Can you put the result of a blackdetect filter in a textfile using ffmpeg?

Question

I'm testing out the "blackdetect" filter in ffmpeg. I want to have the times when the video is black to be read by a script (like actionscript or javascript). I tried:

ffmpeg -i video1.mp4 -vf "blackdetect=d=2:pix_th=0.00" -an -f null -

And I get a nice result in the ffmpeg log:

ffmpeg version N-55644-g68b63a3 Copyright (c) 2000-2013 the FFmpeg developers
  built on Aug 19 2013 20:32:00 with gcc 4.7.3 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetyp
e --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --ena
ble-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-l
ibopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libsp
eex --enable-libtheora --enable-libtwolame --enable-libvo-aacenc --enable-libvo-
amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --
enable-libxvid --enable-zlib
  libavutil      52. 42.100 / 52. 42.100
  libavcodec     55. 28.100 / 55. 28.100
  libavformat    55. 13.103 / 55. 13.103
  libavdevice    55.  3.100 / 55.  3.100
  libavfilter     3. 82.100 /  3. 82.100
  libswscale      2.  5.100 /  2.  5.100
  libswresample   0. 17.103 /  0. 17.103
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video1.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 1970-01-01 00:00:00
    encoder         : Lavf53.13.0
  Duration: 00:02:01.54, start: 0.000000, bitrate: 275 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 768x432 [
SAR 1:1 DAR 16:9], 211 kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 59
 kb/s
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : SoundHandler
Output #0, null, to 'pipe:':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.13.103
    Stream #0:0(eng): Video: rawvideo (I420 / 0x30323449), yuv420p, 768x432 [SAR
 1:1 DAR 16:9], q=2-31, 200 kb/s, 90k tbn, 25 tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    : VideoHandler
Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> rawvideo)
Press [q] to stop, [?] for help
[null @ 00000000003279a0] Encoder did not produce proper pts, making some up.
[blackdetect @ 0000000004d5e800] black_start:0 black_end:17.08 black_duration:17
.08
[blackdetect @ 0000000004d5e800] black_start:62.32 black_end:121.48 black_durati
on:59.16
frame= 3038 fps=2317 q=0.0 Lsize=N/A time=00:02:01.52 bitrate=N/A
video:285kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.007543%

And I'm particularly interested in this part:

[blackdetect @ 0000000004e2e340] black_start:0 black_end:17.08 black_duration:17.08
[blackdetect @ 0000000004e2e340] black_start:62.32 black_end:121.48 black_duration:59.16

So my question:

1. Is there a way to only take the blackdetect filter output and put it in a .txt file?
2. And if this is possible, is there a way to do this in a statement with multiple video inputs? Like in this example

---

example:

ffmpeg -f concat -i mylist.txt -c copy concat.mp4

Where mylist.txt is a list of videos:

file 'video1.mp4'
file 'video2.mp4'
file 'video3.mp4'
file 'video4.mp4'

---

Basically what I want to have is one or more text files containing information about the black frames in every video in this list to be used by another program

Answer 1

If using ffprobe is acceptable there is now a simpler way to do it with metadata injection:

ffprobe -f lavfi -i "movie=input.mp4,blackdetect[out0]" -show_entries tags=lavfi.black_start,lavfi.black_end -of default=nw=1 -v quiet

The command outputs only the lavfi tags black_start and black_end.

The advantage is you can also get frame information etc, or multiple tags in a format which is easier to parse.

ffprobe can also output JSON or XML but I can't find a way to skip the frames with no tags which produce empty nodes. This should be easy to patch though.

Answer 2

See aergistal's answer for a newer, simpler method that was not available at the time this answer was made

Is there a way to only take the blackdetect filter output and put it in a .txt file?

By default ffmpeg logs to stderr. You can output to stdout and then use grep to isolate the blackdetect lines:

$ ffmpeg -i video1.mp4 -vf "blackdetect=d=2:pix_th=0.00" -an -f null - 2>&1 | grep blackdetect > output.txt

Resulting in:

$ cat output.txt 
[blackdetect @ 0x1d2b980] black_start:5.16 black_end:10.24 black_duration:5.08 

If you want to append to output.txt instead of overwriting for each instance then use >> instead of >, as in: blackdetect >> output.txt.

---

Is there a way to do this in a statement with multiple video inputs?

You can use blackdetect with the concat demuxer as shown in your question, but be aware that the black_start and black_end times will be cumulative, and not independent to each input, because you are concatenating all inputs.

Alternatively you can use a bash "for loop" if you want each input to be independently run through blackdetect:

for f in *.mp4; do ffmpeg -i "$f" -vf "blackdetect=d=2:pix_th=0.00" -an -f null - 2>&1 | grep blackdetect > "$f".txt; done

This may create empty txt files.