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• Decoding mJPEG with libavcodec

  5 mai 2014, par dtumaykin

  I am creating video conference application. I have discovered that webcams (at least 3 I have) provide higher resolutions and framerates for mJPEG output format. So far I was using YUY2, converted in I420 for compression with X264. To transcode mJPEG to I420, I need to decode it first. I am trying to decode images from webcam with libavcodec. This is my code.

  Initialization :

  // mJPEG to I420 conversion
  
  AVCodecContext * _transcoder = nullptr;
  
  AVFrame * _outputFrame;
  
  AVPacket _inputPacket;
  
  
  
  avcodec_register_all();
  
  _outputFrame = av_frame_alloc();
  
  av_frame_unref(_outputFrame);
  
  av_init_packet(&_inputPacket);
  
  
  
  AVCodec * codecDecode = avcodec_find_decoder(AV_CODEC_ID_MJPEG);
  
  
  
  _transcoder = avcodec_alloc_context3(codecDecode);
  
  avcodec_get_context_defaults3(_transcoder, codecDecode);
  
  _transcoder->flags2 |= CODEC_FLAG2_FAST;
  
  _transcoder->pix_fmt = AVPixelFormat::AV_PIX_FMT_YUV420P;
  
  _transcoder->width = width;
  
  _transcoder->height = height;
  
  avcodec_open2(_transcoder, codecDecode, nullptr);

  Decoding :

  _inputPacket.size = size;
  
  _inputPacket.data = data;
  
  
  
  int got_picture;
  
  int decompressed_size = avcodec_decode_video2(_transcoder, _outputFrame, &got_picture, &_inputPacket);

  But so far, what I am getting is a green screen. Where am I wrong ?

  UPD :
  I have enabled libavcodec logging, but there are not warnings or errors.
  Also I have discovered that _outputframe has AV_PIX_FMT_YUVJ422P as format and colorspace, which does not fit any on values in libavcodec’s enums (the actual value is 156448160).

• Extract bitstream from a lossless WebP file ?

  2 mars 2021, par malat

  I am interested in removing the RIFF header from a lossless WebP image. I was hoping to use ffmpeg to create the bitstream directly :

  


  % ffmpeg -i input.png -f image2 -vcodec vp8 -q:v 0 lossless.vp8


  


  But quickly looking at the documentation, it seems the lossless mode is totally different from VP8 :

  


  


  Lossless images are a separate codec developed by Google.

  


  


  ref :

  


  

  • http://ffmpeg.org/ffmpeg-codecs.html#libwebp

  


  


  Are there any command line tool to remove RIFF header from an existing webp file (Debian/Linux) ? If not, can I use ffmpeg to create the bitstream (without RIFF container) ?

  


  % webpinfo -bitstream_info test.webp
File: test.webp
RIFF HEADER:
  File size: 176516
Chunk VP8L at offset     12, length 176504
  Width: 1280
  Height: 872
  Alpha: 0
  Animation: 0
  Format: Lossless (2)
  Parsing lossless bitstream...
  Width:            1280
  Height:           872
  Alpha:            0
  Version:          0
  Use transform:    Yes
  1st transform:    Subtract Green (2)
No error detected.


  


• php auto FFmeg encoding shell scrip failing - but command works in cmd [closed]

  11 mars 2020, par altruios

  $dateString = strtotime("now");
  
  $cmd = 'C:\xampp\htdocs\gameOfLife\images>ffmpeg -r 19 -f image2  -i image_%d.png output_'.$dateString.'.mov';
  
  shell_exec($cmd);

  the above works in the command line-but when trying to execute in php it does nothing.
  I am not getting errors, just no result.
  but when I navigate to the folder in the command line, and run that line - it creates a video correctly.
  I’ve checked that shell_exec() is in fact available to use.
  the string outputs fine - so it’s not the date.
  is the start of the cmd correct ?
  I’m a bit green on the command line in general, and could use some clarification on where I’m going wrong.

  EDIT1 : shell_exe() executes stuff, but is not executing ffmpeg ;

  shell_exec("C:/xampp/htdocs/gameOfLife/images/image_1.png");

  opens the image.
  so the shell_exec as been changed to :

  shell_exec("C:/xampp/htdocs/gameOfLife/images/ffmpeg -r 19 -f image2  -i image_%d.png output_t2.mov");

  I’ve tried static names instead of having any date variable in there to keep things simple.

  when that text runs in the console : it works.

  however, when running it though php and the server - I can’t access ffmpeg, where I can access the calc and the image just fine through php.
  So I think I ruled out the path and access issues...

  and i can type the above and get an encoded video back if i do it though the console.
  but I can’t seem to make it run via php.