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• WebM file not seekable in Chrome, when generated with ffmpeg

  11 avril 2014, par Zoon

  I am having a mind-boggling problem, I just can't seem to resolve.

  Providing a WebM file through PHP is nothing new in my world, and I even know how to work with HTTP 206 Partial Content. But for some reason Chrome does not like it.

  A simple HTML5 video playback

  <video width="640" height="360" poster="picture/preview/V00000006.jpg" controls="controls" preload="preload">
  
      <source type="video/webm" src="/video/V00000006.webm">
  
  </source></video>

  where /video/V00000006.webm is rewritten to a PHP-file in Apache, will playback just fine.
  But in Chrome the seekbar is not effective. When clicking on the seekbar the player will freeze and no longer playback until page is refreshed. Firefox handles it just fine !

  If I change /video/V00000006.webm to be a direct link to the same video it works just fine. I even compared the network requests between the two versions (with and without PHP) and there is barely any difference in the first request, but the second is failing in the PHP-delivered video.

  Initial request and seek request for Apache-delivered video file :

      Request URL :http://mytestserver.net/movie1152x720.webm
      Request Method:GET
      Status Code:206 Partial Content
      Request Headers
      Accept :*/*
      Accept-Encoding:identity ;q=1, * ;q=0
      Accept-Language:da-DK,da ;q=0.8,en-US ;q=0.6,en ;q=0.4
      Cache-Control:no-cache
      Connection:keep-alive
      Cookie:PHPSESSID=i562540rek172mnv3nk528acj0 ; userPassword= ; userEmail=
      Host:mytestserver.net
      Pragma:no-cache
      Range:bytes=0-
      Referer :http://mytestserver.net/video.html
      User-Agent:Mozilla/5.0 (X11 ; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.146 Safari/537.36
      Response Headers
      Accept-Ranges:bytes
      Connection:close
      Content-Length:4446451
      Content-Range:bytes 0-4446450/4446451
      Content-Type:video/webm
      Date:Fri, 11 Apr 2014 13:07:30 GMT
      ETag :"d2d0027-43d8f3-b91417c0"
      Last-Modified:Fri, 11 Apr 2014 12:46:31 GMT
      Server:Apache/2.2.3 (CentOS)
      —
      Request URL :http://mytestserver.net/movie1152x720.webm
  
      Request Method:GET
  
      Status Code:206 Partial Content
  
      Request Headers
  
      Accept :*/*
  
      Accept-Encoding:identity ;q=1, * ;q=0
  
      Accept-Language:da-DK,da ;q=0.8,en-US ;q=0.6,en ;q=0.4
  
      Cache-Control:no-cache
  
      Connection:keep-alive
  
      Cookie:PHPSESSID=i562540rek172mnv3nk528acj0 ; userPassword= ; userEmail=
  
      Host:mytestserver.net
  
      Pragma:no-cache
  
      Range:bytes=4445881-
  
      Referer :http://mytestserver.net/video.html
  
      User-Agent:Mozilla/5.0 (X11 ; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.146 Safari/537.36
  
      Response Headers
  
      Accept-Ranges:bytes
  
      Connection:close
  
      Content-Length:570
  
      Content-Range:bytes 4445881-4446450/4446451
  
      Content-Type:video/webm
  
      Date:Fri, 11 Apr 2014 13:09:02 GMT
  
      ETag :"d2d0027-43d8f3-b91417c0"
  
      Last-Modified:Fri, 11 Apr 2014 12:46:31 GMT
  
      Server:Apache/2.2.3 (CentOS)
  

  Initial request and seek request for PHP-streamed video :

      Request URL :http://mytestserver.net/video/V00000006.webm
      Request Method:GET
      Status Code:206 Partial Content
      Request Headers
      Accept :*/*
      Accept-Encoding:identity ;q=1, * ;q=0
      Accept-Language:da-DK,da ;q=0.8,en-US ;q=0.6,en ;q=0.4
      Cache-Control:no-cache
      Connection:keep-alive
      Cookie:PHPSESSID=i562540rek172mnv3nk528acj0 ; userPassword= ; userEmail=
      Host:mytestserver.net
      Pragma:no-cache
      Range:bytes=0-
      Referer :http://mytestserver.net/video.html
      User-Agent:Mozilla/5.0 (X11 ; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.146 Safari/537.36
      Response Headers
      Accept-Ranges:bytes
      Cache-Control:no-store, no-cache, must-revalidate, post-check=0, pre-check=0
      Connection:close
      Content-Length:8566268
      Content-Range:bytes 0-8566267/8566268
      Content-Type:video/webm
      Date:Fri, 11 Apr 2014 13:31:27 GMT
      Expires:Thu, 19 Nov 1981 08:52:00 GMT
      Pragma:no-cache
      Server:Apache/2.2.3 (CentOS)
      X-Powered-By:PHP/5.3.27
      —
      Request URL :http://mytestserver.net/video/V00000006.webm
  
      Request Headers CAUTION : Provisional headers are shown.
  
