utils.py

import numpy as np


def nms_fast(boxes, overlapThresh = 0.5):
    # if there are no boxes, return an empty list
    if len(boxes) == 0:
        return []
    boxes = np.array(boxes)
    # if the bounding boxes integers, convert them to floats --
    # this is important since we'll be doing a bunch of divisions
    if boxes.dtype.kind == "i":
        boxes = boxes.astype("float")

    # initialize the list of picked indexes
    pick = []

    # grab the coordinates of the bounding boxes
    x1 = boxes[:, 0]
    y1 = boxes[:, 1]
    x2 = boxes[:, 2]
    y2 = boxes[:, 3]
    area = np.multiply(x2-x1, y2-y1)

    # compute the area of the bounding boxes and sort the bounding
    # boxes by the bottom-right y-coordinate of the bounding box
    area = (x2 - x1 + 1) * (y2 - y1 + 1)
    idxs = np.argsort(area)

    # keep looping while some indexes still remain in the indexes
    # list
    while len(idxs) > 0:
        # grab the last index in the indexes list and add the
        # index value to the list of picked indexes
        last = len(idxs) - 1
        i = idxs[last]
        pick.append(i)

        # find the largest (x, y) coordinates for the start of
        # the bounding box and the smallest (x, y) coordinates
        # for the end of the bounding box
        xx1 = np.maximum(x1[i], x1[idxs[:last]])
        yy1 = np.maximum(y1[i], y1[idxs[:last]])
        xx2 = np.minimum(x2[i], x2[idxs[:last]])
        yy2 = np.minimum(y2[i], y2[idxs[:last]])

        # compute the width and height of the bounding box
        w = np.maximum(0, xx2 - xx1 + 1)
        h = np.maximum(0, yy2 - yy1 + 1)

        # compute the ratio of overlap
        overlap = (w * h) / area[idxs[:last]]

        # delete all indexes from the index list that have
        idxs = np.delete(idxs, np.concatenate(([last],
                                               np.where(overlap > overlapThresh)[0])))

    # return only the bounding boxes that were picked using the
    # integer data type
    return boxes[pick]

def format_boundingbox(boxes, width, height):
	# Convert from topleft bottomright to center, width, height
	w = boxes[:, 2] - boxes[:, 0]
	h = boxes[:, 3] - boxes[:, 1]
	x = (boxes[:, 0] + w/2)/width
	y = (boxes[:, 1] + h/2)/height
	w = w/width
	h = h/height
	x = x.reshape((len(x), 1))
	y = y.reshape((len(y), 1))
	w = w.reshape((len(w), 1))
	h = h.reshape((len(h), 1))
	boxes = np.concatenate([x, y, w, h], axis = 1)
	return boxes