title: OpenCV VideoWriter produces "can't find starting number" error
body: <p>I am trying to write 16-bit grayscale video using the FFV1 codec and opencv.ImageWriter on Windows 10</p>
  
  <p>Here is my code:</p>
  
  <pre><code>import numpy as np
  import cv2, pdb
  
  print(cv2.getBuildInformation())
  
  def to8(img):
      return (img/256).astype('uint8')
  
  cap = cv2.VideoCapture(0+cv2.CAP_DSHOW)
  cap.set(cv2.CAP_PROP_FOURCC, cv2.VideoWriter_fourcc('Y','1','6',' '))
  cap.set(cv2.CAP_PROP_CONVERT_RGB, False)
  
  out = cv2.VideoWriter('out.avi', cv2.VideoWriter_fourcc('F','F','V','1'), cap.get(cv2.CAP_PROP_FPS), (int(cap.get(cv2.CAP_PROP_FRAME_WIDTH)),int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))))
  
  while True:
      ret, frame = cap.read()
      frame = cv2.normalize(frame,None,0,65535,cv2.NORM_MINMAX)
  
      cv2.imshow('Video',to8(frame))
      out.write(frame)
  
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  cap.release()
  out.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>And here is my error:</p>
  
  <pre><code>[ERROR:0] global C:\projects\opencv-python\opencv\modules\videoio\src\cap.cpp (415) cv::VideoWriter::open VIDEOIO(CV_IMAGES): raised OpenCV exception:
  
  OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\videoio\src\cap_images.cpp:253: error: (-5:Bad argument) CAP_IMAGES: can't find starting number (in the name of file): out.avi in function 'cv::icvExtractPattern'
  </code></pre>
  
  <p>I am running this script from a command window with admin privileges. I've tried both making sure the output file does and does not exist before running.</p>
  
  <p>My OpenCV build information is here: <a href="https://pastebin.com/whtF6ixG" rel="nofollow noreferrer">https://pastebin.com/whtF6ixG</a></p>
  
  <p>Thanks!</p>
  
  <p><strong>EDIT:</strong>
  Based on Rotem's suggestion, instead of using VideoWriter I piped directly to FFMPEG using ffmpeg-python:</p>
  
  <pre><code>import numpy as np
  import cv2, pdb
  import ffmpeg
  
  def to8(img):
      return (img/256).astype('uint8')
  
  cap = cv2.VideoCapture(0+cv2.CAP_DSHOW)
  cap.set(cv2.CAP_PROP_FOURCC, cv2.VideoWriter_fourcc('Y','1','6',' '))
  cap.set(cv2.CAP_PROP_CONVERT_RGB, False)
  
  ff_proc = (
      ffmpeg
      .input('pipe:',format='rawvideo',pix_fmt='gray16le',s='%sx%s'%(int(cap.get(cv2.CAP_PROP_FRAME_WIDTH)),int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))),r='60')
      .output('out3.avi',vcodec='ffv1',an=None)
      .run_async(pipe_stdin=True)
  )
  
  while True:
      ret, frame = cap.read()
  
      cv2.imshow('Video',to8(frame))
      ff_proc.stdin.write(frame)
  
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  
  out.terminate()
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>