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• OpenCV returns an Array of All Zeros on video.read

  19 janvier 2020, 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

  EDIT : The Frame is not empty but all the values in the array seem to be ’0’

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

• AVCodecContex returns zero for width and height in android

  7 mars 2014, par Whoami

  Not sure what was my mistake in the below code. I m trying with ffmpeg 0.11 and SDL2.0 in android.

  QUESTION :
  Why Width and Height of the CodecContext gives me always zero ?..

  int main(int argc, char *argv[])
  
  {
  
  
  
      int flags;
  
      flags = SDL_INIT_VIDEO | SDL_INIT_TIMER;
  
  
  
      if (SDL_Init (flags)) {
  
          LOGD ("Could not intialize Video for SDL: %s \n", SDL_GetError());
  
      }
  
      else 
  
          LOGD (" SUCCESS: SDL_Init ");
  
  
  
      // ffmpeg Register all services..
  
      ffmpeg_register_all (); 
  
  
  
  
  
      pFrame = avcodec_alloc_frame ();
  
      context = avformat_alloc_context();
  
  
  
      err = avformat_open_input (&context, "rtsp:ip:port", NULL, NULL);
  
      if ( err < 0) {
  
          __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Unable to open rtsp... ");
  
  
  
          return -1;
  
      }
  
  
  
      for (i = 0; i < context->nb_streams; i++)
  
      {               
  
          // Find the Decoder.
  
          codec = avcodec_find_decoder(context->streams[i]->codec->codec_id);
  
          if (codec->type  == AVMEDIA_TYPE_VIDEO ) {
  
              __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Found Video Streaming..  ");
  
              videoStreamIndex = i;
  
  
  
          }
  
      }
  
  
  
      // Play RTSP
  
      av_read_play(context);
  
  
  
      // Get Codec Context.
  
      pCodecCtx = context->streams[videoStreamIndex]->codec;
  
      if ( pCodecCtx == NULL )
  
          __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "CodecCtx is NULL>>> ");
  
      else
  
          __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "CodecCtx is &lt;&lt;<ok>>> ");
  
  
  
  
  
      //Find the Decoder.
  
      pCodec = avcodec_find_decoder (pCodecCtx->codec_id);
  
      avcodec_open2 (pCodecCtx, pCodec, NULL);
  
  
  
  
  
      int w = pCodecCtx->width;  // Why me getting 0 ? 
  
      int h = pCodecCtx->height;
  
  
  
      window = SDL_CreateWindow ("Test ffmpeg",SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, w, h, SDL_WINDOW_SHOWN|SDL_WINDOW_ALLOW_HIGHDPI);
  
      // What this HIGHDPI Means ??
  
  
  
      if ( window != NULL ) 
  
      {
  
          LOGD (" WINDOW CREATED.. , create Renderer ..");
  
          renderer = SDL_CreateRenderer (window, -1, 0);  
  
      }
  
      else
  
      {
  
          LOGD (" Invalid SDL Window ");  
  
      }
  
  __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Width and Height of PCodeccCtx.. %d .. %d " , w, h); 
  
      return 0;
  
  }
  
  </ok>

• AVCodecContex returns zero for width and height in android

  17 février 2023, par Whoami

  Not sure what was my mistake in the below code. I m trying with ffmpeg 0.11 and SDL2.0 in android.

  



  QUESTION :
Why Width and Height of the CodecContext gives me always zero ?..

  



  int main(int argc, char *argv[])&#xA;{&#xA;&#xA;    int flags;&#xA;    flags = SDL_INIT_VIDEO | SDL_INIT_TIMER;&#xA;&#xA;    if (SDL_Init (flags)) {&#xA;        LOGD ("Could not intialize Video for SDL: %s \n", SDL_GetError());&#xA;    }&#xA;    else &#xA;        LOGD (" SUCCESS: SDL_Init ");&#xA;&#xA;    // ffmpeg Register all services..&#xA;    ffmpeg_register_all (); &#xA;&#xA;&#xA;    pFrame = avcodec_alloc_frame ();&#xA;    context = avformat_alloc_context();&#xA;&#xA;    err = avformat_open_input (&amp;context, "rtsp:ip:port", NULL, NULL);&#xA;    if ( err &lt; 0) {&#xA;        __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Unable to open rtsp... ");&#xA;&#xA;        return -1;&#xA;    }&#xA;&#xA;    for (i = 0; i &lt; context->nb_streams; i&#x2B;&#x2B;)&#xA;    {               &#xA;        // Find the Decoder.&#xA;        codec = avcodec_find_decoder(context->streams[i]->codec->codec_id);&#xA;        if (codec->type  == AVMEDIA_TYPE_VIDEO ) {&#xA;            __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Found Video Streaming..  ");&#xA;            videoStreamIndex = i;&#xA;&#xA;        }&#xA;    }&#xA;&#xA;    // Play RTSP&#xA;    av_read_play(context);&#xA;&#xA;    // Get Codec Context.&#xA;    pCodecCtx = context->streams[videoStreamIndex]->codec;&#xA;    if ( pCodecCtx == NULL )&#xA;        __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "CodecCtx is NULL>>> ");&#xA;    else&#xA;        __android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "CodecCtx is &lt;&lt;<ok>>> ");&#xA;&#xA;&#xA;    //Find the Decoder.&#xA;    pCodec = avcodec_find_decoder (pCodecCtx->codec_id);&#xA;    avcodec_open2 (pCodecCtx, pCodec, NULL);&#xA;&#xA;&#xA;    int w = pCodecCtx->width;  // Why me getting 0 ? &#xA;    int h = pCodecCtx->height;&#xA;&#xA;    window = SDL_CreateWindow ("Test ffmpeg",SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, w, h, SDL_WINDOW_SHOWN|SDL_WINDOW_ALLOW_HIGHDPI);&#xA;    // What this HIGHDPI Means ??&#xA;&#xA;    if ( window != NULL ) &#xA;    {&#xA;        LOGD (" WINDOW CREATED.. , create Renderer ..");&#xA;        renderer = SDL_CreateRenderer (window, -1, 0);  &#xA;    }&#xA;    else&#xA;    {&#xA;        LOGD (" Invalid SDL Window ");  &#xA;    }&#xA;__android_log_print(ANDROID_LOG_DEBUG, "ffmpegguard", "Width and Height of PCodeccCtx.. %d .. %d " , w, h); &#xA;    return 0;&#xA;}&#xA;</ok>