docs: Update interlaced video design document

The RFF flag is to be reused for buffers in the telecine state to
indicate that the buffer contains only unneeded repeated fields that are
present in other buffers and as such this buffer can be dropped.

diff --git a/docs/design/part-interlaced-video.txt b/docs/design/part-interlaced-video.txt
index caef533..6593146 100644 (file)
--- a/docs/design/part-interlaced-video.txt
+++ b/docs/design/part-interlaced-video.txt
@@ -77,9 +77,20 @@ Telecine
 If the caps have interlaced=true and interlacing-method=telecine then the
 buffers are in some form of telecine state.
 
-The TFF, RFF and ONEFIELD flags have the same semantics as for the plain
-interlaced state, however, for the telecine state require one additional flag to
-be able to identify progressive buffers.
+The TFF and ONEFIELD flags have the same semantics as for the plain interlaced
+state.
+
+GST_VIDEO_BUFFER_RFF in the telecine state indicates that the buffer contains
+only repeated fields that are present in other buffers and are as such
+unneeded. For example, in a sequence of three telecined frames, we might have:
+
+AtAb AtBb BtBb
+
+In this situation, we only need the first and third buffers as the second
+buffer contains fields present in the first and third.
+
+The telecine state require one additional flag to be able to identify
+progressive buffers.
 
 GST_VIDEO_BUFFER_PROGRESSIVE means that the buffer containing two fields is a
 progressive frame. The implication is that if this flag is not set, the buffer