      Accept-Encoding:identity ;q=1, * ;q=0
  
      Cache-Control:no-cache
  
      Pragma:no-cache
  
      Range:bytes=4338314-
  
      Referer :http://mytestserver.net/video.html
  
      User-Agent:Mozilla/5.0 (X11 ; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.146 Safari/537.36
  

  Notice how the second request does not complete, Provisional headers are shown.

  I have tried altering the cache headers, setting it to the future, setting them blank and using file attachment headers.

  I tried fiddling around a lot with the serving code, but lately I have ended up with a simple example.

  <?php
  
  
  
  $path = 'test.webm';
  
  
  
  $size=filesize($path);
  
  
  
  $fm=@fopen($path,'rb');
  
  if(!$fm) {
  
    header ("HTTP/1.0 404 Not Found");
  
    die();
  
  }
  
  
  
  $begin=0;
  
  $end = $size-1;
  
  
  
  if(isset($_SERVER['HTTP_RANGE'])) {
  
    if(preg_match('/bytes=\h*(\d+)-(\d*)[\D.*]?/i', $_SERVER['HTTP_RANGE'], $matches)) {
  
      $begin=intval($matches[0]);
  
      if(!empty($matches[1])) {
  
        $end=intval($matches[1]);
  
      }
  
    }
  
  }
  
  
  
  if($begin>0||$end<$size)
  
    header('HTTP/1.0 206 Partial Content');
  
  else
  
    header('HTTP/1.0 200 OK');
  
  
  
  header("Content-Type: video/webm");
  
  header('Accept-Ranges: bytes');
  
  header('Content-Length:'.($end-$begin+1));
  
  header("Content-Disposition: inline;");
  
  header("Content-Range: bytes $begin-$end/$size");
  
  header("Content-Transfer-Encoding: binary\n");
  
  header('Connection: close');
  
  
  
  ob_get_clean();
  
  flush();
  
  
  
  $f = fopen($path, 'r');
  
  fseek($f, $offset);
  
  
  
  $pos = 0;
  
  $length = $end-$begin;
  
  
  
  while($pos < $length)
  
  {
  
      $chunk = min($length-$pos, 1024);
  
  
  
      echo fread($f, $chunk);
  
      flush();
  
  
  
      $pos += $chunk;
  
  }
  
  ?>

  Please note, entering the PHP-delivered video URL directly into the browser does not make a difference from showing it in a HTML page.

  I hope someone has an answer to why seeking might not work. Let me know if you have any suggestions.

  Thanks !

• How to Read DJI FPV Feed as OpenCV Object ?

  24 mai 2019, par Walter Morawa

  I’ve officially spent a lot of time looking for a solution to reading DJI’s FPV feed as an OpenCV Mat object. I am probably overlooking something simple, since I am not too familiar with Image Encoding/Decoding.

  I apologize if I am missing something very basic, but I know I’m not the first person to have issues getting DJI’s FPV feed, and answering this question, especially if option 1 is possible, would be extremely valuable to many developers. Please consider upvoting this question, as I’ve thoroughly researched this issue and future developers who come across it will likely run into a bunch of the same issues I had.

  I’m willing to use ffmpeg or Javacv if necessary, but that’s quite the hurdle for most Android developers as we’re going to have to use cpp, ndk, terminal for testing, etc. That seems like overkill.

  I believe the issue lies in the fact that we need to decode both the byte array of length 6 (info array) and the byte array with current frame info simultaneously. Thanks in advance for your time.

  Basically, DJI’s FPV feed comes in a number of formats.

