如何利用OpenCV和Python在视频流中逐帧处理视频图像

Question

下面是我的代码：

import cv2
cap = cv2.VideoCapture("./out.mp4")

while True:
    if cap.grab():
        flag, frame = cap.retrieve()
        if not flag:
            continue
        else:
            cv2.imshow('video', frame)
    if cv2.waitKey(10) == 27:
        break

所以问题是cap.grab()调用！当没有框架时，它将返回false！而且它不会再捕捉帧了，即使我等了很长时间。

Answer 1

在阅读了videocapture的文档之后。我发现您可以告诉videocapture，下次调用videocapture.read()（或videocapture.grab())时要处理哪个帧。

问题是，当您要读取()一个尚未准备好的帧时，videocapture对象会停留在该帧上，永远不会继续。所以你必须强迫它从上一帧重新开始。

这是代码

import cv2

cap = cv2.VideoCapture("./out.mp4")
while not cap.isOpened():
    cap = cv2.VideoCapture("./out.mp4")
    cv2.waitKey(1000)
    print "Wait for the header"

pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
while True:
    flag, frame = cap.read()
    if flag:
        # The frame is ready and already captured
        cv2.imshow('video', frame)
        pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
        print str(pos_frame)+" frames"
    else:
        # The next frame is not ready, so we try to read it again
        cap.set(cv2.cv.CV_CAP_PROP_POS_FRAMES, pos_frame-1)
        print "frame is not ready"
        # It is better to wait for a while for the next frame to be ready
        cv2.waitKey(1000)

    if cv2.waitKey(10) == 27:
        break
    if cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES) == cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT):
        # If the number of captured frames is equal to the total number of frames,
        # we stop
        break