Add transcoding example #54

leandromoreira · 2020-01-01T22:57:52Z

After I learned a little I think this is the best examples to write/show the transcoding chapter:

1 - a transmuxing example - mp4 -> mp4 dbab188
2 - a transmuxing example - mp4 -> fmp4 9f31610
3 - a transcoding example - h264 -> h264 (fixed gop, no scenecut) 6ad6a82
4 - a transrating example - h264 -> h264 (fixed gop & CBR bit rate) 8ed2c1f

-b:v 1M -minrate 1M -maxrate 1M -bufsize 2M
5 - a transcoding example - h264 -> h265 6060871
6 - add the markdown chapter explanning the process
7 - (Bonus) a complete transmux|cod|ing example - mp4 -> webm, h264 -> vp9, aac -> opus

Issues

Trying to achieve 3 & 4

high bit rate (10.6 Mb/s) fixed ✅ by b5378f3 (had to use force-cfr 😢 I really don't understand rate control at this level)
warning messages (forced frame type (5) at 80 was changed to frame type (3)) fixed ✅ by 0abee08
variable FPS (59.99 fps) fixed ✅ by 8ed2c1f

Trying to achieve 3 & 4

warning messages specified frame type (5) at 180 is not compatible with keyframe interval ✅ by 0abee08

Wanna help me?

If you want to test or run it locally make sure you have docker installed and that you had ran make fetch_small_bunny_video to download the sample video.

git clone --single-branch --branch transcoding-chapter https://github.com/leandromoreira/ffmpeg-libav-tutorial.git

make run_transcoding

# this is going to create the file bunny_1s_gop.mp4

leandromoreira · 2020-01-01T23:13:08Z

Simple transmuxing and fmp4 is working!

leandromoreira · 2020-01-02T00:09:51Z

#52

leandromoreira · 2020-01-02T14:41:26Z

While coding the 3 38c6c78 I faced the following issues but it works:

the fps has changed 59.99 fps

Video: h264, yuv420p, 1920x1080, 7632 kb/s, 59.99 fps, 60 tbr, 16384 tbn, 0.03 tbc (default)

a single warning message has appeared

[libx264 @ 0x1e78200] forced frame type (5) at 597 was changed to frame type (3)

✅ Playable in VLC

leandromoreira · 2020-01-02T14:51:01Z

While dealing with 3 6ad6a82 now it seems that the gop is fixed but other issues arrived:

bit rate is high

Bit rate                                 : 10.6 Mb/s

fps still seems to be variable

Frame rate mode                          : Variable
Frame rate                               : 60.000 FPS
Minimum frame rate                       : 56.302 FPS
Maximum frame rate                       : 60.015 FPS

a lot of warning messages have appeared:

[libx264 @ 0x1a5a200] forced frame type (5) at 80 was changed to frame type (3)
[libx264 @ 0x1a5a200] forced frame type (3) at 81 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 140 was changed to frame type (3)
[libx264 @ 0x1a5a200] forced frame type (3) at 141 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 200 was changed to frame type (3)
[libx264 @ 0x1a5a200] forced frame type (3) at 201 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 310 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 369 was changed to frame type (3)
[libx264 @ 0x1a5a200] forced frame type (5) at 370 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 429 was changed to frame type (3)
[libx264 @ 0x1a5a200] forced frame type (5) at 430 was changed to frame type (1)
[libx264 @ 0x1a5a200] forced frame type (5) at 489 was changed to frame type (3)
...

✅ Playable in VLC

leandromoreira · 2020-01-02T17:34:05Z

When trying to change the bit rate. If you change only the AVCodecContext->bit_rate this is going to only change the nominal bit rate:

mediainfo bunny_1s_gop.mp4
Bit rate                                 : 39.9 Mb/s
Nominal bit rate                         : 2 000 kb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels

Changing more AVCodecContext rate controls parameters don't seem to affect:

AVCodecContext->bit_rate = 2000 * 1000;
AVCodecContext->rc_max_rate = 2000 * 1000;
AVCodecContext->rc_min_rate = 2000 * 1000;
AVCodecContext->rc_buffer_size = 2 * 2000 * 1000;

mediainfo
Bit rate                                 : 27.8 Mb/s
Nominal bit rate                         : 2 000 kb/s

Or even the x264opts bitrate:

  av_opt_set(sc->video_avcc->priv_data, "x264opts", "bitrate=2000:keyint=60:min-keyint=60:scenecut=-1", 0);

