[FFmpeg-user] HEVC decoding of first frame

[Christian Herglotz]

Hello everyone,

I've got a question regarding the decoding process for HEVC-coded 
sequences. When timing the process I found that the decoding of the 
first frame always takes twice as long as the decoding of all the other 
frames. This seems to be true independent from the resolution. It is 
also not  related to inter coding, as all frames are intra coded with 
the same QP in the example given below. Other decoders (e.g. HM or 
libde265) do not show this behavior for the same input sequences. Here 
is an examplary output for a single frame and for two frames. It is 
timed using the linux time-function, furthermore single core processing 
is forced and priority is set to the highest level.

System: Ubuntu 12.04, Pandaboard with OMAP4430 SoC

Command line for single frame:
  time -p taskset -c 0 nice -n-30 /root/bin/ffmpeg -i 
PeopleOnStreet_intra_1frames_QP10.bin -f rawvideo -y /dev/null

Output:
ffmpeg version 2.1.git Copyright (c) 2000-2014 the FFmpeg developers
   built on Feb 20 2014 16:27:13 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
   configuration: --prefix=/root/ffmpeg_build 
--extra-cflags=-I/root/ffmpeg_build/include 
--extra-ldflags=-L/root/ffmpeg_build/lib --bindir=/root/bin 
--extra-libs=-ldl --enable-gpl --enable-libass --enable-nonfree
   libavutil      52. 64.100 / 52. 64.100
   libavcodec     55. 52.102 / 55. 52.102
   libavformat    55. 33.100 / 55. 33.100
   libavdevice    55. 10.100 / 55. 10.100
   libavfilter     4.  1.103 /  4.  1.103
   libswscale      2.  5.101 /  2.  5.101
   libswresample   0. 17.104 /  0. 17.104
   libpostproc    52.  3.100 / 52.  3.100
Input #0, hevc, from 'PeopleOnStreet_intra_1frames_QP10.bin':
   Duration: N/A, bitrate: N/A
     Stream #0:0: Video: hevc (Main), yuv420p(tv), 2560x1600, 25 tbr, 
1200k tbn, 25 tbc
Output #0, rawvideo, to '/dev/null':
   Metadata:
     encoder         : Lavf55.33.100
     Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 
2560x1600, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
   Stream #0:0 -> #0:0 (hevc -> rawvideo)
Press [q] to stop, [?] for help
frame=    1 fps=0.3 q=0.0 size=    6000kB time=00:00:00.04 
bitrate=1228800.0kbitframe=    1 fps=0.3 q=0.0 Lsize=    6000kB 
time=00:00:00.04 bitrate=1228800.0kbits/s
video:6000kB audio:0kB subtitle:0 data:0 global headers:0kB muxing 
overhead 0.000000%
real 5.93
user 5.69
sys 0.22


Command line two frames:
  time -p taskset -c 0 nice -n-30 /root/bin/ffmpeg -i 
PeopleOnStreet_intra_2frames_QP10.bin -f rawvideo -y /dev/null

Output:
ffmpeg version 2.1.git Copyright (c) 2000-2014 the FFmpeg developers
   built on Feb 20 2014 16:27:13 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
   configuration: --prefix=/root/ffmpeg_build 
--extra-cflags=-I/root/ffmpeg_build/include 
--extra-ldflags=-L/root/ffmpeg_build/lib --bindir=/root/bin 
--extra-libs=-ldl --enable-gpl --enable-libass --enable-nonfree
   libavutil      52. 64.100 / 52. 64.100
   libavcodec     55. 52.102 / 55. 52.102
   libavformat    55. 33.100 / 55. 33.100
   libavdevice    55. 10.100 / 55. 10.100
   libavfilter     4.  1.103 /  4.  1.103
   libswscale      2.  5.101 /  2.  5.101
   libswresample   0. 17.104 /  0. 17.104
   libpostproc    52.  3.100 / 52.  3.100
Input #0, hevc, from 'PeopleOnStreet_intra_2frames_QP10.bin':
   Duration: N/A, bitrate: N/A
     Stream #0:0: Video: hevc (Main), yuv420p(tv), 2560x1600, 25 tbr, 
1200k tbn, 25 tbc
Output #0, rawvideo, to '/dev/null':
   Metadata:
     encoder         : Lavf55.33.100
     Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 
2560x1600, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
   Stream #0:0 -> #0:0 (hevc -> rawvideo)
Press [q] to stop, [?] for help
frame=    2 fps=0.7 q=0.0 size=   12000kB time=00:00:00.08 
bitrate=1228800.0kbitframe=    2 fps=0.3 q=0.0 size=   12000kB 
time=00:00:00.08 bitrate=1228800.0kbitframe=    2 fps=0.3 q=0.0 Lsize=   
12000kB time=00:00:00.08 bitrate=1228800.0kbits/s
video:12000kB audio:0kB subtitle:0 data:0 global headers:0kB muxing 
overhead 0.000000%
real 8.88
user 8.58
sys 0.28

Apparently, decoding of a single frame takes about 6s, decoding of two 
frames about 9s. Is it possible that the first frame is always decoded 
twice in this software?

Thanks for your help!
Christian Herglotz



-- 
Dipl.-Ing. Dipl.-Wirt.Ing. Christian Herglotz

Chair of Multimedia Communications and Signal Processing
Friedrich-Alexander University Erlangen-Nürnberg
Cauerstr. 7, D-91058 Erlangen, Germany