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New item: "Choosing resolution and bitrate", from Rich's encoding guide 

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@16086 b3059339-0415-0410-9bf9-f77b7e298cf2
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gpoirier 2005-07-24 20:53:54 +00:00
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DOCS/xml/en/encoding-guide.xml
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@ -950,7 +950,77 @@
Unfortunately, not all players enforce this auto-scaling information,
therefore you may still want to rescale.
</para>
</sect2>
<sect2 id="menc-feat-dvd-mpeg4-resolution-bitrate">
<title>Choosing resolution and bitrate</title>
<para>
If you will not be encoding in constant quantizer mode, you need to
select a bitrate.
The concept of bitrate is quite simple.
It's the (average) number of bits that will be consumed to store your
movie, per second.
Normally bitrate is measured in kilobits (1000 bits) per second.
The size of your movie on disk is the bitrate times the length of the
movie in time, plus a small amount of "overhead" (see the section on
<link linkend="menc-feat-dvd-mpeg4-muxing-avi-limitations">the AVI container</link>
for instance).
Other parameters such as scaling, cropping, etc. will
<emphasis role="bold">not</emphasis> alter the file size unless you
change the bitrate as well!.
</para>
<para>
Bitrate does <emphasis role="bold">not</emphasis> scale proportional
to resolution.
That is to say, a 320x240 file at 200 kbit/sec will not be the same
quality as the same movie at 640x480 and 800 kbit/sec!
There are two reasons for this:
<orderedlist>
<listitem><para>
<emphasis role="bold">Perceptual</emphasis>: You notice MPEG
artifacts more if they're scaled up bigger!
Artifacts appear on the scale of blocks (8x8).
Your eye will not see errors in 4800 small blocks as easily as it
sees errors in 1200 large blocks (assuming you'll be scaling both
to fullscreen).
</para></listitem>
<listitem><para>
<emphasis role="bold">Theoretical</emphasis>: When you scale down
an image but still use the same size (8x8) blocks for the frequency
space transform, you move more data to the high frequency bands.
Roughly speaking, each pixel contains more of the detail than it
did before.
So even though your scaled-down picture contains 1/4 the information
in the spacial directions, it could still contain a large portion
of the information in the frequency domain (assuming that the high
frequencies were underutilized in the original 640x480 image).
</para></listitem>
</orderedlist>
</para>
<para>
Past guides have recommended choosing a bitrate and resolution based
on a "bits per pixel" approach, but this is usually not valid due to
the above reasons.
A better estimate seems to be that bitrates scale proportional to the
square root of resolution, so that 320x240 and 400 kbit/sec would be
comparable to 640x480 at 800 kbit/sec.
However this has not been verified with theoretical or empirical
rigor.
Further, given that movies vary greatly with regard to noise, detail,
degree of motion, etc., it's futile to make general recommendations
for bits per length-of-diagonal (the analogue of bits per pixel,
using the square root).
</para>
<para>
So far we have discussed the difficulty of choosing a bitrate and
resolution.
</para>
<sect3 id="menc-feat-dvd-mpeg4-resolution-bitrate-compute">
<title>Computing the resolution</title>
<para>
First, you should compute the encoded aspect ratio:
<systemitem>ARc = (Wc x (ARa / PRdvd )) / Hc</systemitem>
@ -1008,6 +1078,7 @@
On the other hand, it is worthless to raise CQ higher than 0.30 as you would
be wasting bits without any noticeable quality gain.
</para>
</sect3>
</sect2>