  1. Raw H264 (MPEG4) in VideoFeeder.VideoDataListener

      // The callback for receiving the raw H264 video data for camera live view
  
      mReceivedVideoDataListener = new VideoFeeder.VideoDataListener() {
  
          @Override
  
          public void onReceive(byte[] videoBuffer, int size) {
  
              //Log.d("BytesReceived", Integer.toString(videoStreamFrameNumber));
  
              if (videoStreamFrameNumber++%30 == 0){
  
                  //convert video buffer to opencv array
  
                  OpenCvAndModelAsync openCvAndModelAsync = new OpenCvAndModelAsync();
  
                  openCvAndModelAsync.execute(videoBuffer);
  
              }
  
              if (mCodecManager != null) {
  
                  mCodecManager.sendDataToDecoder(videoBuffer, size);
  
              }
  
          }
  
      };

  2. DJI also has it’s own Android decoder sample with FFMPEG to convert to YUV format.

      @Override
  
      public void onYuvDataReceived(final ByteBuffer yuvFrame, int dataSize, final int width, final int height) {
  
          //In this demo, we test the YUV data by saving it into JPG files.
  
          //DJILog.d(TAG, "onYuvDataReceived " + dataSize);
  
          if (count++ % 30 == 0 && yuvFrame != null) {
  
              final byte[] bytes = new byte[dataSize];
  
              yuvFrame.get(bytes);
  
              AsyncTask.execute(new Runnable() {
  
                  @Override
  
                  public void run() {
  
                      if (bytes.length >= width * height) {
  
                          Log.d("MatWidth", "Made it");
  
                          YuvImage yuvImage = saveYuvDataToJPEG(bytes, width, height);
  
                          Bitmap rgbYuvConvert = convertYuvImageToRgb(yuvImage, width, height);
  
  
  
                          Mat yuvMat = new Mat(height, width, CvType.CV_8UC1);
  
                          yuvMat.put(0, 0, bytes);
  
                          //OpenCv Stuff
  
                      }
  
                  }
  
              });
  
          }
  
      }

  3. DJI also appears to have a "getRgbaData" function, but there is literally not a single example online or by DJI. Go ahead and Google "DJI getRgbaData"... There’s only the reference to the api documentation that explains the self explanatory parameters and return values but nothing else. I couldn’t figure out where to call this and there doesn’t appear to be a callback function as there is with YUV. You can’t call it from the h264b byte array directly, but perhaps you can get it from the yuv data.

  Option 1 is much more preferable to option 2, since YUV format has quality issues. Option 3 would also likely involve a decoder.

  Here’s a screenshot that DJI’s own YUV conversion produces.

  I’ve looked at a bunch of things about how to improve the YUV, remove green and yellow colors and whatnot, but at this point if DJI can’t do it right, I don’t want to invest resources there.

  Regarding Option 1, I know there’s FFMPEG and JavaCV that seem like good options if I have to go the video decoding route. However, both options seem quite time consuming. This JavaCV H264 conversion seems unnecessarily complex. I found it from this relevant question.

  Moreover, from what I understand, OpenCV can’t handle reading and writing video files without FFMPEG, but I’m not trying to read a video file, I am trying to read an H264/MPEG4 byte[] array. The following code seems to get positive results.

      /* Async OpenCV Code */
  
      private class OpenCvAndModelAsync extends AsyncTask {
  
          @Override
  
          protected double[] doInBackground(byte[]... params) {//Background Code Executing. Don't touch any UI components
  
              //get fpv feed and convert bytes to mat array
  
              Mat videoBufMat = new Mat(4, params[0].length, CvType.CV_8UC4);
  
              videoBufMat.put(0,0, params[0]);
  
              //if I add this in it says the bytes are empty.
  
              //Mat videoBufMat = Imgcodecs.imdecode(encodeVideoBuf, Imgcodecs.IMREAD_ANYCOLOR);
  
              //encodeVideoBuf.release();
  
              Log.d("MatRgba", videoBufMat.toString());
  
              for (int i = 0; i< videoBufMat.rows(); i++){
  
                  for (int j=0; j< videoBufMat.cols(); j++){
  
                      double[] rgb = videoBufMat.get(i, j);
  
                      Log.i("Matrix", "red: "+rgb[0]+" green: "+rgb[1]+" blue: "+rgb[2]+" alpha: "
  
                              + rgb[3] + " Length: " + rgb.length + " Rows: "
  
                              + videoBufMat.rows() + " Columns: " + videoBufMat.cols());
  
                  }
  
              }
  
              double[] center = openCVThingy(videoBufMat);
  
              return center;
  
          }
  
          protected void onPostExecute(double[] center) {
  
              //handle ui or another async task if necessary
  
          }
  
      }

  Rows = 4, Columns > 30k. I get lots of RGB values that seem valid, such as red = 113, green=75, blue=90, alpha=220 as a made up example ; however, I get a ton of 0,0,0,0 values. That should be somewhat okay, since Black is 0,0,0 (although I would have thought the alpha would be higher) and I have a black object in my image.

