Writing video with OpenCV + Python + Mac

Question

I keep getting an assertion error when I'm trying to write frames to video. The error I'm getting is this:

Traceback (most recent call last):
  File "VideoMixer.py", line 23, in <module>
    cv.WriteFrame(writer, cv.LoadImage(fileName))
cv.error: dst.data == dst0.data

Here's my script:

import cv
import sys

files = sys.argv[1:]

for f in files:
    capture = cv.CaptureFromFile(f)
    height = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_WIDTH)
    width = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FRAME_HEIGHT)
    fps = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FPS)
    fourcc = cv.GetCaptureProperty(capture, cv.CV_CAP_PROP_FOURCC)
    print fourcc
    writer = cv.CreateVideoWriter('ok.mov', int(fourcc),fps,(int(width),int(height)),1)
    print writer
    for i in range(30):
        frame = cv.QueryFrame(capture)
        print frame
        if frame:
            cv.WriteFrame(writer, frame)

Saving the frames as images works fine so I know there's nothing wrong with the capture. Am I creating the writer wrong? The 'print fourcc' outputs 0.0 but I've tried with many FOUR_CC values.

Thanks!

samplebias · Accepted Answer · 2011-03-25T18:28:56.907

6

Do some of your frames have different colorspaces or depths? A few observations:

You have swapped the height and width in your code, is that on purpose?
Your fourcc should be an integer > 0. See my example below.

I haven't personally generated Quicktime video using OpenCV, but this worked for me generating an uncompressed AVI file. I choose the I420 fourcc using the cv.CV_FOURCC function:

import cv
import sys

# standard RGB png file
path = 'stack.png'
cap = cv.CaptureFromFile(path)
fps = 24
width = int(cv.GetCaptureProperty(cap, cv.CV_CAP_PROP_FRAME_WIDTH))
height = int(cv.GetCaptureProperty(cap, cv.CV_CAP_PROP_FRAME_HEIGHT))
# uncompressed YUV 4:2:0 chroma subsampled
fourcc = cv.CV_FOURCC('I','4','2','0')
writer = cv.CreateVideoWriter('out.avi', fourcc, fps, (width, height), 1)
for i in range(90):
    cv.GrabFrame(cap)
    frame = cv.RetrieveFrame(cap)
    cv.WriteFrame(writer, frame)

Update: Screencapture of VLC playing out.avi:

enter image description here

In Quicktime:

enter image description here

edited Mar 25 '11 at 18:28

answered Mar 25 '11 at 00:02

samplebias

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2

Thanks a lot but are you able to play the out.avi? The script runs fine for me and makes the file but VLC gives me an error when I try to play it: ps error: cannot peek main debug: no demux module matching "avi" could be loaded main debug: TIMER module_need() : 303.765 ms - Total 303.765 ms / 1 intvls (Avg 303.765 ms) main error: no suitable demux module for `file/:///Users/***/Documents/Code/Video/out.avi' – rgbrgb Mar 25 '11 at 06:27
I can play the file, I'm using latest VLC on Mac. I'll update my answer with a screencap. – samplebias Mar 25 '11 at 15:46
I was also able to open and play the out.avi file using [AviDemux](http://avidemux.sourceforge.net/) and Quicktime. – samplebias Mar 25 '11 at 18:28
3

Hmm couldn't get AviDemux to work either. I think out.avi is corrupted because file is 8kb no matter how many frames I try to make. Are you using Quicktime or ffmpeg? I'm using quicktime so mayube this is the problem. Thanks again for your help. – rgbrgb Mar 26 '11 at 19:54

score 5 · Answer 2 · answered Nov 20 '12 at 17:05

5

I tried various codecs including 'MJPG' and 'I420' and none of them worked on my Mac OpenCV build. They produced tiny unviewable output files without complaining.

Then I found this page which lists some codecs that worked for me. E.g. 'mp4v' works fine on my Mac and QuickTime is able to play it.

answered Nov 20 '12 at 17:05

mtoossi

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@mtoosi, what file extension did you save your file as? I tried `codec = cv.CV_FOURCC('M','P','4','V')` and saving the file as `/full/path/to/file/out.mp4v` which produced a valid file, but not one that opens in QuickTime or VLC. – cooncesean Dec 06 '12 at 07:24
@cooncesean, I used `.avi` as the file extension in all cases IIRC. Also, I was on OS X 10.8 and built OpenCV 2.4.3 myself with all the default flags and no third party libraries installed on the system. – mtoossi Dec 06 '12 at 17:23
FYI I'm on OS X 10.11 with OpenCV 2.4.13.2, and I was able to make this work with `cv2.cv.CV_FOURCC('M','P','4','V')` and an output filename with extension `.avi`. An extension of `.mp4` produced a video with a single frame and no motion. – trianta2 Apr 07 '17 at 20:35
With OpenCV 4.1.0 on OS X 10.14.5, `cv2.VideoWriter_fourcc(*"MP4V")` worked for me but only if I made the file an `.mp4`. If I tried `.avi`, QuickTime wouldn't play it. – Nathan Jul 12 '19 at 16:42

score 0 · Answer 3 · answered Mar 25 '11 at 00:44

0

This file in the OpenCV source implies on line #2598 - if this assert fails: "dst.data == dst0.data"

it means that the destination size or type was incorrect

answered Mar 25 '11 at 00:44

lunixbochs

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Writing video with OpenCV + Python + Mac

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