[MPlayer-DOCS] More XviD encoding options documented for XML doc

[Guillaume POIRIER]

Hello folks,

Here are some additional encoding option which are in their way to the XML docs.
Any comments would be greatly appreciated:

<listitem><para>
  <emphasis role="bold">chroma_opt</emphasis>
    Is intended to increase chroma image quality around pure
    white/black edges, rather than improving compression.
    This can help to reduce the "red stairs" effect.
</para></listitem>

<listitem><para>
  <emphasis role="bold">lumi_mask</emphasis>
    Tries to give less bitrate to part of the picture that the
    human eye cannot see very well, which should allow the encoder
    to spend the saved bits on more important parts of the picture.
    The quality of the encode yielded by this option highly depends
    on personal preferences and on the type and monitor settings
    used to watch it (typically, it will not look as good if it is
    bright or if it is a TFT monitor.
</para></listitem>

<listitem><para>
  <emphasis role="bold">qpel</emphasis>
    Increases the precision of the motion estimation from halfpel to
    quarterpel.
    The idea is to find better motion vectors which will in return
    reduce bitrate (hence increasing quality).
    However, quarterpel precision motion vectors are coded in the
    bitstream with a few bits, so if the content do not feature
    enough possibilities to find better motion vectors through
    <option>qpel</option>, it will in fact hurt compressibility.
    Unfortunately, there is no way to foresee the possible gains of
    <option>qpel</option>, so you need to actually encode with and
    without it to know for sure.
  </para><para>
    <option>qpel</option> can be almost double encoding time, and
    requires as much as 30-60% more processing power to decode.
    It is not supported by all standalone players.
</para></listitem>

<listitem><para>
  <emphasis role="bold">gmc</emphasis>
    Tries to save bits on panning scenes by using a single motion
    vector for the whole frame.
    This almost always raise PSNR, but significant slows down
    encoding (and also needs more processing power to decode).
    Therefore, you should only use it when you have turned
    <option>vhq</option> to the maximum.
    <systemitem class="library">XviD</systemitem>'s GMC (which are
    more sophisticated than DivX's are not supported by all
    standalone players.
</para></listitem>

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