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• Use ffmpeg libraries to convert stream formats

  17 septembre 2021, par Syrinx

  I'm attempting to write a small program and link it to a minimal set of ffmpeg libraries, like libavformat and whatever other libraries I need.

  


  I am looking for documentation to get me started, or maybe a quick fix to the example program I am using.

  


  I know ffmpeg (the project) provides example programs to help developers get started. I'm using the transcoding example, as it's close to my end goal, but it exits during init with an error about an audio issue.

  


  Here I am using the transcoding example program that come with ffmpeg v4.4, on Ubuntu 18.04. My input source has one video channel (h264) and one audio channel (pcm_mulaw).

  


  $ LD_LIBRARY_PATH=../../dist/lib ./transcoding rtsp://ip-camera/stream out.flv
...
  Stream #0:0: Video: h264, yuv420p, 1280x720, q=2-31, 20 tbn
  Stream #0:1: Audio: pcm_mulaw, 8000 Hz, 0 channels, s16
auto_resampler_0 @ 0x55da787fa140] [SWR @ 0x55da787fa5c0] Rematrix is needed between mono and 0 channels but there is not enough information to do it
[auto_resampler_0 @ 0x55da787fa140] Failed to configure output pad on auto_resampler_0


  


  In libswresample/swresample.c :

  


  320      if ((!s->out_ch_layout || !s->in_ch_layout) && s->used_ch_count != s->out.ch_count && !s->rematrix_custom) {
321          av_log(s, AV_LOG_ERROR, "Rematrix is needed between %s and %s "
322                 "but there is not enough information to do it\n", l1, l2);
323          ret = AVERROR(EINVAL);
324          goto fail;
325      }


  


  I'd really like it if I could make the transcoding example program work (fix it, or maybe use it appropriately if I am misunderstanding something). But short of that, where should I look for documentation about using the ffmpeg libraries ?

  


  I don't even care about the audio. If I can just disable the audio, I would be happy with that solution. I tried tracking the "-an" option to ffmpeg (the program) to see how it does that in source code, but the options handling is a mess and I can't distinguish the parts of the code that I need from all the noise.

  


  ffmpeg has web pages like this that aren't useful at all. There is documentation in the source code that looks like it should be viewed as HTML, but I don't see it exported anywhere. "make doc" generates a very small set of man pages that are insufficient to get me started.

  


• build : add support for building CUDA files with clang

  30 juillet 2019, par Rodger Combs

  build : add support for building CUDA files with clang
  This avoids using the CUDA SDK at all ; instead, we provide a minimal
  
  reimplementation of the basic functionality that lavfi actually uses.
  
  It generates very similar code to what NVCC produces.
  The header contains no implementation code derived from the SDK.
  
  The function and type declarations are derived from the SDK only to the
  
  extent required to build a compatible implementation. This is generally
  
  accepted to qualify as fair use.
  Because this option does not require the proprietary SDK, it does not require
  
  the "—enable-nonfree" flag in configure.
  Signed-off-by : Timo Rothenpieler <timo@rothenpieler.org>
  

  • [DH] Changelog
  • [DH] compat/cuda/cuda_runtime.h
  • [DH] configure
  • [DH] ffbuild/common.mak

• gcc "relocation R_X86_64_PC32 against symbol `ff_M24A'" error when linking statically against ffmpeg on linux

  22 janvier 2016, par YB_Evil

  I am trying to build a JNI shared library which statically links to ffmpeg.

  But at the linking stage, gcc fails with the following error :

  /usr/bin/ld: ./lib_lin64/libswscale.a(swscale.o): relocation R_X86_64_PC32 against symbol `ff_M24A' can not be used when making a shared object; recompile with -fPIC

  I am using the following commands to compile my jni library :

  gcc -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -I ./include -fPIC -c *.c
  

  gcc -shared -Wl,--no-undefined -o libnv_avc_dec.so *.o -Wl,-Bstatic -L./lib_lin64 -lavcodec -lavutil -lswresample -lswscale -Wl,-Bdynamic -lm

  And I only use h264 decoding feature, so I am also building ffmpeg from source with the minimal required feature set. The ./configure command I use is :

  ./configure \
--enable-pic --prefix=ffmpeg-dist \
--disable-debug --enable-version3 --enable-gpl \
--disable-everything --enable-hwaccel=h264_vdpau --enable-hwaccel=h264_vaapi --enable-hwaccel=h264_qsv --enable-hwaccel=h264_mmal \
--enable-decoder=h264 --enable-decoder=h264_vdpau --enable-decoder=h264_crystalhd --enable-decoder=h264_mmal --enable-decoder=h264_qsv \
--disable-iconv --disable-securetransport --disable-xlib --disable-zlib --disable-lzma  --disable-bzlib  --disable-doc --disable-programs --disable-avformat --disable-avfilter --disable-postproc

  So, as I understand, the linker tells me that ffmpeg should be compiled with -fPIC flag in order to make a shared library. But I believe that I’ve already done so by specifying the —enable-pic configure flag. And I am pretty much stuck here because I am not very familiar with autotools, nor with ffmpeg build process in particular.

  If this is the issue of ffmpeg .a libs not being compiled with -fPIC flag, how can i force it ? And if this is not the case, what am i doing wrong and how can i fix this error ?

  Environment details : Ubuntu 14.04.3 64-bit in Virtualbox, gcc 4.8.5 and 5.3 (both give the same results), ffmpeg v.2.8.5