如何使矩形透明？

Question

so i'm using opencv and i want to make a sort of selection tool but the problem is can't make the rectangle transparent. here's the code:

import numpy as np
import cv2 as cv
drawing = False

def draw_rec(event,x,y,flags,param):
    global ix,iy,drawing
    if event == cv.EVENT_LBUTTONDOWN:
        drawing = True
        ix,iy = x,y
    elif event == cv.EVENT_LBUTTONUP:
        drawing = False
        cv.rectangle(img,(ix,iy),(x,y),(0,0,0),-1)
    elif event == cv.EVENT_MOUSEMOVE:
        if drawing == True:
            cv.rectangle(img, (ix, iy), (x, y), (0, 255, 0), 5)
img = cv.imread('baboon.jpg', -1)
cv.namedWindow('image')
cv.setMouseCallback('image',draw_rec)
while(1):
    cv.imshow('image',img)
    k = cv.waitKey(1) & 0xFF
    if k == 27:
        break
cv.destroyAllWindows()

Answer 1

The first mistake in the code is:

elif event == cv.EVENT_LBUTTONUP:
        drawing = False
        cv.rectangle(img,(ix,iy),(x,y),(0,0,0),-1)

The -1 parameter means to fill the rectangle. source If we change -1 to 1:

From my point of view, the result is not satisfactory. The multiple rectangle display is caused by the mouse_movement .

elif event == cv.EVENT_MOUSEMOVE:
        if drawing == True:
            cv.rectangle(img, (ix, iy), (x, y), (0, 255, 0), 5)

Each time the mouse moves, the rectangle will be drawn. I think it is better if we draw when the mouse movement finishes:

Code:

---

import numpy as np
import cv2 as cv
drawing = False

def draw_rec(event,x,y,flags,param):
    global ix,iy,drawing
    if event == cv.EVENT_LBUTTONDOWN:
        drawing = True
        ix,iy = x,y
    elif event == cv.EVENT_LBUTTONUP:
        drawing = False
        cv.rectangle(img,(ix,iy),(x,y),(0,255,0),5)


img = cv.imread('27BR1.jpg', -1)
cv.namedWindow('image')
cv.setMouseCallback('image',draw_rec)
while(1):
    cv.imshow('image',img)
    k = cv.waitKey(1) & 0xFF
    if k == 27:
        break
cv.destroyAllWindows()