Why is my OpenCV Video Refusing to Write to Disk? - python-2.7

Question

So I am starting to get very confused by the openCV libraries ability to write out video to disk, because even the openCV documentation is not terribly clear as to how the video actually gets written in this case. The code I have below seems to collect the data just fine but the video file it tries to write has no data in it. All I want to do is take a video that I know I can, change the data within it to a ramp between 0 and 255, and then write that data back out to disk. However, the final I/O step is not cooperating for reasons I don't understand. Can anyone help? Find the code below:
import numpy as np
import cv2
import cv2.cv as cv
cap = cv2.VideoCapture("/Users/Steve/Documents/TestVideo.avi") #The video
height = cap.get(cv.CV_CAP_PROP_FRAME_HEIGHT) #We get some properties of the video
width = cap.get(cv.CV_CAP_PROP_FRAME_WIDTH)
fps = cap.get(cv.CV_CAP_PROP_FPS)
fourcc = cv2.cv.CV_FOURCC(*'PDVC') #This is essential for testing
out = cv2.VideoWriter('output.avi',fourcc, int(fps), (int(width),int(height)))
xaxis = np.arange(width,dtype='int')
yaxis = np.arange(height,dtype='int')
xx,yy = np.meshgrid(xaxis,yaxis)
ramp=256*xx/int(width) #This is a horizontal ramp image that scales from 0-255 across the width of the image
i=0
while(cap.isOpened()):
if i%100==0: print i
i+=1
ret, frame = cap.read() #Grab a frame
if ret==True:
# Change the frame data to the ramp instead of the original video
frame[:,:,0]=ramp #The camera is B/W so the image is in B/W
frame[:,:,1]=ramp
frame[:,:,2]=ramp
out.write(frame) #Write to disk?
cv2.imshow('frame',frame) # I see the ramp as an imshow
if cv2.waitKey(1) & 0xFF == ord('q'):
break
else:
break
cap.release() #Clear windows
out.release()
cv2.destroyAllWindows()

Answer 1

Your code is generally correct, but is likely silently failing at some step along the way.
try adding some debug lines:
out = cv2.VideoWriter('output2.avi',fourcc, int(fps), (int(width),int(height)))
or
else:
print "frame %d is false" % i
break
When I was testing your code locally I found the fps was set to 0 for most .avi files I read. Manually setting it to 15 or 30 worked.
I also didn't have any luck getting your fourcc to work on my machine (osx), but this one worked fine.
fourcc = cv2.cv.CV_FOURCC('m', 'p', '4', 'v')