OpenCV/Python：使用 VideoCapture 读取特定帧

OpenCV/Python: read specific frame using VideoCapture

提问by zinon

Is there a way to get a specific frame using VideoCapture()method?

有没有办法使用VideoCapture()方法获取特定帧？

My current code is:

我目前的代码是：

import numpy as np
import cv2

cap = cv2.VideoCapture('video.avi')

Thisis my reference tutorial.

这是我的参考教程。

采纳答案by zinon

Thank you GPPK.

谢谢 GPPK。

The video parameters should be given as integers. Each flag has its own value. See herefor the codes.

视频参数应以整数形式给出。每个标志都有自己的值。有关代码，请参见此处。

The correct solution is:

正确的解决办法是：

import numpy as np
import cv2

#Get video name from user
#Ginen video name must be in quotes, e.g. "pirkagia.avi" or "plaque.avi"
video_name = input("Please give the video name including its extension. E.g. \"pirkagia.avi\":\n")

#Open the video file
cap = cv2.VideoCapture(video_name)

#Set frame_no in range 0.0-1.0
#In this example we have a video of 30 seconds having 25 frames per seconds, thus we have 750 frames.
#The examined frame must get a value from 0 to 749.
#For more info about the video flags see here: https://stackoverflow.com/questions/11420748/setting-camera-parameters-in-opencv-python
#Here we select the last frame as frame sequence=749. In case you want to select other frame change value 749.
#BE CAREFUL! Each video has different time length and frame rate. 
#So make sure that you have the right parameters for the right video!
time_length = 30.0
fps=25
frame_seq = 749
frame_no = (frame_seq /(time_length*fps))

#The first argument of cap.set(), number 2 defines that parameter for setting the frame selection.
#Number 2 defines flag CV_CAP_PROP_POS_FRAMES which is a 0-based index of the frame to be decoded/captured next.
#The second argument defines the frame number in range 0.0-1.0
cap.set(2,frame_no);

#Read the next frame from the video. If you set frame 749 above then the code will return the last frame.
ret, frame = cap.read()

#Set grayscale colorspace for the frame. 
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

#Cut the video extension to have the name of the video
my_video_name = video_name.split(".")[0]

#Display the resulting frame
cv2.imshow(my_video_name+' frame '+ str(frame_seq),gray)

#Set waitKey 
cv2.waitKey()

#Store this frame to an image
cv2.imwrite(my_video_name+'_frame_'+str(frame_seq)+'.jpg',gray)

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

回答by JParrish88

If you want an exact frame, you could just set the VideoCapture session to that frame. It's much more intuitive to automatically call on that frame. The "correct" solution requires you to input known data: like fps, length, and whatnot. All you need to know with the code below is the frame you want to call.

如果您想要一个精确的帧，您可以将 VideoCapture 会话设置为该帧。自动调用该框架要直观得多。“正确”的解决方案要求您输入已知数据：例如 fps、长度等等。通过下面的代码，您只需要知道要调用的框架。

import numpy as np
import cv2
cap = cv2.VideoCapture(video_name) #video_name is the video being called
cap.set(1,frame_no); # Where frame_no is the frame you want
ret, frame = cap.read() # Read the frame
cv2.imshow('window_name', frame) # show frame on window

If you want to hold the window, until you press exit:

如果要按住窗口，直到按退出：

while True:
    ch = 0xFF & cv2.waitKey(1) # Wait for a second
    if ch == 27:
        break

回答by Alexey Antonenko

Yes, it's very straight forward:

是的，这是非常直接的：

import cv2
cap = cv2.VideoCapture(videopath)
cap.set(cv2.CV_CAP_PROP_POS_FRAMES, frame_number-1)
res, frame = cap.read()

'frame_number' is an integer in range 0...amount_of_frames. Notice: you should set 'frame_number-1' to force reading frame 'frame_number'. It's not documented well but test shows that behavior of VideoCapture module.

'frame_number' 是 0...amount_of_frames 范围内的整数。注意：你应该设置'frame_number-1'来强制读取帧'frame_number'。它没有很好地记录，但测试表明 VideoCapture 模块的行为。

'res' is boolean result of operation, one may use it to check if frame was successfully read. One may obtain amount of frames by:

'res' 是操作的布尔结果，可以用它来检查帧是否被成功读取。可以通过以下方式获得帧数：

amount_of_frames = cap.get(cv2.CV_CAP_PROP_FRAME_COUNT)

回答by Dawid Pustówka

For example, to start reading 15th frame of the video you can use:

例如，要开始阅读视频的第 15 帧，您可以使用：

frame = 15
cap.set(cv2.CAP_PROP_POS_FRAMES, frame-1)

回答by Vittorio

Set a specific frame

设置特定帧

From the documentation of the VideoCaptureProperties (docs) is possible to see that the way to set the frame in the VideoCapture is:

从 VideoCaptureProperties ( docs) 的文档中可以看出在 VideoCapture 中设置帧的方法是：

frame = 30
cap.set(cv2.CAP_PROP_POS_FRAMES, frame)

Notice that you don't have to pass to the function frame - 1because, as the documentation says, the flag CAP_PROP_POS_FRAMESrapresent the "0-based index of the frame to be decoded/captured next".

请注意，您不必传递给该函数，frame - 1因为正如文档所说，该标志CAP_PROP_POS_FRAMES表示“接下来要解码/捕获的帧的基于 0 的索引”。

Concluding a full example where i want to read a frame at each second is:

总结一个我想每秒读取一帧的完整示例是：

import cv2

cap = cv2.VideoCapture('video.avi')

# Get the frames per second
fps = cap.get(cv2.CAP_PROP_FPS) 

# Get the total numer of frames in the video.
frame_count = cap.get(cv2.CAP_PROP_FRAME_COUNT)

frame_number = 0
cap.set(cv2.CAP_PROP_POS_FRAMES, frame_number) # optional
success, image = cap.read()

while success and frame_number <= frame_count:

    # do stuff

    frame_number += fps
    cap.set(cv2.CAP_PROP_POS_FRAMES, frame_number)
    success, image = cap.read()

Set a specific time

设置特定时间

In the documentation linked above is possible to see that the way to set a specific time in the VideoCapture is:

在上面链接的文档中可以看到在 VideoCapture 中设置特定时间的方法是：

milliseconds = 1000
cap.set(cv2.CAP_PROP_POS_MSEC, milliseconds)

And like before a full example that read a frame each second che be achieved in this way:

就像之前一个完整的例子一样，每秒读取一个帧可以通过这种方式实现：

import cv2

cap = cv2.VideoCapture('video.avi')

# Get the frames per second
fps = cap.get(cv2.CAP_PROP_FPS) 

# Get the total numer of frames in the video.
frame_count = cap.get(cv2.CAP_PROP_FRAME_COUNT)

# Calculate the duration of the video in seconds
duration = frame_count / fps

second = 0
cap.set(cv2.CAP_PROP_POS_MSEC, second * 1000) # optional
success, image = cap.read()

while success and second <= duration:

    # do stuff

    second += 1
    cap.set(cv2.CAP_PROP_POS_MSEC, second * 1000)
    success, image = cap.read()