"""Operations for [N, 4] numpy arrays representing bounding boxes.
Example box operations that are supported:
  * Areas: compute bounding box areas
  * IOU: pairwise intersection-over-union scores
"""
import numpy as np
def area(boxes):
	"""Computes area of boxes.
  Args:
    boxes: Numpy array with shape [N, 4] holding N boxes
  Returns:
    a numpy array with shape [N*1] representing box areas
  """
	return (boxes[:, 2] - boxes[:, 0]) * (boxes[:, 3] - boxes[:, 1])
def intersection(boxes1, boxes2):
	"""Compute pairwise intersection areas between boxes.
  Args:
    boxes1: a numpy array with shape [N, 4] holding N boxes
    boxes2: a numpy array with shape [M, 4] holding M boxes
  Returns:
    a numpy array with shape [N*M] representing pairwise intersection area
  """
	[y_min1, x_min1, y_max1, x_max1] = np.split(boxes1, 4, axis=1)
	[y_min2, x_min2, y_max2, x_max2] = np.split(boxes2, 4, axis=1)
	all_pairs_min_ymax = np.minimum(y_max1, np.transpose(y_max2))
	all_pairs_max_ymin = np.maximum(y_min1, np.transpose(y_min2))
	intersect_heights = np.maximum(np.zeros(all_pairs_max_ymin.shape), all_pairs_min_ymax - all_pairs_max_ymin)
	all_pairs_min_xmax = np.minimum(x_max1, np.transpose(x_max2))
	all_pairs_max_xmin = np.maximum(x_min1, np.transpose(x_min2))
	intersect_widths = np.maximum(np.zeros(all_pairs_max_xmin.shape), all_pairs_min_xmax - all_pairs_max_xmin)
	return intersect_heights * intersect_widths
def iou(boxes1, boxes2):
	"""Computes pairwise intersection-over-union between box collections.
  Args:
    boxes1: a numpy array with shape [N, 4] holding N boxes.
    boxes2: a numpy array with shape [M, 4] holding N boxes.
  Returns:
    a numpy array with shape [N, M] representing pairwise iou scores.
  """
	intersect = intersection(boxes1, boxes2)
	area1 = area(boxes1)
	area2 = area(boxes2)
	union = np.expand_dims(area1, axis=1) + np.expand_dims(area2, axis=0) - intersect
	return intersect / union
def ioa(boxes1, boxes2):
	"""Computes pairwise intersection-over-area between box collections.
  Intersection-over-area (ioa) between two boxes box1 and box2 is defined as
  their intersection area over box2's area. Note that ioa is not symmetric,
  that is, IOA(box1, box2) != IOA(box2, box1).
  Args:
    boxes1: a numpy array with shape [N, 4] holding N boxes.
    boxes2: a numpy array with shape [M, 4] holding N boxes.
  Returns:
    a numpy array with shape [N, M] representing pairwise ioa scores.
  """
	intersect = intersection(boxes1, boxes2)
	areas = np.expand_dims(area(boxes2), axis=0)
	return intersect / areas