OpenCV：如何仅在视频文件的第一帧上绘制，然后继续显示整个视频

Question

我在 python 中使用 opencv 库。我打开了一个现有的视频文件并编写了一个小脚本，允许我在视频的任何位置绘制一个矩形。问题是：我想在视频的第一帧上绘制这个矩形，然后把它留在那里为我标记一个感兴趣的区域。

我正在使用cv2.imshow(winname, frame) 来展示我的视频。由于它以每秒非常高的帧速率运行/显示视频（我不想改变它，因为我的视频很长），当我开始绘制矩形时，已经显示了许多帧。

因为我认为这可能会有所帮助，所以这是我目前的代码： 导入cv2

#mouse callback function#
def draw_rectangle(event, x, y, flags, param):

    global pt1, pt2, topLeft_clicked, bottomRight_clicked

    #mouse click
    if event == cv2.EVENT_LBUTTONDOWN:
        #reset
        if topLeft_clicked and bottomRight_clicked:
            topLeft_clicked = False
            bottomRight_clicked = False
            pt1 = (0,0)
            pt2 = (0,0)
        #get coordinates of top left corner
        if not topLeft_clicked:
            pt1 = (x,y)
            topLeft_clicked = True
        #get coordinates of bottom right corner
        elif not bottomRight_clicked:
            pt2 = (x,y)
            bottomRight_clicked = True

#start actual program 

#initially we haven't drawn anything
pt1 = (0,0)
pt2 = (0,0)
topLeft_clicked = False
bottomRight_clicked = False

#capture video
cap = cv2.VideoCapture('Path to video')
cv2.namedWindow(winname='myName')
cv2.setMouseCallback('myName', draw_rectangle)

firstFrame = True
while True: 
    ret, frame = cap.read()
    gray_frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    while ret and firstFrame: 
        cv2.imshow('myName', gray_frame)

        if topLeft_clicked: 
            cv2.circle(gray_frame, center=pt1, radius=5, color=(255,0,0), thickness=-1)

        if topLeft_clicked and bottomRight_clicked: 
            cv2.rectangle(gray_frame, pt1, pt2, (255,0,0), 2)
            firstFrame = False

    cv2.imshow('Estimate_Velocity', gray_frame)

    if cv2.waitKey(1) &0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Answer 1

我做了一点实验，找到了以下解决我的问题的方法：

import cv2

#mouse callback function#
def draw_rectangle(event, x, y, flags, param):

    global pt1, pt2, topLeft_clicked, bottomRight_clicked

    #mouse click
    if event == cv2.EVENT_LBUTTONDOWN:
        #reset
        if topLeft_clicked and bottomRight_clicked:
            topLeft_clicked = False
            bottomRight_clicked = False
            pt1 = (0,0)
            pt2 = (0,0)
        #get coordinates of top left corner
        if not topLeft_clicked:
            pt1 = (x,y)
            topLeft_clicked = True
        #get coordinates of bottom right corner
        elif not bottomRight_clicked:
            pt2 = (x,y)
            bottomRight_clicked = True

#start actual program 

#initially we haven't drawn anything
pt1 = (0,0)
pt2 = (0,0)
topLeft_clicked = False
bottomRight_clicked = False

#capture video
cap = cv2.VideoCapture('Video path')
cv2.namedWindow(winname='myName')
cv2.setMouseCallback('myName', draw_rectangle)

firstFrame = True
while True: 
    ret, frame = cap.read()
    gray_frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    while firstFrame: 
        cv2.imshow('myName', gray_frame)

        if topLeft_clicked: 
            cv2.circle(gray_frame, center=pt1, radius=5, color=(255,0,0), thickness=-1)

        if topLeft_clicked and bottomRight_clicked: 
            cv2.rectangle(gray_frame, pt1, pt2, (255,0,0), 2)

        if cv2.waitKey(1) &0xFF == ord('c'):
            firstFrame = False
            break

    if topLeft_clicked and bottomRight_clicked: 
        cv2.rectangle(gray_frame, pt1, pt2, (255,0,0), 2)

    cv2.imshow('myName', gray_frame)

    if cv2.waitKey(1) &0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Answer 2

我还没有测试过这个，但是你可以修改你的while循环中断条件：

if cv2.waitKey(1) &0xFF == ord('q'):
          break

到

if (topLeft_clicked and bottomRight_clicked):
    if cv2.waitKey(1)  &0xFF == ord('q'):
        break
else:
     # change 1 to 0
     cv2.waitKey(0)

你需要在画完方框后按一个键。