How to join split flv files?

Question

I'm in my 3rd day of trying to download one 4-minute video. It's a Flash FLV that's split into three parts, the first two are about 5MB, the third about 3MB.

Only the first part has normal FLV headers, the next two have mostly just an FLV at the start. I haven't resorted to trying to write something myself, just downloaded and tried a couple hundred megs of junky Windows software that doesn't actually do it.

As it turns out there's no need to download the video parts, you can pull them out of your browser's cache once you've played it through once. I've tried CamStudio to capture the video from the screen as it's being played, but my fastest computer isn't fast enough.

I can't do this realtime. I'd like to be doing this from OpenBSD but I've got one machine booted into Windows because there's no Flash for OpenBSD. I can play it with MPlayer (Windows or OpenBSD), trying to do a simple join of the parts using MEncoder yields a video that stops at the first split. Tried FFmpeg, too. I'd like to convert it to anything more normal, like MP4 or AVI.

The 10.1 spec looks a lot different from 10.0 (hotlink to PDF)

The three FLVs in MC viewer (picture); the first is quite different:

Answer 1

Not a complete answer yet but FLVMeta and a partial reading of Adobe's spec are beginning to shine light on things. From an FLVMeta full dump the tags or sections of data look like this:

    --- Tag #1019 at 0xCBEB5 (835253) ---
    Tag type: audio
    Body length: 213
    Timestamp: 124957
    * Sound type: stereo
    * Sound size: 16
    * Sound rate: 44
    * Sound format: AAC
    Previous tag size: 224
    --- Tag #1020 at 0xCBF99 (835481) ---
    Tag type: video
    Body length: 1201
    Timestamp: 124960
    * Video codec: AVC
    * Video frame type: inter frame
    Previous tag size: 1212
    --- Tag #1021 at 0xCC459 (836697) ---
    Tag type: audio
    Body length: 225
    Timestamp: 124980
    * Sound type: stereo
    * Sound size: 16
    * Sound rate: 44
    * Sound format: AAC
    Previous tag size: 236
    --- Tag #1022 at 0xCC549 (836937) ---
    Tag type: video
    Body length: 542
    Timestamp: 125000
    * Video codec: AVC
    * Video frame type: inter frame
    Previous tag size: 553

So you could read the header into one file, put all the tags into 1 file each, then reconstruct however many input files into however many output files you wanted. Each "tag" I'd call a block, but it's a chunk of data. You don't have to manipulate them in real time. Each has a timestamp, you just have to put them together in that order and don't split a tag across files.

I wish FLVMeta had more useful output for moving things around under program control like tab or comma-delimited data. Maybe even create an SQLite database for each project, put all the audio tags in one table, video in another, script in another. Maybe I'll do that yet since it's open source and on Github. It'd be simpler if flvs weren't big-endian and I'm on a little-endian machine. All integers are big-endian, like on a Mac.

Answer 2

Done finally. I ended up using a C program I wrote to concatenate them once I solved the bug that EOF on the first input file ended up as an FF or -1 in the output file, which stopped players and converters at that point.

flvmeta still gave warnings on the output of this but I was able to convert it to an mp4 using:

    ffmpeg -i out4.flv -vcodec copy -acodec copy out4.mp4


    /*
       My flv concat, a single-use program
    */

    #include <stdio.h>  // don't need most of these headers
    #include <stdlib.h>
    #include <string.h>
    #include <endian.h>  // FLVs have big endian values
    #include <unistd.h>
    #include <fcntl.h>
    #include <inttypes.h>

    FILE *opf;

    void docopy(char *fn, uint ofs) {
      FILE *ipf;
      unsigned char ch;
      ipf = fopen(fn,"r");
      if (ipf == NULL) {
        printf("Failed to open %s\n",fn);
        fclose(opf);
        exit(1);
      }
      fseek(ipf,ofs,SEEK_SET);  // jump to passed in offset
      while (!feof(ipf)) {  // not super efficient
        ch = fgetc(ipf);
      // this DOES have an effect, it stops the -1 from being written
        if (!feof(ipf))
          fputc(ch,opf);
      }
      printf("outfile now at %x\n",(unsigned int) ftell(opf));
      fclose(ipf);
    }

    void outhdr(void) { // write boilerplate flv header
      opf = fopen("out4.flv","w");
      if (opf == NULL) {
        printf("Error creating new output file.\n");
        exit(1);
      }
      fprintf(opf,"FLV%c%c%c%c%c%c",1,5,0,0,0,9); // audio and video enabled
      // This becomes the first PreviousTagSize:
      fprintf(opf,"%c%c%c%c",0,0,0,0);  // flvmeta seems to approve
    }

    int main(void) {
      outhdr();  // write a vanilla header
      docopy("media1.flv", 13);  // copy, starting at byte 13
      docopy("media2.flv", 13);
      docopy("media3.flv", 13);
      fclose(opf);
      return 0;
    }