[FFmpeg-user] Automatically ensuring level-conformance for H.264 encodes

[Francois Visagie]

Here is something I started, partly to fulfil a need and partly to explore
Windows shell scripting. As it may be useful to others too, I'm taking the
liberty of sharing it here.

 

The main aim of the attached utilities is to automatically ensure the H.264
level-conformance of arbitrary encodes. In other words, the utilities make
it unnecessary to guess and/or discover the required settings through
trial-and-error test encodes.

 

Automatically ensuring level-conformance is useful for various reasons:

* It ensures maximum compatibility with the widest range of players (even in
cases where strict settings for a specific target are not required)

* Encoding time can be reduced by as much as 50% by using only as many
reference frames as a particular encode's level allows (relevant to
(lib)x264 presets slower than medium, and esp. true for veryslow)

* It takes the guesswork and test encodes out of changing between sources
with different frame sizes, frame rates, bitrates, etc.

 

h264levl calculates, for the given output video parameters, the expected
H.264 level and corresponding maximum reference frames. It supports both
manual and unattended operation, providing appropriate exit codes and
diagnostic output to STDERR for the latter.

 

Here is an example of how h264level can be called from another script:

 

...

setlocal enableextensions

setlocal enabledelayedexpansion

...

rem Calculate H.264 level and maxref

rem --------------------------------

for /f "usebackq tokens=1* delims==" %%i in (`"call h264levl %WIDTH%
%HEIGHT% %FPS% %BITRATE% %PROFILE% 1 1 "`) do (

    if /i [%%i] == [level] (

        set LEVEL=%%j

    ) else if /i [%%i] == [maxref] (

        set MAXREF=%%j

    ) else if /i [%%i] == [x264maxref] (

        set XMAXREF=%%j

    )

)

 

rem Check successful parsing

set PARLIST=

for %%i in (LEVEL MAXREF XMAXREF) do (

    if [!%%i!] == [] (

        if not [!PARLIST!] == [] set PARLIST=!PARLIST!, 

        set PARLIST=!PARLIST!'%%i'

    )

)

if not "%PARLIST%" == "" (

    set ERRMSG=Unable to calculate legal value/s for %PARLIST%^^!

    goto ERRPAUSE

)

...

 

FFx264 is an example of such an encoder wrapper script. In addition to
h264levl, it also uses ffprobe to establish video characteristics such as
frame rate and width and height (the latter two when not over-ridden by the
user).

 

Although FFx264 uses ffmpeg for encoding, its syntax is readily adapted for
e.g. x264.

 

Extract the files from the attachment, rename extensions from *.bta to *.bat
and double-click for instructions.

 

Known restrictions

* These utilities have only been properly tested on 8-bit video

 

François Visagie

 

 

T: +27 (83) 326-4358

F: +27 (86) 511-3490

E: francois.visagie at gmail.com

 

 

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