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!

Answer 1

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:

In Quicktime:

Answer 2

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.