Or the generic options (minrate, maxrate, bufsize) via priv_data:

  av_opt_set(sc->video_avcc->priv_data, "minrate", "2M", 0);
  av_opt_set(sc->video_avcc->priv_data, "maxrate", "2M", 0);
  av_opt_set(sc->video_avcc->priv_data, "bufsize", "4M", 0);

mediainfo 
Bit rate                                 : 10.6 Mb/s

Tried codec private options, it didn't work as well:

  av_dict_set(&encoder_private_options , "b", "2.0M", 0);
  av_dict_set(&encoder_private_options , "minrate", "2.0M", 0);
  av_dict_set(&encoder_private_options , "maxrate", "2.0M", 0);
  av_dict_set(&encoder_private_options , "bufsize", "4.0M", 0);

leandromoreira · 2020-01-03T02:16:25Z

I tried to control rate by:

  int M = 100; // when I use 1000 it gets worst.
  sc->video_avcc->bit_rate = 2100 * M;
  sc->video_avcc->rc_max_rate = 2600 * M;
  sc->video_avcc->rc_min_rate = 2600 * M;
  sc->video_avcc->rc_buffer_size = 2 * 2600 * M;

The final bit rate is still no how it was suppose to be:

Bit rate                                 : 4 932 kb/s
Nominal bit rate                         : 210 kb/s

When I set `force-crf

av_opt_set(sc->video_avcc->priv_data, "x264opts", "keyint=60:min-keyint=60:scenecut=-1:bitrate=1700:force-cfr=1", 0);

It behaves better but not as expected:

Bit rate                                 : 1 388 kb/s
Nominal bit rate                         : 1 700 kb/s

Only after I set up force-cfrplus AVCodecContext->rc_*:

av_opt_set(sc->video_avcc->priv_data, "x264opts", "keyint=60:min-keyint=60:scenecut=-1:force-cfr=1", 0);

AVCodecContext->video_avcc->bit_rate = 2 * 1000 * 1000;
AVCodecContext->video_avcc->rc_buffer_size = 4 * 1000 * 1000;
AVCodecContext->video_avcc->rc_max_rate = 2 * 1000 * 1000;
AVCodecContext->video_avcc->rc_min_rate = 2.5 * 1000 * 1000;

Bit rate                                 : 2 000 kb/s

leandromoreira · 2020-01-04T01:52:51Z

While I'm trying to figure out all the issues I went deeper into ffmpeg (the command line) "transcode" path source code:

main
- transcode
  - transcode_init
    - init_output_stream
      - init_output_stream_encode
  - transcode_step

And I noticed that there are lots of lines of code to take care of guessing the frame rate, set up the context (timing, streaming params) to encode a video stream and many others checks and adjustments.

I think maybe it'll be too time-consuming for me and too hard for the readers to grasp 😞 therefore I think I'll reduce the chapter to: transmuxing (mp4->mp4, mp4->fmp4) and transcoding (h264->h264 fixed gop, h264->h265)

it was tested with aac

leandromoreira · 2020-01-05T17:26:09Z

I just put libvpx-vp9 as the video codec and it worked BUT again with a huge bit rate 😕 and it took too long even for a 10 second video.

Stream #0:0(und): Video: vp9, yuv420p(tv), 1920x1080, 93847 kb/s, 60 fps, 60 tbr, 15360 tbn, 15360 tbc (default)

leandromoreira added 2 commits January 1, 2020 19:49

first stepo transmuxing

dbab188

add fragmented mp4

9f31610

add basic transcoding

38c6c78

fix gop size

6ad6a82

leandromoreira added 2 commits January 2, 2020 22:04

removing comments

a527920

fix variable framerate

8ed2c1f

leandromoreira added 3 commits January 2, 2020 23:40

uses CBR

b5378f3

uses preset fast and new priv options key name

bdb9d60

transcode to x265

6060871

leandromoreira added 2 commits January 5, 2020 09:39

freeing all used components

3b6a644

add audio transcoding

0d25752

it was tested with aac

leandromoreira added 6 commits January 5, 2020 15:29

close open avformat

a28cd42

add chapter 3

98c7a56

add link to the chapter 3

fec8291

fix img location

dced354

add warning about how hard it was

59a9f02

add references to used functions

7343ccc

leandromoreira merged commit cff48de into master Jan 6, 2020

leandromoreira mentioned this pull request Feb 5, 2020

how to pass the input parameter mpegts_service_id #58

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Add transcoding example #54

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Add transcoding example #54

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Issues

Trying to achieve 3 & 4

Trying to achieve 3 & 4

Wanna help me?

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