[M4IF Technotes] Offscreen MVs

[the_ether]

 Is it possible to have a motion vector to point outside the screen? I know
you can point outside a VOP (with long header only if I remember correctly),
but off-screen?
Say there is a feature at position (0,0) in frame 1 (F1) and in F2 that
feature moves up by 1 pixel so that we lose the top row.
What I'd like to do is to signal the decoder a motion vector of (0,-1), but
the decoder would then need to know what pixels to put in the bottom row of
the macroblock in F2.
I could always just pass a MV of (0,0) buit if we take the worst case
whereby every row of that feature had a different luminance value (eg
stripped) then I would get a larger error value than if I sent a MV of
(0,-1) and the decoder put in some random pixels into the lowest row of that
macroblock.
I don't think I can pass MVs that point off screen but I thought I'd check.
regards
Graham