box_utils.py

import numpy as np
from PIL import Image


def nms(boxes, overlap_threshold = 0.5, mode = 'union'):
    """Non-maximum suppression.

    Arguments:
        boxes: a float numpy array of shape [n, 5],
            where each row is (xmin, ymin, xmax, ymax, score).
        overlap_threshold: a float number.
        mode: 'union' or 'min'.

    Returns:
        list with indices of the selected boxes
    """

    # if there are no boxes, return the empty list
    if len(boxes) == 0:
        return []

    # list of picked indices
    pick = []

    # grab the coordinates of the bounding boxes
    x1, y1, x2, y2, score = [boxes[:, i] for i in range(5)]

    area = (x2 - x1 + 1.0)*(y2 - y1 + 1.0)
    ids = np.argsort(score)  # in increasing order

    while len(ids) > 0:

        # grab index of the largest value
        last = len(ids) - 1
        i = ids[last]
        pick.append(i)

        # compute intersections
        # of the box with the largest score
        # with the rest of boxes

        # left top corner of intersection boxes
        ix1 = np.maximum(x1[i], x1[ids[:last]])
        iy1 = np.maximum(y1[i], y1[ids[:last]])

        # right bottom corner of intersection boxes
        ix2 = np.minimum(x2[i], x2[ids[:last]])
        iy2 = np.minimum(y2[i], y2[ids[:last]])

        # width and height of intersection boxes
        w = np.maximum(0.0, ix2 - ix1 + 1.0)
        h = np.maximum(0.0, iy2 - iy1 + 1.0)

        # intersections' areas
        inter = w * h
        if mode == 'min':
            overlap = inter/np.minimum(area[i], area[ids[:last]])
        elif mode == 'union':
            # intersection over union (IoU)
            overlap = inter/(area[i] + area[ids[:last]] - inter)

        # delete all boxes where overlap is too big
        ids = np.delete(
            ids,
            np.concatenate([[last], np.where(overlap > overlap_threshold)[0]])
        )

    return pick


def convert_to_square(bboxes):
    """Convert bounding boxes to a square form.

    Arguments:
        bboxes: a float numpy array of shape [n, 5].

    Returns:
        a float numpy array of shape [n, 5],
            squared bounding boxes.
    """

    square_bboxes = np.zeros_like(bboxes)
    x1, y1, x2, y2 = [bboxes[:, i] for i in range(4)]
    h = y2 - y1 + 1.0
    w = x2 - x1 + 1.0
    max_side = np.maximum(h, w)
    square_bboxes[:, 0] = x1 + w*0.5 - max_side*0.5
    square_bboxes[:, 1] = y1 + h*0.5 - max_side*0.5
    square_bboxes[:, 2] = square_bboxes[:, 0] + max_side - 1.0
    square_bboxes[:, 3] = square_bboxes[:, 1] + max_side - 1.0
    return square_bboxes


def calibrate_box(bboxes, offsets):
    """Transform bounding boxes to be more like true bounding boxes.
    'offsets' is one of the outputs of the nets.

    Arguments:
        bboxes: a float numpy array of shape [n, 5].
        offsets: a float numpy array of shape [n, 4].

    Returns:
        a float numpy array of shape [n, 5].
    """
    x1, y1, x2, y2 = [bboxes[:, i] for i in range(4)]
    w = x2 - x1 + 1.0
    h = y2 - y1 + 1.0
    w = np.expand_dims(w, 1)
    h = np.expand_dims(h, 1)

    # this is what happening here:
    # tx1, ty1, tx2, ty2 = [offsets[:, i] for i in range(4)]
    # x1_true = x1 + tx1*w
    # y1_true = y1 + ty1*h
    # x2_true = x2 + tx2*w
    # y2_true = y2 + ty2*h
    # below is just more compact form of this

    # are offsets always such that
    # x1 < x2 and y1 < y2 ?

    translation = np.hstack([w, h, w, h])*offsets
    bboxes[:, 0:4] = bboxes[:, 0:4] + translation
    return bboxes


def get_image_boxes(bounding_boxes, img, size = 24):
    """Cut out boxes from the image.

