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• network : prevent SIGPIPE on OSX

  30 mai 2015, par wm4

  network : prevent SIGPIPE on OSX
  OSX does not know MSG_NOSIGNAL, and provides its own non-standard
  
  mechanism instead. I guess Apple hates standards.
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavformat/network.c

• Lossless avi encoding on linux

  31 août 2012, par JohnSavage

  I am trying to write video using opencv. It is important for me to do this precisely - so it has to be a lossless codec. I am working with OpenCV 2.4.1 on Ubuntu 12.04

  Previously, I was using the fourcc code 0. This gave me the exact result I wanted, and I was able to recover the images perfectly.

  I am not sure what happened, but as of a recent update (around Jul 20th 2012), something went wrong and I am no longer able to write files with this fourcc code. I really don't remember what it was, but it could have come from doing an update, removing some software from my software center, and some other things I did during general cleaning...

  When I check an older file with mediainfo (http://www.fourcc.org/identifier/) I see the following result :

  Complete name                            : oldsample.avi
  
  Format                                   : AVI
  
  Format/Info                              : Audio Video Interleave
  
  Format profile                           : OpenDML
  
  File size                                : 1.07 GiB
  
  Duration                                 : 41s 467ms
  
  Overall bit rate                         : 221 Mbps
  
  Writing application                      : Lavf53.5.0
  
  Video
  
  ID                                       : 0
  
  Format                                   : RGB
  
  Codec ID                                 : 0x00000000
  
  Codec ID/Info                            : Basic Windows bitmap format. 1, 4 and 8 bpp     versions are palettised. 16, 24 and 32bpp contain raw RGB samples
  
  Duration                                 : 41s 467ms
  
  Bit rate                                 : 221 Mbps
  
  Width                                    : 640 pixels
  
  Height                                   : 4294966 816 pixels
  
  Display aspect ratio                     : 0.000
  
  Frame rate                               : 30.000 fps
  
  Bit depth                                : 8 bits
  
  Stream size                              : 1.07 GiB (100%)

  Now, I see that when I write using the 0 fourcc codec, the program actually defaults to the i420 codec. Here is the output from one of the files I try to write now :

  Complete name                            : newsample.avi
  
  Format                                   : AVI
  
  Format/Info                              : Audio Video Interleave
  
  File size                                : 73.0 MiB
  
  Duration                                 : 5s 533ms
  
  Overall bit rate                         : 111 Mbps
  
  Writing application                      : Lavf54.6.100
  
  Video
  
  ID                                       : 0
  
  Format                                   : YUV
  
  Codec ID                                 : I420
  
  Codec ID/Info                            : 8 bit Y plane followed by 8 bit 2x2 subsampled U and V planes.
  
  Duration                                 : 5s 533ms
  
  Bit rate                                 : 111 Mbps
  
  Width                                    : 640 pixels
  
  Height                                   : 480 pixels
  
  Display aspect ratio                     : 4:3
  
  Frame rate                               : 30.000 fps
  
  Compression mode                         : Lossless
  
  Bits/(Pixel*Frame)                       : 12.000
  
  Stream size                              : 72.9 MiB (100%)

  This format, and other formats I try to use (like huffyuv HFYU), do not work for me because I end up with effects like this http://imgur.com/a/0OC4y - you see the bright artifacts coming in due to what I assume is either lossy compression or chroma subsampling in the case of HFYU which is supposed to be lossless. What you are looking at is the red channel from one of my videos. The perceptual effect is negligible when you look at all 3 channels simultaneously but it is essential that I reconstruct the images exactly.

  Furthermore, while I am able to play my old files in media players like vlc, I suddenly find them to be completely incompatible with opencv. When I try to open the older files with a videocapture, the open step works fine, but trying to do a read operation results in a segfault. Furthermore, When I try to write with either :

  CV_FOURCC(0,0,0,0)
  
  0

  Opencv defaults to I420 for some reason.

  Next, I tried using some alternate codecs. 'DIB ' seems like something that should work for me, and on the opencv website (http://opencv.willowgarage.com/wiki/VideoCodecs) it is listed as a 'recommended' codec. However, trying to use this results in the following message :

  OpenCV-2.4.1/modules/highgui/src/cap_gstreamer.cpp:483: error: (-210) Gstreamer Opencv backend doesn't support this codec acutally. in function CvVideoWriter_GStreamer::open
  
  
  
  Aborted (core dumped)

  I checked the opencv source for this codec, and stumbled across the following :

  cd OpenCV-2.4.1/modules
  
  grep -i -r "CV_FOURCC" ./*
  
  ...
  
  ./highgui/src/cap_qt.cpp:    /*if( fourcc == CV_FOURCC( 'D', 'I', 'B', ' ' ))
  
  ./highgui/include/opencv2/highgui/highgui_c.h:#define CV_FOURCC_DEFAULT CV_FOURCC('I', 'Y', 'U', 'V') /* Use default codec for specified filename (Linux only) */

  I tried installing qt4 and reconfiguring with the WITH_QT flag, but that did not change anything. I also tried uncommenting that part of the code and reinstalling opencv, but that also did not work.

  My ultimate goal is for any way to efficiently store and retrieve a video stream with 16 bits for every pixel (like 32float would work fine, and then it wouldn't need to be perfect). Right now I am unpacking the 16 bits into the red and green channels, which is why I need it to be perfect - since an error of 1 in the red channel is multiplied by 256 in the final result. I am not having success with any of the fourcc codes available to me.

• Is it possible to have two images in a single frame in motion jpeg ?

  29 avril 2015, par skipper

  I have a quick time video file, video stream is in motion jpeg format, I extract every frame in the file with

  ffmpeg -i a.mov -vcodec copy -f image2 %d.jpg

  I found that in every jpeg file, there are actually two FFD8 marker, which means there are actually two images in one single jpeg file.

  Is this correct ? Is the file interlaced ? Anything special need to pass to codec ?