doc/encoders: document libtheora encoder

doc/encoders.texi

@@ -420,6 +420,45 @@ Selected by Encoder (default)
 A description of some of the currently available video encoders
 follows.
 
+@section libtheora
+
+Theora format supported through libtheora.
+
+Requires the presence of the libtheora headers and library during
+configuration. You need to explicitly configure the build with
+@code{--enable-libtheora}.
+
+@subsection Options
+
+The following global options are mapped to internal libtheora options
+which affect the quality and the bitrate of the encoded stream.
+
+@table @option
+@item b
+Set the video bitrate, only works if the @code{qscale} flag in
+@option{flags} is not enabled.
+
+@item flags
+Used to enable constant quality mode encoding through the
+@option{qscale} flag, and to enable the @code{pass1} and @code{pass2}
+modes.
+
+@item g
+Set the GOP size.
+
+@item global_quality
+Set the global quality in lambda units, only works if the
+@code{qscale} flag in @option{flags} is enabled. The value is clipped
+in the [0 - 10*@code{FF_QP2LAMBDA}] range, and then multiplied for 6.3
+to get a value in the native libtheora range [0-63].
+
+For example, to set maximum constant quality encoding with
+@command{ffmpeg}:
+@example
+ffmpeg -i INPUT -flags:v qscale -global_quality:v "10*QP2LAMBDA" -codec:v libtheora OUTPUT.ogg
+@end example
+@end table
+
 @section libvpx
 
 VP8 format supported through libvpx.