rectangle(...)
    rectangle(img, pt1, pt2, color[, thickness[, lineType[, shift]]]) -> img
    .   @brief Draws a simple, thick, or filled up-right rectangle.
    .   
    .   The function cv::rectangle draws a rectangle outline or a filled rectangle whose two opposite corners
    .   are pt1 and pt2.
    .   
    .   @param img Image.
    .   @param pt1 Vertex of the rectangle.
    .   @param pt2 Vertex of the rectangle opposite to pt1 .
    .   @param color Rectangle color or brightness (grayscale image).
    .   @param thickness Thickness of lines that make up the rectangle. Negative values, like #FILLED,
    .   mean that the function has to draw a filled rectangle.
    .   @param lineType Type of the line. See #LineTypes
    .   @param shift Number of fractional bits in the point coordinates.
    
    
    
    rectangle(img, rec, color[, thickness[, lineType[, shift]]]) -> img
    .   @overload
    .   
    .   use `rec` parameter as alternative specification of the drawn rectangle: `r.tl() and
    .   r.br()-Point(1,1)` are opposite corners

# -*- coding: utf-8 -*-

import cv2
import numpy as np

if __name__ == '__main__':
    image = np.zeros((256, 256, 3), np.uint8)
    color = (0, 255, 0)
    cv2.rectangle(image, (20, 20), (60, 60), (0, 255, 0))
    cv2.rectangle(image, (120, 120), (80, 80), (255, 0, 0), thickness=-1)
    cv2.rectangle(image, (140, 200), (200, 140), (0, 0, 255), thickness=5)
    cv2.namedWindow('rect', 1)
    cv2.imshow('rect', image)
    cv2.waitKey(0)
    cv2.destroyAllWindows()

# -*- coding: utf-8 -*-

import copy
import cv2
import numpy as np

WIN_NAME = 'draw_rect'


class Rect(object):
    def __init__(self):
        self.tl = (0, 0)
        self.br = (0, 0)

    def regularize(self):
        """
        make sure tl = TopLeft point, br = BottomRight point
        """
        pt1 = (min(self.tl[0], self.br[0]), min(self.tl[1], self.br[1]))
        pt2 = (max(self.tl[0], self.br[0]), max(self.tl[1], self.br[1]))
        self.tl = pt1
        self.br = pt2


class DrawRects(object):
    def __init__(self, image, color, thickness=1):
        self.original_image = image
        self.image_for_show = image.copy()
        self.color = color
        self.thickness = thickness
        self.rects = []
        self.current_rect = Rect()
        self.left_button_down = False

    @staticmethod
    def __clip(value, low, high):
        """
        clip value between low and high

        Parameters
        ----------
        value: a number
            value to be clipped
        low: a number
            low limit
        high: a number
            high limit

        Returns
        -------
        output: a number
            clipped value
        """
        output = max(value, low)
        output = min(output, high)
        return output

    def shrink_point(self, x, y):
        """
        shrink point (x, y) to inside image_for_show

        Parameters
        ----------
        x, y: int, int
            coordinate of a point

        Returns
        -------
        x_shrink, y_shrink: int, int
            shrinked coordinate
        """
        height, width = self.image_for_show.shape[0:2]
        x_shrink = self.__clip(x, 0, width)
        y_shrink = self.__clip(y, 0, height)
        return (x_shrink, y_shrink)

    def append(self):
        """
        add a rect to rects list
        """
        self.rects.append(copy.deepcopy(self.current_rect))

    def pop(self):
        """
        pop a rect from rects list
        """
        rect = Rect()
        if self.rects:
            rect = self.rects.pop()
        return rect

    def reset_image(self):
        """
        reset image_for_show using original image
        """
        self.image_for_show = self.original_image.copy()

    def draw(self):
        """
        draw rects on image_for_show
        """
        for rect in self.rects:
            cv2.rectangle(self.image_for_show, rect.tl, rect.br,
                          color=self.color, thickness=self.thickness)

    def draw_current_rect(self):
        """
        draw current rect on image_for_show
        """
        cv2.rectangle(self.image_for_show,
                      self.current_rect.tl, self.current_rect.br,
                      color=self.color, thickness=self.thickness)


def onmouse_draw_rect(event, x, y, flags, draw_rects):
    if event == cv2.EVENT_LBUTTONDOWN:
        # pick first point of rect
        print('pt1: x = %d, y = %d' % (x, y))
        draw_rects.left_button_down = True
        draw_rects.current_rect.tl = (x, y)
    if draw_rects.left_button_down and event == cv2.EVENT_MOUSEMOVE:
        # pick second point of rect and draw current rect
        draw_rects.current_rect.br = draw_rects.shrink_point(x, y)
        draw_rects.reset_image()
        draw_rects.draw()
        draw_rects.draw_current_rect()
    if event == cv2.EVENT_LBUTTONUP:
        # finish drawing current rect and append it to rects list
        draw_rects.left_button_down = False
        draw_rects.current_rect.br = draw_rects.shrink_point(x, y)
        print('pt2: x = %d, y = %d' % (draw_rects.current_rect.br[0],
                                       draw_rects.current_rect.br[1]))
        draw_rects.current_rect.regularize()
        draw_rects.append()
    if (not draw_rects.left_button_down) and event == cv2.EVENT_RBUTTONDOWN:
        # pop the last rect in rects list
        draw_rects.pop()
        draw_rects.reset_image()
        draw_rects.draw()


if __name__ == '__main__':
    image = np.zeros((256, 256, 3), np.uint8)
    draw_rects = DrawRects(image, (0, 255, 0), 2)
    cv2.namedWindow(WIN_NAME, 0)
    cv2.setMouseCallback(WIN_NAME, onmouse_draw_rect, draw_rects)
    while True:
        cv2.imshow(WIN_NAME, draw_rects.image_for_show)
        key = cv2.waitKey(30)
        if key == 27:  # ESC
            break
    cv2.destroyAllWindows()