在opencv中使用倒带

Question

在opencv中按指定键时是否可以倒带？我有下面的视频，但它没有回放

def run(self, filepath, fps, width, height, monochrome=False):

    video_file = self.read_file(filepath)
    previous_frame = None


    while (video_file.isOpened()):
        ret, frame = video_file.read()

        if previous_frame is not None:
            pass

        previous_frame = frame

        if ret:

            frame = self.color(frame, monochrome)

            final_frame = self.resolution(frame, width, height)

            delaytime = self.frame_per_second(fps)


            cv2.imshow('frame', final_frame)
            key = cv2.waitKey(delaytime)

            if key & 0xFF == ord("p"):
                cv2.waitKey(234320)
                if key & 0xFF == ord("r"):
                    cv2.set(cv2.CV_CAP_PROP_POS_FRAMES(2, previous_frame))


    video_file.release()
    cv2.destroyAllWindows()

上面的代码将它带到上一帧，但不播放

Answer 1

理想的解决方案是设置暂停视频时需要播放的帧数，以实现回放效果。这可以通过video capture properties完成：

cv2.VideoCapture.set(propId, value)

CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.

有用的示例可在“Jumping between frames in video files”中找到

这是一个源代码示例，它暂停视频，并提供一些控制，让您回放到上一帧或跳转到零帧，然后重新启动视频：

import cv2
import sys

# load input video
cap = cv2.VideoCapture('TheMandalorian.mkv')
if (cap.isOpened() == False):
    print("!!! Failed cap.isOpened()")
    sys.exit(-1)

# retrieve the total number of frames
frame_count = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))

# loop to read every frame of the video
while (cap.isOpened()):

    # capture a frame
    ret, frame = cap.read()
    if ret == False:
        print("!!! Failed cap.read()")
        break

    cv2.imshow('video', frame)

    # check if 'p' was pressed and wait for a 'b' press
    key = cv2.waitKey(int(frame_count/1000))
    if (key & 0xFF == ord('p')):

        # sleep here until a valid key is pressed
        while (True):
            key = cv2.waitKey(0)

            # check if 'p' is pressed and resume playing
            if (key & 0xFF == ord('p')):
                break

            # check if 'b' is pressed and rewind video to the previous frame, but do not play
            if (key & 0xFF == ord('b')):
                cur_frame_number = cap.get(cv2.CAP_PROP_POS_FRAMES)
                print('* At frame #' + str(cur_frame_number))

                prev_frame = cur_frame_number
                if (cur_frame_number > 1):
                    prev_frame -= 1

                print('* Rewind to frame #' + str(prev_frame))
                cap.set(cv2.CAP_PROP_POS_FRAMES, prev_frame)

            # check if 'r' is pressed and rewind video to frame 0, then resume playing
            if (key & 0xFF == ord('r')):
                cap.set(cv2.CAP_PROP_POS_FRAMES, 0)
                break

    # exit when 'q' is pressed to quit
    elif (key & 0xFF == ord('q')):
        break

# release resources
cap.release()
cv2.destroyAllWindows()

按键选项：

• 按q退出应用程序
• 按p暂停
• 暂停后，再次按p继续播放
• 暂停时，按b回放单个帧。您必须按p重新开始播放
• 暂停时，按r将回放到第0帧并自动恢复播放

Answer 2

如果只需要上一帧，可以存储在临时数组中。 例如：

prev_frame = curr_image.copy()