How to know total number of Frame in a file with c

问题:

How to know total number of Frame in a file ( .avi) through Python using open cv module.

If possible what all the information (resolution, fps,duration,etc) we can get of a video file through this.

回答1:

With a newer OpenCV version (I use 3.1.0) it works like this:

import cv2

cap = cv2.VideoCapture("video.mp4")
length = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
print( length )

And similar for other video properties cv2.CAP_PROP_*

回答2:

import cv2

cap = cv2.VideoCapture(fn)

if not cap.isOpened(): 
    print "could not open :",fn
    return

length = int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT))
width  = int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_HEIGHT))
fps    = cap.get(cv2.cv.CV_CAP_PROP_FPS)

see here for more info.

also, all of it with a grain of salt, not all those props are mandatory, some might not be available with your capture / video codec

回答3:

Here is how it works with Python 3.6.5 (on Anaconda) and OpenCV 3.4.2. [Note]: You need to drop the "CV_" from the "CV_CAP_PROP_xx" for any property as given on the official OpenCV website.

import cv2
cap = cv2.VideoCapture("video.mp4")
property_id = int(cv2.CAP_PROP_FRAME_COUNT) 
length = int(cv2.VideoCapture.get(cap, property_id))
print( length )

回答4:

Another solution that doesn't depend on the sometimes buggy CV_CAP_PROP getters is to traverse your whole video file in a loop

• Increase a frame counter variable every time a valid frame is encountered and stop when an invalid one comes (end of the video file).

• Gathering information about the resolution is trickier because some codecs support variable resolution (similar to VBR in audio files where the bitrate is not a constant but instead covers some predefined range).

  • constant resolution - you need only the first frame to determine the resolution of the whole video file in this case so traversing the full video is not required
  • variable resolution - you need to get the resolution of every single frame (width and height) and calculate an average to get the average resolution of the video

• FPS can be calculated however here you have the same problem as with the resolution - constant (CFR) vs variable (VFR). This is more of a mutli-threading problem omho. Personally I would use a frame counter, which increased after each valid frame while at an interval of 1 second a timer (running in a background thread) would trigger saving the current counter's value and then resetting it. You can store the values in a list in order to calculate the average/constant frame rate at the end when you will also know the total number of frames the video has.

The disadvantage of this rather simplistic way of doing things is that you have to traverse the whole file, which - in case it's several hours long - will definitely be noticeable by the user. In this case you can be smart about it and do that in a background process while letting the user do something else while your application is gathering this information about the loaded video file.

The advantage is that no matter what video file you have as long as OpenCV can read from it you will get quite accurate results unlike the CV_CAP_PROP which may or may not work as you expect it to.