  However, when I try to compute the contours from this image, I almost always get that the moments (center x, y) are exactly in the center of the image. This error has nothing to do with my color filter or contours algorithm, as I wrote a script in python and tested that I implemented it correctly in Android by reading a still image and getting the exact same number of contours, position, etc in both Python and Android.

  I noticed it has something to do with the videoBuffer byte size (bonus points if you can explain why every other length is 6 !)

  2019-05-23 21:14:29.601 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2425
  
  2019-05-23 21:14:29.802 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2659
  
  2019-05-23 21:14:30.004 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:30.263 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6015
  
  2019-05-23 21:14:30.507 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:30.766 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4682
  
  2019-05-23 21:14:31.005 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:31.234 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2840
  
  2019-05-23 21:14:31.433 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4482
  
  2019-05-23 21:14:31.664 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:31.927 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4768
  
  2019-05-23 21:14:32.174 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:32.433 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4700
  
  2019-05-23 21:14:32.668 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:32.864 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4740
  
  2019-05-23 21:14:33.102 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:33.365 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4640

  My questions :

  I. Is this the correct format way to read an h264 byte as mat ?
  Assuming the format is RGBA, that means row = 4 and columns = byte[].length, and CvType.CV_8UC4. Do I have height and width correct ? Something tells me YUV height and width is off. I was getting some meaningful results, but the contours were exactly in the center, just like with the H264.

  II. Does OpenCV handle MP4 in android like this ? If not, do I need to use FFMPEG or JavaCV ?

  III. Does the int size have something to do with it ? Why is the int size occassionally 6, and other times 2400 to 6000 ? I’ve heard about the difference between this frames information and information about the next frame, but I’m simply not knowledgeable enough to know how to apply that here.
  I’m starting to think this is where the issue lies. Since I need to get the 6 byte array for info about next frame, perhaps my modulo 30 is incorrect. So should I pass the 29th or 31st frame as a format byte for each frame ? How is that done in opencv or are we doomed to use to the the complicated ffmpeg.

  IV. Can I fix this using Imcodecs ? I was hoping opencv would natively handle whether a frame was color from this frame or info about next frame. I added the below code, but I am getting an empty array :

  Mat videoBufMat = Imgcodecs.imdecode(new MatOfByte(params[0]), Imgcodecs.IMREAD_UNCHANGED);

  This also is empty :

  Mat encodeVideoBuf = new Mat(4, params[0].length, CvType.CV_8UC4);
  
  encodeVideoBuf.put(0,0, params[0]);
  
  Mat videoBufMat = Imgcodecs.imdecode(encodeVideoBuf, Imgcodecs.IMREAD_UNCHANGED);

  V. Should I try converting the bytes into Android jpeg and then import it ? Why is djis yuv decoder so complicated looking ? It makes me cautious from wanting to try ffmpeg or Javacv and just stick to Android decoder or opencv decoder.

  VI. At what stage should I resize the frames to speed up calculations ?

• How to Read DJI H264 FPV Feed as OpenCV Mat Object ?

  29 mai 2019, par Walter Morawa

  TDLR : All DJI developers would benefit from decoding raw H264 video stream byte arrays to a format compatible with OpenCV.

  I’ve spent a lot of time looking for a solution to reading DJI’s FPV feed as an OpenCV Mat object. I am probably overlooking something fundamental, since I am not too familiar with Image Encoding/Decoding.

  Future developers who come across it will likely run into a bunch of the same issues I had. It would be great if DJI developers could use opencv directly without needing a 3rd party library.

  I’m willing to use ffmpeg or JavaCV if necessary, but that’s quite the hurdle for most Android developers as we’re going to have to use cpp, ndk, terminal for testing, etc. That seems like overkill. Both options seem quite time consuming. This JavaCV H264 conversion seems unnecessarily complex. I found it from this relevant question.

  I believe the issue lies in the fact that we need to decode both the byte array of length 6 (info array) and the byte array with current frame info simultaneously.

