avfilter: port qp filter from libmpcodecs

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

1 files changed, 32 insertions, 0 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index 42f69eaacb..154508cfff 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -7351,6 +7351,38 @@ telecine NTSC input:
 ffmpeg -i input -vf pullup -r 24000/1001 ...
 @end example
 
+@section qp
+
+Change video quantization parameters (QP).
+
+The filter accepts the following option:
+
+@table @option
+@item qp
+Set expression for quantization parameter.
+@end table
+
+The expression is evaluated through the eval API and can contain, among others,
+the following constants:
+
+@table @var
+@item known
+1 if index is not 129, 0 otherwise.
+
+@item qp
+Sequentional index starting from -129 to 128.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Some equation like:
+@example
+qp=2+2*sin(PI*qp)
+@end example
+@end itemize
+
 @section removelogo
 
 Suppress a TV station logo, using an image file to determine which