    Arguments:
        bounding_boxes: a float numpy array of shape [n, 5].
        img: an instance of PIL.Image.
        size: an integer, size of cutouts.

    Returns:
        a float numpy array of shape [n, 3, size, size].
    """

    num_boxes = len(bounding_boxes)
    width, height = img.size

    [dy, edy, dx, edx, y, ey, x, ex, w, h] = correct_bboxes(bounding_boxes, width, height)
    img_boxes = np.zeros((num_boxes, 3, size, size), 'float32')

    for i in range(num_boxes):
        img_box = np.zeros((h[i], w[i], 3), 'uint8')

        img_array = np.asarray(img, 'uint8')
        img_box[dy[i]:(edy[i] + 1), dx[i]:(edx[i] + 1), :] =\
            img_array[y[i]:(ey[i] + 1), x[i]:(ex[i] + 1), :]

        # resize
        img_box = Image.fromarray(img_box)
        img_box = img_box.resize((size, size), Image.BILINEAR)
        img_box = np.asarray(img_box, 'float32')

        img_boxes[i, :, :, :] = _preprocess(img_box)

    return img_boxes


def correct_bboxes(bboxes, width, height):
    """Crop boxes that are too big and get coordinates
    with respect to cutouts.

    Arguments:
        bboxes: a float numpy array of shape [n, 5],
            where each row is (xmin, ymin, xmax, ymax, score).
        width: a float number.
        height: a float number.

    Returns:
        dy, dx, edy, edx: a int numpy arrays of shape [n],
            coordinates of the boxes with respect to the cutouts.
        y, x, ey, ex: a int numpy arrays of shape [n],
            corrected ymin, xmin, ymax, xmax.
        h, w: a int numpy arrays of shape [n],
            just heights and widths of boxes.

        in the following order:
            [dy, edy, dx, edx, y, ey, x, ex, w, h].
    """

    x1, y1, x2, y2 = [bboxes[:, i] for i in range(4)]
    w, h = x2 - x1 + 1.0,  y2 - y1 + 1.0
    num_boxes = bboxes.shape[0]

    # 'e' stands for end
    # (x, y) -> (ex, ey)
    x, y, ex, ey = x1, y1, x2, y2

    # we need to cut out a box from the image.
    # (x, y, ex, ey) are corrected coordinates of the box
    # in the image.
    # (dx, dy, edx, edy) are coordinates of the box in the cutout
    # from the image.
    dx, dy = np.zeros((num_boxes,)), np.zeros((num_boxes,))
    edx, edy = w.copy() - 1.0, h.copy() - 1.0

    # if box's bottom right corner is too far right
    ind = np.where(ex > width - 1.0)[0]
    edx[ind] = w[ind] + width - 2.0 - ex[ind]
    ex[ind] = width - 1.0

    # if box's bottom right corner is too low
    ind = np.where(ey > height - 1.0)[0]
    edy[ind] = h[ind] + height - 2.0 - ey[ind]
    ey[ind] = height - 1.0

    # if box's top left corner is too far left
    ind = np.where(x < 0.0)[0]
    dx[ind] = 0.0 - x[ind]
    x[ind] = 0.0

    # if box's top left corner is too high
    ind = np.where(y < 0.0)[0]
    dy[ind] = 0.0 - y[ind]
    y[ind] = 0.0

    return_list = [dy, edy, dx, edx, y, ey, x, ex, w, h]
    return_list = [i.astype('int32') for i in return_list]

    return return_list


def _preprocess(img):
    """Preprocessing step before feeding the network.

    Arguments:
        img: a float numpy array of shape [h, w, c].

    Returns:
        a float numpy array of shape [1, c, h, w].
    """
    img = img.transpose((2, 0, 1))
    img = np.expand_dims(img, 0)
    img = (img - 127.5) * 0.0078125
    return img