CVS change done by Guillaume Poirier CVS Update of /cvsroot/mplayer/main/DOCS/xml/en In directory mail:/var2/tmp/cvs-serv5106/en Modified Files: encoding-guide.xml Log Message: New item: "Choosing resolution and bitrate", from Rich's encoding guide Index: encoding-guide.xml =================================================================== RCS file: /cvsroot/mplayer/main/DOCS/xml/en/encoding-guide.xml,v retrieving revision 1.1 retrieving revision 1.2 diff -u -r1.1 -r1.2 --- encoding-guide.xml 24 Jul 2005 14:22:14 -0000 1.1 +++ encoding-guide.xml 24 Jul 2005 20:53:54 -0000 1.2 @@ -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>