grid_size[0], img_size[1] // grid_size[1])

    anchors = [(a[0] / stride[0], a[1] / stride[1]) for a in anchors]

    box_centers, box_sizes, confidence, classes = tf.split(
        predictions, [2, 2, 1, num_classes], axis=-1)

    box_centers = tf.nn.sigmoid(box_centers)
    confidence = tf.nn.sigmoid(confidence)

    grid_x = tf.range(grid_size[0], dtype=tf.float32)
    grid_y = tf.range(grid_size[1], dtype=tf.float32)
    a, b = tf.meshgrid(grid_x, grid_y)

    x_offset = tf.reshape(a, (-1, 1))
    y_offset = tf.reshape(b, (-1, 1))

    x_y_offset = tf.concat([x_offset, y_offset], axis=-1)
    x_y_offset = tf.reshape(tf.tile(x_y_offset, [1, num_anchors]), [1, -1, 2])

    box_centers = box_centers + x_y_offset
    box_centers = box_centers * stride

    anchors = tf.tile(anchors, [dim, 1])
    box_sizes = tf.exp(box_sizes) * anchors
    box_sizes = box_sizes * stride

    detections = tf.concat([box_centers, box_sizes, confidence], axis=-1)

    classes = tf.nn.sigmoid(classes)
    predictions = tf.concat([detections, classes], axis=-1)
    return predictions


def _upsample(inputs, out_shape, data_format='NCHW'):
    # tf.image.resize_nearest_neighbor accepts input in format NHWC
    if data_format == 'NCHW':
        inputs = tf.transpose(inputs, [0, 2, 3, 1])

    if data_format == 'NCHW':
        new_height = out_shape[3]
        new_width = out_shape[2]
    else:
        new_height = out_shape[2]
        new_width = out_shape[1]

    inputs = tf.image.resize_nearest_neighbor(inputs, (new_height, new_width))

    # back to NCHW if needed
    if data_format == 'NCHW':
        inputs = tf.transpose(inputs, [0, 3, 1, 2])

    inputs = tf.identity(inputs, name='upsampled')
    return inputs


def yolo_v3(inputs, num_classes, is_training=False, data_format='NCHW', reuse=False):
    """
    Creates YOLO v3 model.

    :param inputs: a 4-D tensor of size [batch_size, height, width, channels].
        Dimension batch_size may be undefined. The channel order is RGB.
    :param num_classes: number of predicted classes.
    :param is_training: whether is training or not.
    :param data_format: data format NCHW or NHWC.
    :param reuse: whether or not the network and its variables should be reused.
    :return:
    """
    # it will be needed later on
    img_size = inputs.get_shape().as_list()[1:3]

    # transpose the inputs to NCHW
    if data_format == 'NCHW':
        inputs = tf.transpose(inputs, [0, 3, 1, 2])

    # normalize values to range [0..1]
    inputs = inputs / 255

    # set batch norm params
    batch_norm_params = {
        'decay': _BATCH_NORM_DECAY,
        'epsilon': _BATCH_NORM_EPSILON,
        'scale': True,
        'is_training': is_training,
        'fused': None,  # Use fused batch norm if possible.
    }

    # Set activation_fn and parameters for conv2d, batch_norm.
    with slim.arg_scope([slim.conv2d, slim.batch_norm, _fixed_padding], data_format=data_format, reuse=reuse):
        with slim.arg_scope([slim.conv2d], normalizer_fn=slim.batch_norm,
                            normalizer_params=batch_norm_params,
                            biases_initializer=None,
                            activation_fn=lambda x: tf.nn.leaky_relu(x, alpha=_LEAKY_RELU)):
            with tf.variable_scope('darknet-53'):
                route_1, route_2, inputs = darknet53(inputs)

            with tf.variable_scope('yolo-v3'):
                route, inputs = _yolo_block(inputs, 512)
                detect_1 = _detection_layer(inputs, num_classes, _ANCHORS[6:9], img_size, data_format)
                detect_1 = tf.identity(detect_1, name='detect_1')
                print("detect_1.shape =", detect_1.shape)

                inputs = _conv2d_fixed_padding(route, 256, 1)
                upsample_size = route_2.get_shape().as_list()
                inputs = _upsample(inputs, upsample_size, data_format)
                inputs = tf.concat([inputs, route_2], axis=1 if data_format == 'NCHW' else 3)

                route, inputs = _yolo_block(inputs, 256)

                detect_2 = _detection_layer(inputs, num_classes, _ANCHORS[3:6], img_size, data_format)
                detect_2 = tf.identity(detect_2, name='detect_2')
                print("detect_2.shape =", detect_2.shape)

                inputs = _conv2d_fixed_padding(route, 128, 1)
                upsample_size = route_1.get_shape().as_list()
                inputs = _upsample(inputs, upsample_size, data_format)
                inputs = tf.concat([inputs, route_1], axis=1 if data_format == 'NCHW' else 3)

                _, inputs = _yolo_block(inputs, 128)

                detect_3 = _detection_layer(inputs, num_classes, _ANCHORS[0:3], img_size, data_format)
                detect_3 = tf.identity(detect_3, name='detect_3')
                print("detect_3.shape =", detect_3.shape)

                detections = tf.concat([detect_1, detect_2, detect_3], axis=1)
                #detections = tf.reshape(detections, [1, detect_1.shape[1]+detect_2.shape[1]+detect_3.shape[1], num_classes+5])
                detections = tf.identity(detections, name='detections')
                return detections