  Basically, DJI’s FPV feed comes in a number of formats.

  1. Raw H264 (MPEG4) in VideoFeeder.VideoDataListener

      // The callback for receiving the raw H264 video data for camera live view
  
      mReceivedVideoDataListener = new VideoFeeder.VideoDataListener() {
  
          @Override
  
          public void onReceive(byte[] videoBuffer, int size) {
  
              //Log.d("BytesReceived", Integer.toString(videoStreamFrameNumber));
  
              if (videoStreamFrameNumber++%30 == 0){
  
                  //convert video buffer to opencv array
  
                  OpenCvAndModelAsync openCvAndModelAsync = new OpenCvAndModelAsync();
  
                  openCvAndModelAsync.execute(videoBuffer);
  
              }
  
              if (mCodecManager != null) {
  
                  mCodecManager.sendDataToDecoder(videoBuffer, size);
  
              }
  
          }
  
      };

  2. DJI also has it’s own Android decoder sample with FFMPEG to convert to YUV format.

      @Override
  
      public void onYuvDataReceived(final ByteBuffer yuvFrame, int dataSize, final int width, final int height) {
  
          //In this demo, we test the YUV data by saving it into JPG files.
  
          //DJILog.d(TAG, "onYuvDataReceived " + dataSize);
  
          if (count++ % 30 == 0 && yuvFrame != null) {
  
              final byte[] bytes = new byte[dataSize];
  
              yuvFrame.get(bytes);
  
              AsyncTask.execute(new Runnable() {
  
                  @Override
  
                  public void run() {
  
                      if (bytes.length >= width * height) {
  
                          Log.d("MatWidth", "Made it");
  
                          YuvImage yuvImage = saveYuvDataToJPEG(bytes, width, height);
  
                          Bitmap rgbYuvConvert = convertYuvImageToRgb(yuvImage, width, height);
  
  
  
                          Mat yuvMat = new Mat(height, width, CvType.CV_8UC1);
  
                          yuvMat.put(0, 0, bytes);
  
                          //OpenCv Stuff
  
                      }
  
                  }
  
              });
  
          }
  
      }

  Edit : For those who want to see DJI’s YUV to JPEG function, here it is from the sample application :

  private YuvImage saveYuvDataToJPEG(byte[] yuvFrame, int width, int height){
  
          byte[] y = new byte[width * height];
  
          byte[] u = new byte[width * height / 4];
  
          byte[] v = new byte[width * height / 4];
  
          byte[] nu = new byte[width * height / 4]; //
  
          byte[] nv = new byte[width * height / 4];
  
  
  
          System.arraycopy(yuvFrame, 0, y, 0, y.length);
  
          Log.d("MatY", y.toString());
  
          for (int i = 0; i < u.length; i++) {
  
              v[i] = yuvFrame[y.length + 2 * i];
  
              u[i] = yuvFrame[y.length + 2 * i + 1];
  
          }
  
          int uvWidth = width / 2;
  
          int uvHeight = height / 2;
  
          for (int j = 0; j < uvWidth / 2; j++) {
  
              for (int i = 0; i < uvHeight / 2; i++) {
  
                  byte uSample1 = u[i * uvWidth + j];
  
                  byte uSample2 = u[i * uvWidth + j + uvWidth / 2];
  
                  byte vSample1 = v[(i + uvHeight / 2) * uvWidth + j];
  
                  byte vSample2 = v[(i + uvHeight / 2) * uvWidth + j + uvWidth / 2];
  
                  nu[2 * (i * uvWidth + j)] = uSample1;
  
                  nu[2 * (i * uvWidth + j) + 1] = uSample1;
  
                  nu[2 * (i * uvWidth + j) + uvWidth] = uSample2;
  
                  nu[2 * (i * uvWidth + j) + 1 + uvWidth] = uSample2;
  
                  nv[2 * (i * uvWidth + j)] = vSample1;
  
                  nv[2 * (i * uvWidth + j) + 1] = vSample1;
  
                  nv[2 * (i * uvWidth + j) + uvWidth] = vSample2;
  
                  nv[2 * (i * uvWidth + j) + 1 + uvWidth] = vSample2;
  
              }
  
          }
  
          //nv21test
  
          byte[] bytes = new byte[yuvFrame.length];
  
