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• lavc/ffjni : remove use of private JniInvocation API to retreive the Java VM

  11 mars 2016, par Matthieu Bouron

  lavc/ffjni : remove use of private JniInvocation API to retreive the Java VM
  Android N will prevent users from loading non-public APIs.
  Users should only rely on the av_jni_set_java_vm function to set the
  
  Java VM.
  

  • [DH] libavcodec/ffjni.c

• JavaCV grab frame method delays and returns old frames

  27 janvier 2019, par Null Pointer

  I’m trying to create a video player in Java using JavaCV and its FFmpegFrameGrabber class. Simply, Inside a loop, I use

  .grab() to get a frame and then paint it on a panel

  Problem is, player gets delayed. For example, after 30 seconds passes, in video only 20 seconds passes.

  Source is ok. Other players can play the stream normally. Problem is possibly the long printing time.

  What I do not understand is that : "why does .grab() method brings me a frame from 10 seconds ago ?" Shouldn’t it just grab the frame which is being streamed at the moment ?

  (Sorry for not providing a working code, it’s all over different huge classes)

  I use the following grabber options (selected by some other colleague) :

  grabber.setImageHeight(480);
  
  grabber.setImageWidth(640);
  
  grabber.setOption("reconnect", "1");
  
  grabber.setOption("reconnect_at_eof", "1");
  
  grabber.setOption("reconnect_streamed", "1");
  
  grabber.setOption("reconnect_delay_max", "2");
  
  grabber.setOption("preset", "veryfast");
  
  grabber.setOption("probesize", "192");
  
  grabber.setOption("tune", "zerolatency");
  
  grabber.setFrameRate(30.0);
  
  grabber.setOption("buffer_size", "" + this.bufferSize);
  
  grabber.setOption("max_delay", "500000");
  
  grabber.setOption("stimeout", String.valueOf(6000000));
  
  grabber.setOption("loglevel", "quiet");
  
  grabber.start();

  Thanks

• 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 ?