OpenCV 使用 cap.set(cv2.CAP_PROP_POS_FRAMES

Question

I'm using the following code to create a simple video player but I've seen that when I introduce the cap.set(cv2.CAP_PROP_POS_FRAMES,arg) line, the all process is getting very slow while playing the video. The player works correctly with its trackbar but the speed is very slow. In general I noted that every time you use the cap.set(cv2.CAP_PROP_POS_FRAMES,...) command the speed get much slower than leaving the player going with ret, frame = cap.read() without setting the framenumber I need to use the cv2 because the purpose of job is to treat all frames by overlapping a text to every frame and show on the player (the text overlay code is not yet written here)

import cv2
    
    
def on_trackbar(arg):
    global cap
    cap.set(cv2.CAP_PROP_POS_FRAMES,arg)

    filevideo = r'D:\Documenti\Regate\Progetti\VideoOverlayData\Sviluppo\VideoOverlayData\INPUTFILES\28AugRace5.MP4'
    cap = cv2.VideoCapture(filevideo)
    Videofps = cap.get(cv2.CAP_PROP_FPS)
    nr_of_frames = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
    
    if (cap.isOpened() == False):
            print("Error opening video stream or file")
            
    cv2.namedWindow('Frame', cv2.WINDOW_KEEPRATIO)
    cv2.createTrackbar("F", "Frame", 0, nr_of_frames, on_trackbar)
    videoCurrentFrameNumber = 0
    while(cap.isOpened()):
           # cap.set(1,videoCurrentFrameNumber)
            ret, frame = cap.read()
            if ret is True:
                cv2.imshow('Frame',frame)
                videoCurrentFrameNumber = int(cap.get(cv2.CAP_PROP_POS_FRAMES))
                #cap.set(cv2.CAP_PROP_POS_FRAMES,videoCurrentFrameNumber)
                #cap.set(5,50)
                cv2.setTrackbarPos('F','Frame',videoCurrentFrameNumber)
                #videoCurrentFrameNumber = videoCurrentFrameNumber +1
                frameclick = cv2.waitKey(1) & 0xFF
                if frameclick == ord('q'):
                       break
    cap.release()
    cv2.destroyAllWindows()

Answer 1

I had the same problem using python3 with opencv-python 4.5.5.64 on an m1 mac. ( Last known version to work with trackbar , as of writing this article ) The best explanation i've found is that random access it is naturally slow. Though, I recall using older versions of opencv-python with your code with absolutely no problem. Needless to say, the solution that made most sense to my case was to load the entire video into memory and then display it. The code for my use-case is shown below, where I've build a video player with frame by frame ( a , b ) movement and play/pause ( spacebar ). The default position is pause:

import cv2
import numpy as np
import datetime
import os
import sys

# import the video
videoFile = sys.argv[1];

# load capture
cap = cv2.VideoCapture(videoFile)
if (cap.isOpened()== False): 
  print("Error opening video stream or file")

# prepare view
view_name = "Video in memory"
total_frames = cap.get(cv2.CAP_PROP_FRAME_COUNT)
cv2.namedWindow(view_name)
cv2.createTrackbar('S',view_name, 0,int(total_frames)-1, lambda x:x)
cv2.setTrackbarPos('S',view_name,0)

# read the entire video to memory
print("[INFO] loading frames to memory")
all_frames = []
while(cap.isOpened()):
    frame_exists, frame = cap.read()
    if frame_exists == True:
        all_frames.append(frame)
    else:
        break

# When everything done, release the video capture object
cap.release()
print("[INFO] load compelte");

# set default play state to pause
status = 'pause'
previous_status = 'pause'
frame_idx = 0
while True:
    try:
        if frame_idx==total_frames-1:
            frame_idx=0

        frame = all_frames[frame_idx]
        cv2.imshow(view_name,frame)

        # event handling
        keyPress = cv2.waitKey(1)
        if keyPress & 0xFF == ord(' '):
            status = 'play_pause_click'
        elif keyPress & 0xFF == ord('a'):
            status = 'prev_frame'
        elif keyPress & 0xFF == ord('d'):
            status = 'next_frame'
        elif keyPress & 0xFF == -1:
            status = status
        elif keyPress & 0xFF == ord('q'):
            status = 'exit'

        if (status == 'play_pause_click' and previous_status == 'play_pause_click'):
            status = 'pause'
            previous_status = 'pause'
        if (status == 'play_pause_click'):
            status = 'play'
            previous_status = 'play_pause_click'

        if status == 'play':
            frame_idx += 1
            cv2.setTrackbarPos('S',view_name,frame_idx)
            continue
        if status == 'pause':
            frame_idx = cv2.getTrackbarPos('S',view_name)
        if status=='prev_frame':
            frame_idx-=1
            frame_idx = max(0, frame_idx)
            cv2.setTrackbarPos('S',view_name,frame_idx)
            status='pause'
        if status=='next_frame':
            frame_idx+=1
            cv2.setTrackbarPos('S',view_name,frame_idx)
            status='pause'
        if status == 'exit':
            break

    except KeyError:
        print("Invalid Key was pressed")
 
# Closes the generated window
cv2.destroyWindow(view_name)