anchors.py

import numpy as np
from config import INPUT_SIZE

_default_anchors_setting = (
    dict(layer='p3', stride=32, size=48, scale=[2 ** (1. / 3.), 2 ** (2. / 3.)], aspect_ratio=[0.667, 1, 1.5]),
    dict(layer='p4', stride=64, size=96, scale=[2 ** (1. / 3.), 2 ** (2. / 3.)], aspect_ratio=[0.667, 1, 1.5]),
    dict(layer='p5', stride=128, size=192, scale=[1, 2 ** (1. / 3.), 2 ** (2. / 3.)], aspect_ratio=[0.667, 1, 1.5]),
)


def generate_default_anchor_maps(anchors_setting=None, input_shape=INPUT_SIZE):
    """
    generate default anchor

    :param anchors_setting: all informations of anchors
    :param input_shape: shape of input images, e.g. (h, w)
    :return: center_anchors: # anchors * 4 (oy, ox, h, w)
             edge_anchors: # anchors * 4 (y0, x0, y1, x1)
             anchor_area: # anchors * 1 (area)
    """
    if anchors_setting is None:
        anchors_setting = _default_anchors_setting

    center_anchors = np.zeros((0, 4), dtype=np.float32)
    edge_anchors = np.zeros((0, 4), dtype=np.float32)
    anchor_areas = np.zeros((0,), dtype=np.float32)
    input_shape = np.array(input_shape, dtype=int)

    for anchor_info in anchors_setting:

        stride = anchor_info['stride']
        size = anchor_info['size']
        scales = anchor_info['scale']
        aspect_ratios = anchor_info['aspect_ratio']

        output_map_shape = np.ceil(input_shape.astype(np.float32) / stride)
        output_map_shape = output_map_shape.astype(np.int)
        output_shape = tuple(output_map_shape) + (4,)
        ostart = stride / 2.
        oy = np.arange(ostart, ostart + stride * output_shape[0], stride)
        oy = oy.reshape(output_shape[0], 1)
        ox = np.arange(ostart, ostart + stride * output_shape[1], stride)
        ox = ox.reshape(1, output_shape[1])
        center_anchor_map_template = np.zeros(output_shape, dtype=np.float32)
        center_anchor_map_template[:, :, 0] = oy
        center_anchor_map_template[:, :, 1] = ox
        for scale in scales:
            for aspect_ratio in aspect_ratios:
                center_anchor_map = center_anchor_map_template.copy()
                center_anchor_map[:, :, 2] = size * scale / float(aspect_ratio) ** 0.5
                center_anchor_map[:, :, 3] = size * scale * float(aspect_ratio) ** 0.5

                edge_anchor_map = np.concatenate((center_anchor_map[..., :2] - center_anchor_map[..., 2:4] / 2.,
                                                  center_anchor_map[..., :2] + center_anchor_map[..., 2:4] / 2.),
                                                 axis=-1)
                anchor_area_map = center_anchor_map[..., 2] * center_anchor_map[..., 3]
                center_anchors = np.concatenate((center_anchors, center_anchor_map.reshape(-1, 4)))
                edge_anchors = np.concatenate((edge_anchors, edge_anchor_map.reshape(-1, 4)))
                anchor_areas = np.concatenate((anchor_areas, anchor_area_map.reshape(-1)))

    return edge_anchors