lavfi: add field filter

The filter is a port of libmpcodecs/vf_field.c, since there is no common
code I relicensed it as LGPL, while keeping the original author
copyright.

1 files changed, 25 insertions, 0 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index 81918dec36..7bf3896282 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -2128,6 +2128,31 @@ fade=in:5:20
 fade=in:0:25:alpha=1
 @end example
 
+@section field
+
+Extract a single field from an interlaced image using stride
+arithmetic to avoid wasting CPU time. The output frames are marked as
+non-interlaced.
+
+This filter accepts the following named options:
+@table @option
+@item type
+Specify whether to extract the top (if the value is @code{0} or
+@code{top}) or the bottom field (if the value is @code{1} or
+@code{bottom}).
+@end table
+
+If the option key is not specified, the first value sets the @var{type}
+option. For example:
+@example
+field=bottom
+@end example
+
+is equivalent to:
+@example
+field=type=bottom
+@end example
+
 @section fieldorder
 
 Transform the field order of the input video.