Scanning directory and processing multiple files with cv2.VideoCapture()

JSMelone · April 14, 2022, 8:17pm

Hi all,

I’m totally stuck on this, I just can’t find what’s wrong. Not sure if this is OpenCV specific or just my limited python skills. But I’m hoping someone can help me out.

What I would like to do is:

scan a directory for (mp4) video files
make a list of the found files
process each file with cv2.VideoCapture()

This is my code:

# file operations
import os
# Motion detection
import cv2


# Scan target directory
# os.scandir returns an iterator, make it a list to be re-usable

# Target directory is in the same directory as the script
targetDirectory = "videofiles"
dircontent = list(os.scandir(targetDirectory))

# Search for mp4 files and make a list [ [filename, path], [filename, path], ... ]
mp4Files = [[f.name[:-4], os.path.abspath(f.name)] for f in dircontent if f.name.lower().endswith('.mp4')]

for videofile in mp4Files:

    print(videofile[1], " -- ", type(videofile[1]))
    # This prints: E:\Projects\Programming\Python\OpenCV\_CaptureSaveROI\Motiondetection_Batch_Heatmap\Burglary.mp4  --  <class 'str'>

    # Open file to process
    # Replace backslahes with forward slashes
    videofilepath = videofile[1].replace("\\","/")
    print(videofilepath)
    # This prints: E:/Projects/Programming/Python/OpenCV/_CaptureSaveROI/Motiondetection_Batch_Heatmap/Burglary.mp4
    cap = cv2.VideoCapture(videofilepath)

    # the following doesn't give an error:
    # cap = cv2.VideoCapture("E:/Projects/Programming/Python/OpenCV/_CaptureSaveROI/Motiondetection_Batch_Heatmap/videofiles/Burglary.mp4")

    print(cap)
    # This prints: <VideoCapture 018CB4C0>

    if (cap.isOpened() == False):
        print("Error opening the video file")
    else:
        ret, frame1 = cap.read()
        heatmap = frame1.copy() # <<<<<<< ERROR

In the last if/else:
the error I get when not using the if/else :

heatmap = frame1.copy()  
AttributeError: 'NoneType' object has no attribute 'copy'

When using the if/else, cap.isOpened() returns false and I get the “Error opening the video file” message

Thanks for any help in advance!

matti.vuori · April 15, 2022, 6:52am

You know what your variable ret is for? Checking that read succeeds. If it doesn’t, would there be a frame?

JSMelone · April 15, 2022, 7:23am

Well, ret returns false and nonetype indicates there is no frame to perform copy on. The question is:

This works:

cap = cv2.VideoCapture(“E:/Projects/…/Burglary.mp4”) returns a valid cap object

This does not work

cap = cv2.VideoCapture(videofilepath) where videofilepath is the string coming from a list

The strings look the same to me and I have no clue what’s going on.

Thanks

berak · April 15, 2022, 7:59am

there may be non - video files in your folder
(hidden thumbnails, dot files, DS_Store, etc).
use python’s glob to filter filetypes,
and check cap.isOpened() before proceeding

crackwitz · April 15, 2022, 8:31am

but they aren’t equal in the python == sense, are they?

have your script print those strings. show both strings to us.

also, you don’t need to rewrite backslashes to forward slashes.

JSMelone · April 15, 2022, 8:55am

Ok, I did a test using glob and guess what, it works:

# file operations
import glob
# Motion detection
import cv2


# Scan target directory
# os.scandir returns an iterator, make it a list to be re-usable

# Target directory is in the same directory as the script
targetDirectory = "videofiles"
mp4files = glob.glob(targetDirectory + '/*.mp4')

for element in mp4files:
    print(element)
# The above prints all found paths as videofiles\filename.mp4

for videofile in mp4files:

    print(videofile, " -- ", type(videofile))
    # This prints: videofiles\Burglary.mp4  --  <class 'str'>

    cap = cv2.VideoCapture(videofile)

    print(cap)
    # This prints: <VideoCapture 018CB4C0>

    ret, frame1 = cap.read()

    if(ret):
        heatmap = frame1.copy()  # no error
        cv2.imshow('heatmap', heatmap)
    else:
        print('error')

So it works, glob however returns relative paths.

