Writing x264 from OpenCV 3 with FFmpeg on Linux

I think your finding here is key:

FFMPEG backend with MP4 container natively uses other values as fourcc code: see ObjectType, so you may receive a warning message from OpenCV about fourcc code conversion.

The mp4 tag values implemented for ffmpeg confirm this, and are in the ff_mp4_obj_type[], in isom.c. The code in OpenCV's cap_ffmpeg_impl.hpp likely needs to be updated to support this. I poked around for an hour or two, realized it was non-trivial, and bailed.

One work-around is to output to an .avi file. There are numerous examples of people having trouble with OpenCV and mp4, and being told to use .mov or .avi. (Here's one.)

@Greg Kramida: setting isColor = false did not help for me: the message remained, and my output file became only ~48 bytes. According to the documentation it is a Windows flag -- whatever it is doing for me on Linux, it isn't good.

Did you confirm that you could generate x264 with ffmpeg, by itself?

After confirming that libx264 has value 0x21 when I invoke this on the command line:

ffmpeg -i x264-input.mp4 -vcodec libx264 -f mp4 x264-output.mp4

I decided to use 0x21 directly into VideoWriter.open(). That generates a valid and interpretable video file.

For reference, my software is ffmpeg 3.0:

ffmpeg -versionffmpeg version 3.0 Copyright (c) 2000-2016 the FFmpeg developersbuilt with gcc 4.9.2 (Debian 4.9.2-10)configuration: --enable-libx264 --enable-gpl --prefix=/usr/local --enable-shared --cc=`gcc -fPIC` --enable-libfdk-aac --enable-libx265 --enable-nonfree --enable-libmp3lamelibavutil      55. 17.103 / 55. 17.103libavcodec     57. 24.102 / 57. 24.102libavformat    57. 25.100 / 57. 25.100libavdevice    57.  0.101 / 57.  0.101libavfilter     6. 31.100 /  6. 31.100libswscale      4.  0.100 /  4.  0.100libswresample   2.  0.101 /  2.  0.101libpostproc    54.  0.100 / 54.  0.100

And OpenCV 3.1.0 configured with:

cmake \    -D WITH_IPP=ON \    -D INSTALL_CREATE_DISTRIB=ON \    -D CMAKE_BUILD_TYPE=Release \    -D CMAKE_INSTALL_PREFIX=/usr/local ..

x264 is the libx264-142:amd64 release for Debian Jessie.

The problem had nothing to do with the displayed warning. I was trying to write single-channel images, while the VideoWriter was expecting a 3-channel color image (default value of isColor, the 5-th argument to VideoWriter's constructor, is "true"). The solution was setting isColor to false.

The problem on OpenCV which message is:

OpenCV: FFMPEG: tag 0x34363248/'H264' is not supported with codec id 28 and format 'mp4 / MP4 (MPEG-4 Part 14)' OpenCV: FFMPEG: fallback to use tag 0x00000021/'!???'

It is produced by definitions of CV_FOURCC('H','2','6','4') this value do not corresponds with the value defined on isom.c

{ AV_CODEC_ID_H264 , 0x21 }

Then defining fourcc as 0x21 cv::VideoWriter works smooth,

cv::VideoWriter VF;VF.open(filename,0x21,fps,frameSize,true);

as Jameson comments above,

"I decided to use 0x21 directly into VideoWriter.open(). That generates a valid and interpretable video file."