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• OpenCV returns an Empty Frame on video.read

  20 mars 2019, par Ikechukwu Anude

  Below is the relevant code

  import cv2 as cv
  
  import numpy as np
  
  
  
  video = cv.VideoCapture(0) #tells obj to use built in camera\
  
  
  
  #create a face cascade object 
  
  face_cascade  = 
  
  cv.CascadeClassifier(r"C:\Users\xxxxxxx\AppData\Roaming\Python\Python36\site- 
  
  packages\cv2\data\haarcascade_frontalcatface.xml")
  
  
  
  a = 1
  
  #create loop to display a video
  
  while True:
  
      a = a + 1
  
      check, frame = video.read()
  
      print(frame)
  
  
  
      #converts to a gray scale img
  
      gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
  
  
  
      #create the faces
  
      faces = face_cascade.detectMultiScale(gray, scaleFactor=1.5, minNeighbors=5)
  
  
  
      for(x, y, w, h) in faces:
  
          print(x, y, w, h)
  
  
  
      #show the image
  
      cv.imshow('capturing', gray)
  
  
  
      key = cv.waitKey(1) #gen a new frame every 1ms
  
  
  
      if key == ord('q'): #once you enter 'q' the loop will be exited
  
          break
  
  
  
  print(a) #this will print the number of frames
  
  
  
  #captures the first frame
  
  video.release() #end the web cam
  
  
  
  #destroys the windows when you are not defined
  
  cv.destroyAllWindows()

  The code displays a video captured from my webcam camera. Despite that, OpevCV doesn’t seem to be processing any frames as all the frames look like this

   [[0 0 0]
  
    [0 0 0]
  
    [0 0 0]
  
    ...
  
    [0 0 0]
  
    [0 0 0]
  
    [0 0 0]]]

  which I assume means that they are empty.

  This I believe is preventing the algorithm from being able to detect my face in the frame. I have a feeling that the issue lies in the ffmpeg codec, but I’m not entirely sure how to proceed even if that is the case.

  OS : Windows 10
  Language : Python

  Why is the frame empty and how can I get OpenCV to detect my face in the frame ?

• FFMPEG : Is there any property of FFMPEG, which make it start publishing video stream from IDR frame only

  27 mars 2019, par Shrishty Deorari

  I am using FFMPEG to publish video data (h264 encoded data) into Wowza streaming engine. My application captures and writes encoded video data frame by frame in a FIFO(named Pipe) and in another thread ffmpeg reads video data from that particular FIFO and then pushes to the cloud, everything goes smooth as long as network condition is good.

  In Bad network, frames start to drop as FFMPEG is not able to read and push frames to the cloud, in this loss if any I frame gets discarded I get green glitches in the video as expected.
  I am trying to get if there is any option/property in FFMPEG which I can use while pushing the data to the cloud, which will take care of GOP, so FFMPEG always pushes first IDR frame and then respective P frames to the Wowza streaming engine.

  Flow is :
  Application ----writes----->FIFO---reads--->FFMPEG-----pushes--->Wowza

  I am using FFMPEG binary. I can implement the GOP logic in my application, before writing the data in FIFO, but that can be done only If I can somehow know when FFMPEG stops reading data from FIFO due to a network condition and I am not sure I can do that, as it is a binary.

  Any help would be appreciated.

• Invalid key_frame(I) and P- frame sequence in h264 Binary stream

  9 avril 2019, par Syed

  I am getting video from USB webcam and encoding into h.264 raw stream as follows...

  ffmpeg.exe -f dshow -rtbufsize 200M -i video="Logitech HD Webcam C270" -vcodec libx264 -preset ultrafast -tune zerolatency -g 30 -s 480x640 -buf
  size:v 50M output.h264

  I am expecting 1-Key frame and 29-P Frame(with SPS/PPS) in output.h264 stream.But I am not getting the expected result.However video is playing fine.

  I tried to get Metadata of same file using ffprob ..

  ffprobe -show_frames videofilename.h264 > outputlogfile.txt

  Here I can see proper sequence 1-Key frame and 29-P frames.But if we open h264 file in binary reader(I am using HDX) I can see Key/P frames not in proper sequence.

  You can download h264 sample and ffprob logs from below links.

  https://www.dropbox.com/s/3ghpkqdc36wdgxr/TimerSample.h264?dl=0
  https://www.dropbox.com/s/gdn64004o0udrfk/TimerSample.txt?dl=0

  You can find binary sequence of same file from here (filtered by start code)

  Please let me know whether i am missing some filter. Thank you.