How to properly wrap H264 into FLV with FFMPEG?

Question

First of all, the "properly" in the title refers to this related question, of which answer does not solve my problem.

tl;dr: There is a difference between encoding a video and directly storing it into FLV and doing this in two separate steps. I need to do it separately, how do I get the same result as doing it directly?

Nvidia's hardware encoder NVENC produces raw H.264 data without a container, which is difficult to play in most video players. For an Adobe AIR application, I need to wrap the video into the FLV format, for which I wanted to use FFMPEG:

ffmpeg -f h264 -i "input.h264" -c copy -f flv "output.flv"

This did not work as expected, because the first frame of each video treated this way is simply not shown. Each video is only displayed from the second frame, which is a shame for single-frame videos (using the GPU's hardware encoder for lightning-fast image compression only).

For inspection, I now reencode the input video twice: once directly to FLV output

ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f flv "A.flv"

and once to H.264, then shoving it into an FLV afterwards.

ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f h264 "reencode.h264"
ffmpeg -f h264 -i "reencode.h264" -c copy -f flv "B.flv"

The first video plays fine, the second does not. The resulting FLV of the direct approach (A.flv, see below) has a slightly different file structure, especially the NAL unit differs, which I suspect is the reason for the different behavior.

So, my question is: If I already have a H.264 video and only want it to be copied into an FLV container without being transcoded, but the file and frame headers should be filled in correctly as is done when actually transcoding, how do I tell this to FFMPEG? Are there commands for this, such as "-c copy butGenerateValidHeader"?

Here the relevant portions of the files:

Direct approach

ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f flv "A.flv"

A.flv

46 4C 56 01 01 00 00 00 09 00 00 00 00 12 00 00 // FLV header + metadata
B8 00 00 00 00 00 00 00 02 00 0A 6F 6E 4D 65 74
61 44 61 74 61 08 00 00 00 08 00 08 64 75 72 61
74 69 6F 6E 00 3F A0 E5 60 41 89 37 4C 00 05 77
69 64 74 68 00 40 93 80 00 00 00 00 00 00 06 68
65 69 67 68 74 00 40 8E F0 00 00 00 00 00 00 0D
76 69 64 65 6F 64 61 74 61 72 61 74 65 00 40 9E
84 80 00 00 00 00 00 09 66 72 61 6D 65 72 61 74
65 00 40 3E 00 00 00 00 00 00 00 0C 76 69 64 65
6F 63 6F 64 65 63 69 64 00 40 1C 00 00 00 00 00
00 00 07 65 6E 63 6F 64 65 72 02 00 0D 4C 61 76
66 35 37 2E 37 31 2E 31 30 30 00 08 66 69 6C 65
73 69 7A 65 00 40 F9 5C B0 00 00 00 00 00 00 09

00 00 00 C3 09 00 00 2B 00 00 00 00 00 00 00 17 // AVC sequence start
00 00 00 00
            01 4D 40 20 FF E1 00 17             // ?
                                    67 4D 40 20 // Sequence parameter set
95 A0 13 81 F7 EB 01 10 00 00 3E 80 00 0E A6 08
F1 C3 2A
         01 00 04                               // ?
                  68 EE 3C 80                   // Picture parameter set
                              00 00 00 36 09 01 // AVC NALU
94 9A 00 00 00 00 00 00 00 17 01 00 00 00
                                          00 01 // ?
94 91
      65                                        // IDR frame
        [B8 04 1D FF ...]
00 01 94 A5 09 00 00 05 00 00 00 00 00 00 00    // ?
                                             17 // AVC sequence end
02 00 00 00 00 00 00 10

Encoding first

ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f h264 "reencode.h264"

reencode.h264

00 00 00 01 67 4D 40 20 95 A0 13 81 F7 EB 01 10 // Sequence parameter set
00 00 3E 80 00 0E A6 08 F1 C3 2A
                                 00 00 00 01 68 // Picture parameter set
EE 3C 80
         00 00 00 01 65                         // IDR frame
                       [B8 04 1D FF ...]        // Frame data

Squeeze into container

ffmpeg -f h264 -i "reencode.h264" -c copy -f flv "B.flv"

