How to choose between openH264 and x264 decoder

Question

I'm using the dev build from zeranoe.com which has OpenH264 and libx264 in it. How can i choose between these two Decoders to compare the decoding speed ?

avcodec_find_decoder(AVCodecID.AV_CODEC_ID_H264); 

only gives me the name "h264" but which decoder is it ?

And tries to force a specific decoder failed, like:

codec = avcodec_find_decoder _by_name("x264");
codec = avcodec_find_decoder _by_name("libx264");

Which other options i have to improve the decoding speed of avcodec_decode_video2 for highres (4k and higher) RTSP video streams ?

Benally

x264 and openH264 are encoders. AFAIK, ffmpeg only uses its native h264 decoder to decode. – Sappanwood

x264 is encoder only, open OpenH264 both encoder and decoder. So you either decode with built-in ffmpeg h264 decoder or with libopenh264 decoder. Decoding with x264 is currently not possible. – Siloum

Answer 1

FFmpeg can use OpenH264 (named libopenh264 in FFmpeg) to decode in addition to the native FFmpeg H.264 decoder (named h264). There are also several supported hardware assisted H.264 decoders, but I'm not going to go into detail about those.

OpenH264 vs FFmpeg H.264 decoder

• The documentation states that OpenH264 supports decoding of Constrained Baseline profile, so its usefulness may be limited. However it was able to decode a Main and a High profile video I tested. The actual capabilities appear to be undocumented, so it is unclear if it has partial or full support for Main and High profiles.

• The native FFmpeg H.264 decoder has threading capabilities (frame and slice) but OpenH264 does not. So the FFmpeg H.264 decoder is much faster than OpenH264.

• OpenH264 is licensed BSD 2-Clause "Simplified" License. FFmpeg H.264 decoder is LGPL 2.1 or later.

ffmpeg CLI tool examples

It appears you're working with the FFmpeg libraries, but here are some examples using ffmpeg.

To choose the decoder you want use the -c:v input option. Below are two decoding benchmark examples using the null muxer. The -map option is also used so only the (first) video stream is decoded and to ignore other, non-video streams such as audio.

Native FFmpeg H.264 decoder

$ ffmpeg -benchmark -i input.mp4 -map 0:v:0 -f null -
bench: utime=23.557s
bench: maxrss=57752kB
real    0m3.834s
user    0m23.573s
sys     0m0.213s

libopenh264

$ ffmpeg -benchmark -c:v libopenh264 -i input.mp4 -map 0:v:0 -f null -
bench: utime=20.927s
bench: maxrss=37404kB
real    0m21.002s
user    0m20.940s
sys     0m0.060s

Benchmark time

Note the benchmark option outputs user CPU time, not the elapsed real time. If you want real time as well add the time command before the ffmpeg command (if you're on Linux). I added the outputs from time in the example (the last three lines) for comparison purposes.

To verify the desired decoder is being used

View the console output. For example using the native H.264 decoder:

Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> wrapped_avframe (native))

Using libopenh264:

Stream mapping:
  Stream #0:0 -> #0:0 (h264 (libopenh264) -> wrapped_avframe (native))

Infirmary

Weird. ffmpeg -h decoder=h264 does not show openh264 as a decoder. – Sappanwood