OpenCV VideoWriter Not Writing to Output.avi

Question

I'm attempting to write a simple bit of code that takes a video, crops it, and writes to an output file.

System Setup:

OS: Windows 10
Conda Environment Python Version: 3.7
OpenCV Version: 3.4.2
ffmpeg Version: 2.7.0

File Input Specs:

Codec: H264 - MPEG-4 AVC (part 10)(avc1)
Type: Video
Video resolution: 640x360
Frame rate: 5.056860

Code failing to produce output (it creates the file but doesn't write to it):

import numpy as np
import cv2

cap = cv2.VideoCapture('croptest1.mp4')

# Define the codec and create VideoWriter object
fourcc = cv2.VideoWriter_fourcc('F', 'M', 'P', '4')
out = cv2.VideoWriter('output.avi', fourcc, 20.0,
                      (int(cap.get(3)), int(cap.get(4))))

# Verify input shape
width = cap.get(3)
height = cap.get(4)
fps = cap.get(5)
print(width, height, fps)

while(cap.isOpened()):
    ret, frame = cap.read()
    if ret == True:
        # from top left =frame[y0:y1, x0:x1]
        cropped_frame = frame[20:360, 0:640]

        # write the clipped frames
        out.write(cropped_frame)

        # show the clipped video
        cv2.imshow('cropped_frame', cropped_frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()

Variations to fourcc and out variables tried to get codec to work:

fourcc = cv2.cv.CV_FOURCC(*'DIVX')
fourcc = cv2.VideoWriter_fourcc(*'DIVX')

out = cv2.VideoWriter('ditch_effort.avi', -1, 20.0, (640, 360))

Based on this link I should be able to refer to this fourcc reference list to determine the appropriate fourcc compression code to use. I have tried a bunch of variations, but cannot get the output file to be written. When I run the code, the #verify input shape variables print the corresponding 640, 360 and correct Frame Rate.

Can any one tell me what my issue is...would be much appreciated.

Solution 1

The reason of error is the differences between the dimension of the cropped_frame (640,340) and the dimension declared in the writer (640,360).

So the writer should be:

out = cv2.VideoWriter('output.avi', fourcc, 20.0,(640,340))

Solution 2

I had the same problem with OpenCV C++ in Ubuntu ubuntu 18.04. I was processing a video (converting to grayscale and then generating a "paint" effect).

I solved the problem successfully doing the following steps:

1. Use the VideoWriter with the following line:

    VideoWriter videoWriter("effect.avi", VideoWriter::fourcc('m','p','4','v'), 15.0, Size(ancho, alto), true);

2. Convert the image (in my case was in grayscale) to BGR (do the same in case that your image is in RGB color space):

    Mat finalImage;
    cvtColor(imgEffect,finalImage,COLOR_GRAY2BGR);
    videoWriter.write(finalImage);

After the execution, the code generates the video without problems.

You can found my code in the following link: Example Code in C++

Comments

[R.Zane]

I have a question concerning the details. So my out variable must always have the corresponding dimensions such that Y=y1-y0 and X-x1-x0. Is this correct? And lastly, why is the time off by a margin of a few seconds. I tried matching the fps (5.056859927954619) (the 20 I had above was an error), but the time is still off from the original by an added 7 seconds

[Ha Bom]

Yes, the writer out must have the corresponding dimensions with the frame you write.

[Ha Bom]

What do you mean by time off? The fps you set and the actual fps are different?

[R.Zane]

To your first answer. Awesome...thanks again very much. To your second, I had a fps of 20 in my 'out' variable in the code above because I pulled it from the website, however, my actual input file has a fps of 5.056860 (as in my above File Input Specs). I tried matching the decimal place the cap.get method gives me (5.056859927954619) and I still get an added 7 seconds (11:02-->11:09) Any idea why?

[Ha Bom]

I guess that cv2.waitKey(1) makes a delay of 1 ms every frame so your video is a little bit longer than the original. Also, I've never tried to set fps in that such decimal number.

[R.Zane]

I wonder if it has to do with the codec I used (FMP4), I will try some others now like (*'DIVX) now that you informed me on the problem of syncing the net value of the dimensions. The cv2.waitKey(1) likely has no effect, as I tested with out that section of code. Anyways, I really appreciate all the help. Thanks very much for you time, patience and effort.