anchors.py

import numpy as np
import torch
import torch.nn as nn


class Anchors(nn.Module):
    def __init__(self, pyramid_levels=None, strides=None, sizes=None, ratios=None, scales=None):
        super(Anchors, self).__init__()

        if pyramid_levels is None:
            self.pyramid_levels = [3, 4, 5, 6, 7]
        if strides is None:
            self.strides = [2 ** x for x in self.pyramid_levels]
        if ratios is None:
            self.ratios = np.array([2.90, ])
        if scales is None:
            self.scales = [np.array([38.,]), np.array([86.,]), np.array([112.,]), np.array([156.,]), np.array([328.,])]
    def forward(self, image_shape):
        
        image_shape = np.array(image_shape)
        image_shapes = [(image_shape + 2 ** x - 1) // (2 ** x) for x in self.pyramid_levels]

        # compute anchors over all pyramid levels
        all_anchors = np.zeros((0, 4)).astype(np.float32)

        for idx, p in enumerate(self.pyramid_levels):
            anchors         = generate_anchors(ratios=self.ratios, scales=self.scales[idx])
            shifted_anchors = shift(image_shapes[idx], self.strides[idx], anchors)
            all_anchors     = np.append(all_anchors, shifted_anchors, axis=0)

        all_anchors = np.expand_dims(all_anchors, axis=0)

        return torch.from_numpy(all_anchors.astype(np.float32)).cuda()

def generate_anchors(ratios=None, scales=None):
    """
    Generate anchor (reference) windows by enumerating aspect ratios X
    scales w.r.t. a reference window.
    """

    if ratios is None:
        ratios = np.array([0.5, 1, 2])

    if scales is None:
        scales = np.array([2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])

    num_anchors = len(ratios) * len(scales)

    # initialize output anchors
    anchors = np.zeros((num_anchors, 4))

    anchors[:, 2:] = np.tile(scales, (2, len(ratios))).T

    # compute areas of anchors
    areas = anchors[:, 2] * anchors[:, 3]

    # correct for ratios
    anchors[:, 2] = np.sqrt(areas / np.repeat(ratios, len(scales)))
    anchors[:, 3] = anchors[:, 2] * np.repeat(ratios, len(scales))

    # transform from (x_ctr, y_ctr, w, h) -> (x1, y1, x2, y2)
    anchors[:, 0::2] -= np.tile(anchors[:, 2] * 0.5, (2, 1)).T
    anchors[:, 1::2] -= np.tile(anchors[:, 3] * 0.5, (2, 1)).T

    return anchors

def shift(shape, stride, anchors):
    shift_x = (np.arange(0, shape[1]) + 0.5) * stride
    shift_y = (np.arange(0, shape[0]) + 0.5) * stride

    shift_x, shift_y = np.meshgrid(shift_x, shift_y)

    shifts = np.vstack((
        shift_x.ravel(), shift_y.ravel(),
        shift_x.ravel(), shift_y.ravel()
    )).transpose()

    # add A anchors (1, A, 4) to
    # cell K shifts (K, 1, 4) to get
    # shift anchors (K, A, 4)
    # reshape to (K*A, 4) shifted anchors
    A = anchors.shape[0]
    K = shifts.shape[0]
    all_anchors = (anchors.reshape((1, A, 4)) + shifts.reshape((1, K, 4)).transpose((1, 0, 2)))
    all_anchors = all_anchors.reshape((K * A, 4))

    return all_anchors