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Avoid short forms and libavcodec should not the that much singled out

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git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@16179 b3059339-0415-0410-9bf9-f77b7e298cf2
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gpoirier 2005-08-05 21:44:28 +00:00
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DOCS/xml/en/encoding-guide.xml
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@ -292,7 +292,8 @@
<note><title>Note:</title> <note><title>Note:</title>
<para> <para>
Most codecs which support ABR encode only support two pass encode Most codecs which support ABR encode only support two pass encode
while some others such as <systemitem class="library">x264</systemitem> while some others such as <systemitem class="library">x264</systemitem>,
<systemitem class="library">XviD</systemitem>
and <systemitem class="library">libavcodec</systemitem> support and <systemitem class="library">libavcodec</systemitem> support
multipass, which slightly improves quality at each pass, multipass, which slightly improves quality at each pass,
yet this improvement is no longer measurable nor noticeable after the yet this improvement is no longer measurable nor noticeable after the
@ -303,28 +304,31 @@
</note> </note>
<para> <para>
In each of these modes, <systemitem class="library">libavcodec</systemitem> In each of these modes, the video codec (such as
<systemitem class="library">libavcodec</systemitem>)
breaks the video frame into 16x16 pixel macroblocks and then applies a breaks the video frame into 16x16 pixel macroblocks and then applies a
quantizer to each macroblock. The lower the quantizer, the better the quantizer to each macroblock. The lower the quantizer, the better the
quality and higher the bitrate. The method quality and higher the bitrate.
<systemitem class="library">libavcodec</systemitem> uses to determine The method the movie encoder uses to determine
which quantizer to use for a given macroblock varies and is highly which quantizer to use for a given macroblock varies and is highly
tunable. (This is an extreme over-simplification of the actual tunable. (This is an extreme over-simplification of the actual
process, but the basic concept is useful to understand.) process, but the basic concept is useful to understand.)
</para> </para>
<para> <para>
When you specify a constant bitrate, <systemitem When you specify a constant bitrate, the video codec will encode the video,
class="library">libavcodec</systemitem> will encode the video, discarding discarding
detail as much as necessary and as little as possible in order to remain detail as much as necessary and as little as possible in order to remain
lower than the given bitrate. If you truly do not care about file size, lower than the given bitrate. If you truly do not care about file size,
you could as well use CBR and specify a bitrate of infinity. (In you could as well use CBR and specify a bitrate of infinity. (In
practice, this means a value high enough so that it poses no limit, like practice, this means a value high enough so that it poses no limit, like
10000Kbit.) With no real restriction on bitrate, the result is that 10000Kbit.) With no real restriction on bitrate, the result is that
<systemitem class="library">libavcodec</systemitem> will use the lowest the codec will use the lowest
possible quantizer for each macroblock (as specified by possible quantizer for each macroblock (as specified by
<option>vqmin</option>, which is 2 by default). As soon as you specify a <option>vqmin</option> for
low enough bitrate that <systemitem class="library">libavcodec</systemitem> <systemitem class="library">libavcodec</systemitem>, which is 2 by default).
As soon as you specify a
low enough bitrate that the codec
is forced to use a higher quantizer, then you are almost certainly ruining is forced to use a higher quantizer, then you are almost certainly ruining
the quality of your video. the quality of your video.
In order to avoid that, you should probably downscale your video, according In order to avoid that, you should probably downscale your video, according
@ -333,12 +337,13 @@
</para> </para>
<para> <para>
With constant quantizer, <systemitem With constant quantizer, the codec uses the same quantizer, as
class="library">libavcodec</systemitem> uses the same quantizer, as specified by the <option>vqscale</option> option (for
specified by the <option>vqscale</option> option, on every macroblock. If <systemitem class="library">libavcodec</systemitem>), on every macroblock.
you want the highest quality rip possible, again ignoring bitrate, you can If you want the highest quality rip possible, again ignoring bitrate,
use <option>vqscale=2</option>. This will yield the same bitrate and PSNR you can use <option>vqscale=2</option>.
(peak signal-to-noise ratio) as CBR with This will yield the same bitrate and PSNR (peak signal-to-noise ratio)
as CBR with
<option>vbitrate</option>=infinity and the default <option>vqmin</option> <option>vbitrate</option>=infinity and the default <option>vqmin</option>
of 2. of 2.
</para> </para>
@ -960,7 +965,7 @@
If you will not be encoding in constant quantizer mode, you need to If you will not be encoding in constant quantizer mode, you need to
select a bitrate. select a bitrate.
The concept of bitrate is quite simple. The concept of bitrate is quite simple.
It's the (average) number of bits that will be consumed to store your It is the (average) number of bits that will be consumed to store your
movie, per second. movie, per second.
Normally bitrate is measured in kilobits (1000 bits) 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 The size of your movie on disk is the bitrate times the length of the
@ -980,10 +985,10 @@
<orderedlist> <orderedlist>
<listitem><para> <listitem><para>
<emphasis role="bold">Perceptual</emphasis>: You notice MPEG <emphasis role="bold">Perceptual</emphasis>: You notice MPEG
artifacts more if they're scaled up bigger! artifacts more if they are scaled up bigger!
Artifacts appear on the scale of blocks (8x8). Artifacts appear on the scale of blocks (8x8).
Your eye will not see errors in 4800 small blocks as easily as it 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 sees errors in 1200 large blocks (assuming you will be scaling both
to fullscreen). to fullscreen).
</para></listitem> </para></listitem>
<listitem><para> <listitem><para>
@ -1009,7 +1014,7 @@
However this has not been verified with theoretical or empirical However this has not been verified with theoretical or empirical
rigor. rigor.
Further, given that movies vary greatly with regard to noise, detail, Further, given that movies vary greatly with regard to noise, detail,
degree of motion, etc., it's futile to make general recommendations degree of motion, etc., it is futile to make general recommendations
for bits per length-of-diagonal (the analog of bits per pixel, for bits per length-of-diagonal (the analog of bits per pixel,
using the square root). using the square root).
</para> </para>