B.flv

46 4C 56 01 01 00 00 00 09 00 00 00 00 12 00 00 // FLV header + metadata
A4 00 00 00 00 00 00 00 02 00 0A 6F 6E 4D 65 74
61 44 61 74 61 08 00 00 00 07 00 08 64 75 72 61
74 69 6F 6E 00 3F A4 7A E1 47 AE 14 7B 00 05 77
69 64 74 68 00 40 93 80 00 00 00 00 00 00 06 68
65 69 67 68 74 00 40 8E F0 00 00 00 00 00 00 0D
76 69 64 65 6F 64 61 74 61 72 61 74 65 00 00 00
00 00 00 00 00 00 00 0C 76 69 64 65 6F 63 6F 64
65 63 69 64 00 40 1C 00 00 00 00 00 00 00 07 65
6E 63 6F 64 65 72 02 00 0D 4C 61 76 66 35 37 2E
37 31 2E 31 30 30 00 08 66 69 6C 65 73 69 7A 65
00 40 F9 5B 40 00 00 00 00 00 00 09
                                    00 00 00 AF // AVC sequence start
09 00 00 05 00 00 00 00 00 00 00 17 00 00 00 00

00 00 00 10 09 01 94 BD 00 00 00 00 00 00 00 17 // AVC NALU
01 00 00
         00 00 00 00 01 67 4D 40 20 95 A0 13 81 // Sequence parameter set
F7 EB 01 10 00 00 3E 80 00 0E A6 08 F1 C3 2A
                                             00 // Picture parameter set
00 00 01 68 EE 3C 80
                     00 00 00 01 65             // IDR frame
                                   [B8 04 1D FF // Frame data
...]
00 01 94 C8 09 00 00 05 00 00 00 00 00 00 00    // ?
                                             17 // AVC sequence end
02 00 00 00 00 00 00 10

Update 08.08.2017: Added input and output files for examination

• input.h264
• ffmpeg -f h264 -i "input.h264" -c copy "copy.flv"
• ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f flv "A.flv"
• ffmpeg -f h264 -i "input.h264" -c:v h264_nvenc -f h264 "reencode.h264"
• ffmpeg -f h264 -i "reencode.h264" -c copy -f flv "B.flv"
• ffmpeg -f h264 -i "reencode.h264" -c copy -bsf:v extract_extradata -f flv "extradata.flv"

Answer 1

(1)

"If I already have a H.264 video and only want it to be copied into an FLV container without being transcoded, but the file and frame headers should be filled in correctly as is done when actually transcoding, how do I tell this to FFMPEG?"

You want muxing : Try below command (directly copies H.264 data into FLV container).

ffmpeg -i input.h264 -c:v copy output.flv

(2)

First frame of each video treated this way is simply not shown. Each video is only displayed from the second frame, which is a shame for single-frame videos...

How are you checking the frames to know it's showing frame 2 onwards only? Is it by AS3 code netStream.pause() or by media players like VLC?

In FLV each video frame goes into a Video Tag. With other codecs we can say put frame #1 into videoTag #1 etc.. but with H.264 the first tag always holds "AVC Decoder Configuration" data so the pixels of input video frame #1 will exist at FLV's videoTag #2.
Also with your first shown bytes... in "AVC NALU" section that 17 01 let's you know this is a keyframe (first frame is always a keyframe), MPEG decoders would never show the second frame (usually P-frame) if it did not at first decode keyframe (I-frame). Your frame #1 image exists somewhere...

I could not recreate this issue with a H.264 video of some timecode countdown. First frame begins at 00hh:00mm:00ss:00msec and the msec moves up frame-by-frame. After using command shown in point (1), I see first frame in VLC & MPC-HC and also in AS3 if I use appendBytes to feed decoder with just a single frame's data (byteArray packaged as a 1-frame FLV).

Answer 2

First of all your description of file structure contradicts to real structure in your binary A.flv and B.flv (they are backward). So which result do you really want? Like A.flv with start codes: 00 00 00 01 67 .. 00 00 00 01 68 .. 00 00 00 01 65 or like B.flv with avcC + 00 17 67 .. 00 04 68 .. 00 01 94 91 65?

Standard format for FLV is like B.flv with avcC (without start codes) and it is produced by default with: ffmpeg -i "reencode.h264" -c copy "C.flv": here is resulting C.flv.

If this file works for you than try to update your ffmpeg (I used self compiled version from master branch). As for format with start codes than I am dunno how to produce them.