@crackwitz: I tested the following

targetDirectory = "videofiles"
dircontent = list(os.scandir(targetDirectory))

# Search for mp4 files and make a list [ [filename, path], [filename, path], ... ]
mp4Files = [[f.name[:-4], os.path.abspath(f.name)] for f in dircontent if f.name.lower().endswith('.mp4')]

for videofile in mp4Files:

    print(videofile[1], " -- ", type(videofile[1]))
    # This prints: E:\Projects\Programming\Python\OpenCV\_CaptureSaveROI\Motiondetection_Batch_Heatmap\Burglary.mp4  --  <class 'str'>

    pathAsString = 'E:/Projects/Programming/Python/OpenCV/_CaptureSaveROI/Motiondetection_Batch_Heatmap/videofiles/Burglary.mp4'

    print(pathAsString)
    # This prints: E:/Projects/Programming/Python/OpenCV/_CaptureSaveROI/Motiondetection_Batch_Heatmap/videofiles/Burglary.mp4

    print(videofile[1] == pathAsString)
    # This prints: False

    print('types:')
    print(type(videofile[1]))       # This prints: <class 'str'>
    print(type(pathAsString))       # This prints: <class 'str'>

    cap = cv2.VideoCapture(videofile[1])

    # the following lines don't give an error:
    # cap = cv2.VideoCapture('E:/Projects/Programming/Python/OpenCV/_CaptureSaveROI/Motiondetection_Batch_Heatmap/videofiles/Burglary.mp4')
    # cap = cv2.VideoCapture(pathAsString)

    print(cap)
    # This prints: <VideoCapture 018CB4C0>

    ret, frame1 = cap.read()
    print(ret)                  # This prints: False
    heatmap = frame1.copy()     # <<<<<<< ERROR: 'NoneType' object has no attribute 'copy'

videofile[1] == pathAsString returns false because they are two different objects right? What I don’t understand is that they are both strings with the same content but when fed into cv2.VideoCapture() one works and the other doesn’t.

I don’t know if it’s relevant but I’m using pycharm on windows:

crackwitz · April 15, 2022, 10:04am

uh no. that means they are not equal by value. that means their “content” differs.

berak · April 15, 2022, 10:30am

videoFile[1] contains backward slashes
pathAsString has forward slashes

so, not the same string

JSMelone · April 15, 2022, 10:46am

Man… I’ve been staring at this too long, I missed the actual difference in the paths (Motiondetection_Batch_Heatmap\Burglary.mp4 != Motiondetection_Batch_Heatmap/videofiles/Burglary.mp4)

Just changed:

mp4Files = [[f.name[:-4], os.path.abspath(f.name)] for f in dircontent if f.name.lower().endswith('.mp4')]

To:

mp4Files = [[f.name[:-4], os.path.abspath(f)] for f in dircontent if f.name.lower().endswith('.mp4')]

So to conclude, this works as expected:

# file operations
import os
# Motion detection
import cv2


# Scan target directory
# os.scandir returns an iterator, make it a list to be re-usable

# Target directory is in the same directory as the script
targetDirectory = "videofiles"
dircontent = list(os.scandir(targetDirectory))

# Search for mp4 files and make a list [ [filename, path], [filename, path], ... ]
mp4Files = [[f.name[:-4], os.path.abspath(f)] for f in dircontent if f.name.lower().endswith('.mp4')]


for videofile in mp4Files:

    cap = cv2.VideoCapture(videofile[1])

    ret, frame1 = cap.read()
    heatmap = frame1.copy()

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Scanning directory and processing multiple files with cv2.VideoCapture()

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