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anchors.py
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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 = [2, 3, 4, 5, 6, 7]
if strides is None:
self.strides = [2 ** x for x in self.pyramid_levels]
if sizes is None:
self.sizes = [2 ** (x + 2) for x in self.pyramid_levels]
# self.sizes = [2 ** x for x in self.pyramid_levels]
if ratios is None:
# self.ratios = np.array([0.5, 1, 2])
self.ratios = np.array([1.0, 1.5, 2.0, 2.5, 3.0])
if scales is None:
self.scales = np.array([2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])
# self.scales = np.array([2 ** (-2), 2 ** (-1), 2 ** 0, 2 ** (1.0 / 3.0), 2 ** (2.0 / 3.0)])
def forward(self, image):
image_shape = image.shape[2:]
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(base_size=self.sizes[idx], ratios=self.ratios, scales=self.scales)
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(base_size=16, 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))
# scale base_size
anchors[:, 2:] = base_size * 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 compute_shape(image_shape, pyramid_levels):
"""Compute shapes based on pyramid levels.
:param image_shape:
:param pyramid_levels:
:return:
"""
image_shape = np.array(image_shape[:2])
image_shapes = [(image_shape + 2 ** x - 1) // (2 ** x) for x in pyramid_levels]
return image_shapes
def anchors_for_shape(
image_shape,
pyramid_levels=None,
ratios=None,
scales=None,
strides=None,
sizes=None,
shapes_callback=None,
):
image_shapes = compute_shape(image_shape, pyramid_levels)
# compute anchors over all pyramid levels
all_anchors = np.zeros((0, 4))
for idx, p in enumerate(pyramid_levels):
anchors = generate_anchors(base_size=sizes[idx], ratios=ratios, scales=scales)
shifted_anchors = shift(image_shapes[idx], strides[idx], anchors)
all_anchors = np.append(all_anchors, shifted_anchors, axis=0)
return all_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