Changing PAR in H.264 bitstream using FFmpeg

[Selur]

Using a file named 'h264_par_1.mp4' with the following characteristics:

h264_par_1 had:
Sampled_Width : 640
Sampled_Height : 352
Pixel aspect ratio : 1.000 <-
Display aspect ratio : 1.818
Display aspect ratio : 16:9

one can use:
ffmpeg -y -i "E:\Output\h264_par_1.mp4" -map 0:0 -vcodec copy -an -sn -bsf:v h264_metadata=sample_aspect_ratio=2 "E:\Output\h264_par_2.mp4" (seems to work fine with any x264 encode)
to change the PAR from 1 to 2.

h264_par_2 has:
Sampled_Width : 640
Sampled_Height : 352
Pixel aspect ratio : 2.000 <-
Display aspect ratio : 3.636
Display aspect ratio : 3.636

Problem is if the input stream has no PAR data like 'Attica The Movie - Trailer By Brass':
(which I uploaded to my GoogleDrive (https://drive.google.com/open?id=19wKFoFAW3xEUaOuoC0dDSPN3ISEQswv_)

Width : 1280
Width : 1 280 pixels
Height : 720
Height : 720 pixels
Pixel aspect ratio : 1.000
Display aspect ratio : 1.778
Display aspect ratio : 16:9
note: MediaInfo shows no Sampled Width/Height

ffmpeg -y -i "F:\TestClips&Co\files\MPEG-4 H.264\Attica The Movie - Trailer By Brass.mp4" -map 0:0 -vcodec copy -an -sn -bsf:v h264_metadata=sample_aspect_ratio=2 "E:\Output\test.mp4"
fails with

[AVBSFContext @ 000001bcad3b5b00] rbsp_alignment_zero_bit out of range: 1, but must be in [0,0].
[AVBSFContext @ 000001bcad3b5b00] Failed to read unit 0 (type 7).
[AVBSFContext @ 000001bcad3b5b00] Failed to read extradata.
Error initializing bitstream filter: h264_metadata


My current guess is that the original stream has no explicit PAR (/ Sampled Width/Height) info which is why ffmpegs bitstream filter can't modify those values and change the PAR.

-> Does anyone know a way to change the PAR of such a stream without reencoding (preferably with ffmpeg)?

Cu Selur

[Selur]

I found a workaround by first remuxing to mkv using mkvtoolnix and then using
ffmpeg -y -i "E:\Output\Attica The Movie - Trailer By Brass.mkv" -map 0:0 -vcodec copy -an -sn -bsf:v h264_metadata=sample_aspect_ratio=2 "E:\Output\test.mp4"
still would be nice to do this in one go with ffmpeg.

---
for completeness here the calls I used in my workaround:
1. extracting the video stream
MP4Box -raw 1 "F:\TESTCL~1\files\MPEG-4~1.264\ATTICA~1.MP4" -out "E:\Temp\Attica The Movie - Trailer By Brass_new_09_16_09_7710_01.264"
2. muxing to mkv
mkvmerge --ui-language en -o "E:\Output\Attica The Movie - Trailer By Brass.mkv" -d 0 --default-track 0:yes --default-duration 0:30/1fps --aspect-ratio-factor 0:1/1 --no-chapters --compression -1:none --forced-track 0:yes --no-audio --no-subtitles "E:\Temp\Attica The Movie - Trailer By Brass_new_09_16_09_7710_01.264
3. adjusting the PAR
ffmpeg -y -i "E:\Output\Attica The Movie - Trailer By Brass.mkv" -map 0:0 -vcodec copy -an -sn -bsf:v h264_metadata=sample_aspect_ratio=2 "E:\Output\test.mp4"


Cu Selur

[StainlessS]

Some related links here

https://forum.videohelp.com/threads/301849-Change-Aspect-Ratio-of-mp4-without-re-encoding

http://superuser.com/questions/61894/incorrect-aspect-ratio-in-mp4-h-264-generated-by-samsung-camcorder

http://forum.videohelp.com/threads/316259-Can-I-change-the-pixel-aspect-ratio-in-an-x264-AVI-without-reencoding

http://forum.doom9.org/showthread.php?t=152419

[Selur]

@StainlessS: Thanks, sadly
MP4Box only sets the container not the stream PAR, the ffmpeg version in the thread you linked to is dated and the stuff it can do is replaced with the bitstream filters in ffmpeg, see: https://ffmpeg.org/ffmpeg-bitstream-filters.html