Merged video filter description from vop.txt.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@9688 b3059339-0415-0410-9bf9-f77b7e298cf2

DOCS/tech/libmpcodecs.txt

@@ -111,7 +111,30 @@ Short description of video path:
    filter have to implement the whole filter api.
    Leaf filters are now: vf_vo.c (wrapper over libvo) and ve_XXX.c (video
    encoders used by mencoder).
+
    
+Video Filters
+=============
+
+Video filters are plugin-like code modules implementing the interface
+defined in vf.h.
+
+Basically it means video output manipulation, i.e. these plugins can
+modify the image and the image properties (size, colorspace etc) between
+the video decoders (vd.h) and the output layer (libvo or video encoders).
+
+The actual API is a mixture of the video decoder (vd.h) and libvo
+(video_out.h) APIs.
+
+main differences:
+- vf plugins may be "loaded" multiple times, with different parameters
+  and context - it's new in MPlayer, old APIs weren't reentrant.
+- vf plugins don't have to implement all functions - all functions have a
+  'fallback' version, so the plugins only override these if wanted.
+- Each vf plugin has its own get_image context, and they can interchange
+  images/buffers using these get_image/put_image calls.
+
+
 The VIDEO FILTER API:
 =====================
 filename: vf_FILTERNAME.c
@@ -348,6 +371,7 @@ btw, u should avoid using global or static variables, to store filter instance
 specific stuff, as filters might be used multiple times & in the future even
 multiple streams might be possible
 
+
 The AUDIO path:
 ===============
 TODO!!!