          System.arraycopy(y, 0, bytes, 0, y.length);
  
          for (int i = 0; i < u.length; i++) {
  
              bytes[y.length + (i * 2)] = nv[i];
  
              bytes[y.length + (i * 2) + 1] = nu[i];
  
          }
  
          Log.d(TAG,
  
                "onYuvDataReceived: frame index: "
  
                    + DJIVideoStreamDecoder.getInstance().frameIndex
  
                    + ",array length: "
  
                    + bytes.length);
  
          YuvImage yuver = screenShot(bytes,Environment.getExternalStorageDirectory() + "/DJI_ScreenShot", width, height);
  
          return yuver;
  
      }
  
  
  
      /**
  
       * Save the buffered data into a JPG image file
  
       */
  
      private YuvImage screenShot(byte[] buf, String shotDir, int width, int height) {
  
          File dir = new File(shotDir);
  
          if (!dir.exists() || !dir.isDirectory()) {
  
              dir.mkdirs();
  
          }
  
          YuvImage yuvImage = new YuvImage(buf,
  
                  ImageFormat.NV21,
  
                  width,
  
                  height,
  
                  null);
  
  
  
          OutputStream outputFile = null;
  
  
  
          final String path = dir + "/ScreenShot_" + System.currentTimeMillis() + ".jpg";
  
  
  
          try {
  
              outputFile = new FileOutputStream(new File(path));
  
          } catch (FileNotFoundException e) {
  
              Log.e(TAG, "test screenShot: new bitmap output file error: " + e);
  
              //return;
  
          }
  
          if (outputFile != null) {
  
              yuvImage.compressToJpeg(new Rect(0,
  
                      0,
  
                      width,
  
                      height), 100, outputFile);
  
          }
  
          try {
  
              outputFile.close();
  
          } catch (IOException e) {
  
              Log.e(TAG, "test screenShot: compress yuv image error: " + e);
  
              e.printStackTrace();
  
          }
  
  
  
          runOnUiThread(new Runnable() {
  
              @Override
  
              public void run() {
  
                  displayPath(path);
  
              }
  
          });
  
          return yuvImage;
  
      }

  3. DJI also appears to have a "getRgbaData" function, but there is literally not a single example online or by DJI. Go ahead and Google "DJI getRgbaData"... There’s only the reference to the api documentation that explains the self explanatory parameters and return values but nothing else. I couldn’t figure out where to call this and there doesn’t appear to be a callback function as there is with YUV. You can’t call it from the h264b byte array directly, but perhaps you can get it from the yuv data.

  Option 1 is much more preferable to option 2, since YUV format has quality issues. Option 3 would also likely involve a decoder.

  Here’s a screenshot that DJI’s own YUV conversion produces.

  I’ve looked at a bunch of things about how to improve the YUV, remove green and yellow colors and whatnot, but at this point if DJI can’t do it right, I don’t want to invest resources there.

  Regarding Option 1, I know there’s FFMPEG and JavaCV that seem like good options if I have to go the video decoding route.

  Moreover, from what I understand, OpenCV can’t handle reading and writing video files without FFMPEG, but I’m not trying to read a video file, I am trying to read an H264/MPEG4 byte[] array. The following code seems to get positive results.

      /* Async OpenCV Code */
  
      private class OpenCvAndModelAsync extends AsyncTask {
  
          @Override
  
          protected double[] doInBackground(byte[]... params) {//Background Code Executing. Don't touch any UI components
  
              //get fpv feed and convert bytes to mat array
  
              Mat videoBufMat = new Mat(4, params[0].length, CvType.CV_8UC4);
  
              videoBufMat.put(0,0, params[0]);
  
              //if I add this in it says the bytes are empty.
  
              //Mat videoBufMat = Imgcodecs.imdecode(encodeVideoBuf, Imgcodecs.IMREAD_ANYCOLOR);
  
              //encodeVideoBuf.release();
  
              Log.d("MatRgba", videoBufMat.toString());
  
              for (int i = 0; i< videoBufMat.rows(); i++){
  
                  for (int j=0; j< videoBufMat.cols(); j++){
  
                      double[] rgb = videoBufMat.get(i, j);
  
                      Log.i("Matrix", "red: "+rgb[0]+" green: "+rgb[1]+" blue: "+rgb[2]+" alpha: "
  
                              + rgb[3] + " Length: " + rgb.length + " Rows: "
  
                              + videoBufMat.rows() + " Columns: " + videoBufMat.cols());
  
