[MEncoder-users] (work-in-progress) XviD encoding guide

[Guillaume POIRIER]

Hello,

Here's a work-in-progress encoding guide for XviD MPEG4-ASP codec that
should one day make its way to our documentation.
As the world of video encoding is one of those areas where using this
or that set of options is often a matter of choice, I'd like to hear
what you may have to say about that document.

It still lacks some option, and says almost nothing about the PSNR
(in/de)crease to expect from using each option. If you have those
figures on hand, please feel free to send them to me.

It also lack encoding examples, and information about custom
quantization matrices, though it seems to me this subject is too
controversial to get engraved in stone in an official documentation.

All feedbacks are welcome, either technical, or about my louzy English
that you wish to improve.

Regards,
Guillaume

------------------------
<sect1 id="menc-feat-xvid">
<title>Encoding with the <systemitem class="library">XviD</systemitem>
codec</title>
<para>
  <systemitem class="library">XviD</systemitem> is a free library for
  encoding MPEG-4 ASP video streams.
  Before starting to encode, you need to <link linkend="xvid">
  set up <application>MEncoder</application> to support it</link>.
</para>
<para>
  This guide mainly aims at featuring the same kind of information
  as x264's encoding guide.
  Therefore, please begin by reading
  <link linkend="menc-feat-x264-intro">the first part</link> of that
  guide.
</para>

<sect2 id="menc-feat-xvid-intro">
<title>What options should I use to get the best results?</title>

<para>
  Please begin by reviewing the
  <systemitem class="library">XviD</systemitem> section of
  <application>MPlayer</application>'s man page.
  This section is intended to be a supplement to the man page.
</para>
<para>
  The XviD's default settings are already a good trade off speed vs.
  quality, therefore, if you can safely stick to them if the
  following section puzzles you.
</para>
</sect2>

<sect2 id="menc-feat-xvid-encoding-options">
<title>Encoding options of <systemitem class="library">XviD</systemitem></title>

<itemizedlist>

<listitem><para>
  <emphasis role="bold">vhq</emphasis>
    Default setting may safely be used on every encode, when higher
    settings always help quality, but are significantly slower.
    Turning it off will noticeably speed-up encode if you need speed.
</para></listitem>

<listitem><para>
  <emphasis role="bold">bvhq</emphasis>
    This does the same job as vhq, but does it on B-frames.
    It has a negligible impact on speed, and slightly improves quality
    (around +0.1dB PSNR).
</para></listitem>

<listitem><para>
  <emphasis role="bold">max_bframes</emphasis>
    A higher number of consecutive allowed B-frames usually improves
    compressibility while this may lead to more blocking artifacts.
    Default setting is a good trade-off of compressability vs. quality,
    but you may increase it up to 3 if you're bitrate-shorted, while
    you may also decrease it to 1 or 0 if you're aiming at perfect
    quality though this might waste more bits than the increased of
    subjective quality is worth.
</para></listitem>
<listitem><para>
  <emphasis role="bold">bf_threshold</emphasis>
    This controls the B-frame sensibility of the encoder, where a higher
    value leads to more b-frames being used (and vice versa).
    This setting is to be used together with <option>max_bframes</option>,
    so that if you are bitrate-shorted, you should increase both
    <option>max_bframes</option> and <option>bf_threshold</option>,
    while you may <option>max_bframes</option> and reduce
    <option>bf_threshold</option> so that the encoder may use more
    B-frames in places that only really need them.
    A low number of <option>max_bframes</option> and a high value of
    <option>bf_threshold</option> is probably not a wise choice as it
    will force the encoder to put B-frames in places that would not
    benefit from them, therefore reducing visual quality.
    However, if you need to be compatible with standalone players that
    only supports DivX profiles (which only supports up to 1 consecutive
    B-frame), this would be you only possibility to increase
    compressibility through using B-frames.
</para></listitem>

<listitem><para>
  <emphasis role="bold">trellis</emphasis>
    Optimizes the motion estimation and allows significant bit saving
    which would be spent elsewhere on the video, raising overall visual
    quality.
    You can safely leave it on as its impact on quality is very sensible,
    but if you're looking for speed, only disable it if you have turn
    down <option>vhq</option> to the minimum.
</para></listitem>

<listitem><para>
  <emphasis role="bold">cartoon</emphasis>
    Designed to better encode cartoon content, and has no impact on
    speed.
</para></listitem>

<listitem><para>
  <emphasis role="bold">me_quality</emphasis>
    This setting is to control the precision of the motion estimation.
    The higher <option>me_quality</option>, the more
    precise the estimation of the original motion will be, and the
    better the resulting clip will capture the original motion.
  </para>
  <para>
    Default setting is best in pretty much all cases, and it's not
    recommended to turn it down unless you're really looking for speed,
    as all the bits saved by a good motion estimation would be spend
    elsewhere, raising overall quality.
    Therefore, only go as low as 5, and only as a last resort.
</para></listitem>

<listitem><para>
  <emphasis role="bold">chroma_me</emphasis>
    Improves motion estimation by taking the chroma (the color)
    information into account, when <option>me_quality</option> alone
    only uses luma (greyscale).
    This slows down encoding by 5-10% but improves quite a bit visual
    quality by reducing blocking effect.
    If you're looking for speed, you should disable this option before
    starting to consider reducing <option>me_quality</option>.
</para></listitem>

</itemizedlist>
</sect2>
<sect2 id="menc-feat-xvid-example">
<title>Encoding examples</title>
</sect2>

</sect1>
-- 
A woman isn't complete without a man.
But where do you find a man - a real man - these
days?
  -- Lauren Bacall
My answer: Only at #mplayerdev on the freenode IRC network.