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• Get MP3 content in C ? ffmpeg or mpg123lib ?

  20 novembre 2014, par Nicolás Múnera

  I’m trying for a personal project of mine to mix two mp3 files into one. I’ve been investigating for a couple of days how to read an mp3 with C and i’ve come up with two libraries, ffmpeg and mpg3lib, unfortunately the documentation is a little bit confusing.

  For now I’m trying to get the content of one MP3 file, I just want to understand which parts are valuable and which parts of the data I get are the ones i’m supposed to mix together. This is what i’ve got so far :

  int cont = 0;
  
  fprintf(stderr, "Starting decode...\n");
  
  while(1)
  
  {
  
           len = fread(buf, sizeof(unsigned char), INBUFF, in);
  
           if(len <= 0)
  
                   break;
  
           inc += len;
  
           ret = mpg123_feed(m, buf, len);
  
  
  
           while(ret != MPG123_ERR && ret != MPG123_NEED_MORE)
  
           {
  
                   ret = mpg123_decode_frame(m, &num, &audio, &bytes);
  
                   if(ret == MPG123_NEW_FORMAT)
  
                   {
  
                       mpg123_getformat(m, &rate, &channels, &enc);
  
                       initwavformat();
  
                       initwav();
  
                       fprintf(stderr, "New format: %li Hz, %i channels, encoding value %i\n", rate, channels, enc);
  
                   }
  
  
  
                   printf("Frame # %d: %s\n",cont,audio);          
  
                   fwrite(audio, sizeof(unsigned char), bytes, out);
  
                   outc += bytes;
  
           }
  
       if(ret == MPG123_ERR){
  
               fprintf(stderr, "Error: %s", mpg123_strerror(m));
  
               break; 
  
       }
  
   cont++;
  
   }

  With this code, I think i’m getting the content of the MP3, but when I print it I just get some weird characters, so I don’t know which strategy i should take.

  Could you guys give me a little bit of advice with this ?

  Thank you !

• Compress video like whatsapp

  12 février 2016, par Copernic

  I’m not an expert in Video Editing but what I want to understand the logic of Whatsapp video processing.

  First of all I have noticed that whatever the file is, Whatsapp sets the limit of Uploaded videos to 16MO, after which whatsapp crops the video to not exceed the limit. is this a convention or it’s a personal choice ?

  Secondly, When a video is recorded using the Camera it’s not compressed by default, so whatsapp compresses it using FFMPEG I guess, and it takes no time. (tried for a video of 1min 1920x1080 with 125MO of size, becomes 640x360 with 5MO of size in no time, and the upload starts automatically).. how may they do this ? and why the choice of 640x360, It seems to me very fast for 2 asynchronous tasks : Compression + Upload.

  When I run the compression command ffmpeg -y -i in.mp4 -codec:v libx264 -crf 23 -preset medium -codec:a libfdk_aac -vbr 4 -vf scale=-1:640,format=yuv420p out.mp4 it takes approximatively 1 min and the video is being rotated !! :D

  Finally, when we download a video from Youtube it’s already compressed (I guess) and whatsapp doesn’t even try to compress it. So I think that it automatically detects thats the video is compressed. How can we detect this ?

  Thank you.

• pnacl-clang doesn't know where ffmpeg libraries are (but Eclipse does ?)

  10 août 2014, par lavsprat

  I’m trying to make my first "hello world"-like app using ffmpeg libraries. I already got NaCl SDK and downloaded & compiled the ffmpeg port.

  This is my code :

  main.c

  #include <libavformat></libavformat>avformat.h>
  
  
  
  int main()
  
  {
  
      av_register_all();
  
      return 0;
  
  }

  Building with $ (...)/pnacl-clang main.c -o main -lavformat in terminal.

  The output :

  main.c:2:10: fatal error: 'libavformat/avformat.h' file not found
  
  #include <libavformat></libavformat>avformat.h>
  
           ^

  Now, why am I not using -L(...)\lib and -I(...)\include in the build command ? Because it should work without it. In my workplace nacl-clang somehow knows where the libs are and compiles everything successfully. Why is that not working on my personal computer ? How can I permanently let pnacl-clang know where to look for them ?