                  }
  
              }
  
              double[] center = openCVThingy(videoBufMat);
  
              return center;
  
          }
  
          protected void onPostExecute(double[] center) {
  
              //handle ui or another async task if necessary
  
          }
  
      }

  Rows = 4, Columns > 30k. I get lots of RGB values that seem valid, such as red = 113, green=75, blue=90, alpha=220 as a made up example ; however, I get a ton of 0,0,0,0 values. That should be somewhat okay, since Black is 0,0,0 (although I would have thought the alpha would be higher) and I have a black object in my image. I also don’t seem to get any white values 255, 255, 255, even though there is also plenty of white area. I’m not logging the entire byte so it could be there, but I have yet to see it.

  However, when I try to compute the contours from this image, I almost always get that the moments (center x, y) are exactly in the center of the image. This error has nothing to do with my color filter or contours algorithm, as I wrote a script in python and tested that I implemented it correctly in Android by reading a still image and getting the exact same number of contours, position, etc in both Python and Android.

  I noticed it has something to do with the videoBuffer byte size (bonus points if you can explain why every other length is 6)

  2019-05-23 21:14:29.601 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2425
  
  2019-05-23 21:14:29.802 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2659
  
  2019-05-23 21:14:30.004 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:30.263 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6015
  
  2019-05-23 21:14:30.507 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:30.766 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4682
  
  2019-05-23 21:14:31.005 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:31.234 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 2840
  
  2019-05-23 21:14:31.433 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4482
  
  2019-05-23 21:14:31.664 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:31.927 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4768
  
  2019-05-23 21:14:32.174 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:32.433 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4700
  
  2019-05-23 21:14:32.668 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:32.864 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4740
  
  2019-05-23 21:14:33.102 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 6
  
  2019-05-23 21:14:33.365 21431-22086/com.dji.simulatorDemo D/VideoBufferSize: 4640

  My questions :

  I. Is this the correct format to read an h264 byte as mat ?
  Assuming the format is RGBA, that means row = 4 and columns = byte[].length, and CvType.CV_8UC4. Do I have height and width correct ? Something tells me YUV height and width is off. I was getting some meaningful results, but the contours were exactly in the center, just like with the H264.

  II. Does OpenCV handle MP4 in android like this ? If not, do we need to use FFMPEG or JavaCV ?

  III. Does the int size have something to do with it ? Why is the int size occassionally 6, and other times 2400 to 6000 ? I’ve heard about the difference between this frames information and information about the next frame, but I’m simply not knowledgeable enough to know how to apply that here.

  I’m starting to think this is where the issue lies. Since I need to get the 6 byte array for info about next frame, perhaps my modulo 30 is incorrect. So should I pass the 29th or 31st frame as a format byte for each frame ? How is that done in opencv or are we doomed to use the complicated ffmpeg ? How would I go about joining the neighboring frames/ byte arrays ?

  IV. Can I fix this using Imcodecs ? I was hoping opencv would natively handle whether a frame was color from this frame or info about next frame. I added the below code, but I am getting an empty array :

  Mat videoBufMat = Imgcodecs.imdecode(new MatOfByte(params[0]), Imgcodecs.IMREAD_UNCHANGED);

  This also is empty :

  Mat encodeVideoBuf = new Mat(4, params[0].length, CvType.CV_8UC4);
  
  encodeVideoBuf.put(0,0, params[0]);
  
  Mat videoBufMat = Imgcodecs.imdecode(encodeVideoBuf, Imgcodecs.IMREAD_UNCHANGED);

  V. Should I try converting the bytes into Android jpeg and then import it ? Why is djis yuv decoder so complicated looking ? It makes me cautious from wanting to try ffmpeg or Javacv and just stick to Android decoder or opencv decoder.

  VI. At what stage should I resize the frames to speed up calculations ?

  Edit : DJI support got back to me and confirmed they don’t have any samples for doing what I’ve described. This is a time for we the community to make this available for everyone !

  Upon further research, I don’t think opencv will be able to handle this as opencv’s android sdk has no functionality for video files/url’s (apart from a homegrown MJPEG codec).

  So is there a way in Android to convert to mjpeg or similar in order to read ? In my application, I only need 1 or 2 frames per second, so perhaps I can save the image as jpeg.

  But for real time applications we will likely need to write our own decoder. Please help so that we can make this available to everyone ! This question seems promising :