Encoding

Overview¶

We discuss here how to use encoders in GPAC.

GPAC filter graph resolution always targets the shortest possible path between two filters to link them. This implies that for a given source, encoders may or may not be triggered depending on the destination.
For example, if the source is an AAC file and the destination is an ISOBMFF file, since ISOBMFF accepts AAC data inputs, no encoder will be used. But if the source is an AAC file and the destination is an MP3 file, the graph resolver will load a decoder and an MP3 encoder to move source data from AAC to MP3.

Encoding Video¶

Encoding from a raw video file¶

gpac -i source.yuv:size=1280x720 -o test.avc

The above command will encode the source.yuv YUV420 8-bits into AVC|H264 using default encoding parameters of the encoder used. This may load a filter chain to adapt the PCM data to another format (floating point, etc ...).

gpac -i source.yuv:size=1280x720 enc:c=avc -o test.mp4

The above command will encode the source.yuv YUV420 8-bits into AVC|H264 using default encoding parameters of the encoder used and mux this into MP4.

Discussion
Prior to GPAC 2.0, the following command had to be used:
gpac -i source.yuv:size=1280x720 enc:c=avc @ -o test.mp4
The link directive @ is used here to prevent the raw YUV/RGB data PID to link against the test.mp4 destination which would accept it. When a PID can directly connect to a filter without any adaptation chain, the graph resolver will not try to connect it to any other filters unless sourceIDs are explicitly set, as described in the general documentation. GPAC 2.0 and above simplified command line processing to avoid link directives in most cases.

The enc filter is an internal reserved filter class used by GPAC to load an encoder satisfying the format given by c=. This means that if several encoders provide this format, the first one with the highest priority will be used.

You can directly call your preferred encoder rather than letting the link resolver pick it for you:

gpac -i source.yuv:size=1280x720 ffenc:c=avc -o test.mp4

This will explicitly load the FFmpeg encoder to do the job.

Note that for ffenc, since the filter provides all encoders supported by your FFmpeg build, you may have several choices within FFmpeg itself. To specify the true desired codec rather than the format, use the codec name directly:

gpac -i source.yuv:size=1280x720 enc:c=libx264 -o test.mp4
gpac -i source.yuv:size=1280x720 enc:c=libx264rgb -o test.mp4

Warning: Hardware encoders of FFmpeg have not been tested yet!

Transcoding from a non raw video file¶

gpac -i source.mp4 c=avc -o test.avc

The above command will encode the video track in source.mp4 into AVC|H264 using default encoding parameters of the encoder used and dump this as Annex B format (start codes).

Discussion
Prior to GPAC 2.0, the following command had to be used:
gpac -i source.mp4 enc:c=avc @ -o test.avc
As explained previously, the link directive @ ensures that we do the transcoding. If we omit this directive and the source.mp4 file has an AVC|H264 track in it, this track will simply be dumped to test.avcwith no transcoding used. However, if we omit this and the file has video track NOT in AVC|H264 format, the video will be transcoded.

Setting encoder parameters¶

Currently, all encoders audio and video in GPAC are using FFmpeg. We therefore only illustrate encoder parameters with the ffenc filter.

gpac -i source.mp4 c=avc:b=2M -o test.avc

The above command will encode the video track in source.mp4 into AVC|H264 at 2 mbits per seconds.

gpac -i source.mp4 c=avc:fintra=2:b=500k -o test.avc

The above command will encode the video track in source.mp4 into AVC|H264 at 500 kbps and enforce intra frames using fintra every 2 seconds. This doesn't prevent the encoder to produce intra frames within this period (e.g. scene cut), but ensures you always have IDRs at the right period. This is obviously designed for HTTP Adaptive Streaming.

gpac -i source.mp4 c=avc::x264-params=no-mbtree:sync-lookahead=0::profile=baseline -o test.avc

The above command will encode the video track in source.mp4 into AVC|H264 and pass two options to FFmpeg encoder:

x264-params, with value no-mbtree:sync-lookahead=0
profile, with value baseline

Discussion
Note the usage of double colon in the command line. As explained here, this prevents the filter resolver to interpret arguments of x264-params as arguments of the ffenc filter; this is needed here since both gpac and libx264 use :-separated list of key=value pairs. Another possibility would be to change the set of separators used by GPAC using -seps.

To get a list of all generic options supported by the ffenc filter, use gpac -h ffenc.

To get a list of all options supported by a given encoder of the ffenc filter, for example libx264, use gpac -h ffenc:libx264.

gpac -i source.mp4 enc:c=avc:pfmt=yuv2 -o test.avc
The above command will encode the video track in source.mp4 into AVC|H264, converting the source data to YUV 422 8bit planar.

Discussion
FFmpeg usually comes with several variations of the same coding type, with different pixel format supported. For example, AVC|H264 encoding can use both libx264 and libx264_rgb encoders. If the source data is decoded into RGB by the decoder, the ffenc filter will match the libx264_rgb encoder as supporting RGB input and AVC format. The resulting file may be in a profile not supported by most devices !

Setting the pixel format using pfmt will make sure you encode in YUV format; alternatively, you can also enforce the encoder used by setting ffc=libx264.

Note: The pfmt option is a generic option of all present and future encoders in GPAC, while ffc option is specific to FFmpeg encoder.

Changing Resolution¶

You can use the ffsws filter to rescale videos in your pipeline:

gpac -i source.mp4 ffsws:osize=1280x720 c=avc:b=1m -o test.avc

This will resize (downscale or upscale) input to a resolution of 1280x720 without checking aspect ratio, and encode to AVC at 1 mbps. To keep aspect ratio, use ffsws:osize=1280x720:keepar=full.

Encoding Audio¶

Encoding from files¶

This is basically the same as video

gpac -i source.pcm:sr=48k:ch=2 -o test.aac

The above command will encode the source.pcm 48000 Hz stereo signed 16 bit PCM into AAC using default encoding parameters of the encoder used. This may load a filter chain to adapt the PCM data to another format (floating point, etc ...).

gpac -i source.mp4 enc:c=aac -o test.mp4

The above command will encode the source.mp4 audio tracks into AAC, even if the track is already in AAC format, and use FFmpeg aac encoder for the job. Note that the ffc option will only be used by the ffenc filter. If another filter provides AAC encoding and is used to encode the stream, the ffc option will likely be reported as not used by the filter session.

Changing sample rate¶

You can use the audio resampler filter to change your input signal characteristics:

gpac -i source.mp4 resampler:osr=48k:och=2 -o test.aac:b=128k

This will resample the input signal to stereo 48000 Hz and encode to AAC at 128k.

Transcoding AV file¶

Combine the above tow steps:

gpac -i av_source c=avc:b=2m c=aac:b=128k -o test.mp4

If you only want video transcode, omit the audio encoder:

gpac -i av_source c=avc:b=2m -o test.mp4

If you only want audio transcode, omit the video encoder:

gpac -i av_source c=aac:b=128k -o test.mp4