vp9: clip motion vectors in the same way as libvpx does.

The practical effect of this is that the scaling will wrongly not be
applied to the interpolation edge (the 3/4 constants in this patch).
In other words, we clip to the pre-scaling interpolation, even though
these should be clipped post-scaling. The resulting out-of-frame MVs
are thus automatically clipped within the visible portion of the frame,
which is probably not the intention, but is unfortunately what libvpx
does, so we need to copy that behaviour.

libavcodec/vp9.c

@@ -2740,18 +2740,23 @@ static av_always_inline void mc_luma_scaled(VP9Context *s, vp9_scaled_mc_func sm
                                             uint8_t *dst, ptrdiff_t dst_stride,
                                             const uint8_t *ref, ptrdiff_t ref_stride,
                                             ThreadFrame *ref_frame,
-                                            ptrdiff_t y, ptrdiff_t x, const VP56mv *mv,
+                                            ptrdiff_t y, ptrdiff_t x, const VP56mv *in_mv,
                                             int bw, int bh, int w, int h, int bytesperpixel,
                                             const uint16_t *scale, const uint8_t *step)
 {
 #define scale_mv(n, dim) (((int64_t)(n) * scale[dim]) >> 14)
+    int mx, my;
+    int refbw_m1, refbh_m1;
+    int th;
+    VP56mv mv;
+
+    mv.x = av_clip(in_mv->x, -(x + bw + 4) << 3, (s->cols * 8 - x + 3) << 3);
+    mv.y = av_clip(in_mv->y, -(y + bh + 4) << 3, (s->rows * 8 - y + 3) << 3);
     // BUG libvpx seems to scale the two components separately. This introduces
     // rounding errors but we have to reproduce them to be exactly compatible
     // with the output from libvpx...
-    int mx = scale_mv(mv->x * 2, 0) + scale_mv(x * 16, 0);
-    int my = scale_mv(mv->y * 2, 1) + scale_mv(y * 16, 1);
-    int refbw_m1, refbh_m1;
-    int th;
+    mx = scale_mv(mv.x * 2, 0) + scale_mv(x * 16, 0);
+    my = scale_mv(mv.y * 2, 1) + scale_mv(y * 16, 1);
 
     y = my >> 4;
     x = mx >> 4;
@@ -2783,25 +2788,30 @@ static av_always_inline void mc_chroma_scaled(VP9Context *s, vp9_scaled_mc_func
                                               const uint8_t *ref_u, ptrdiff_t src_stride_u,
                                               const uint8_t *ref_v, ptrdiff_t src_stride_v,
                                               ThreadFrame *ref_frame,
-                                              ptrdiff_t y, ptrdiff_t x, const VP56mv *mv,
+                                              ptrdiff_t y, ptrdiff_t x, const VP56mv *in_mv,
                                               int bw, int bh, int w, int h, int bytesperpixel,
                                               const uint16_t *scale, const uint8_t *step)
 {
     int mx, my;
     int refbw_m1, refbh_m1;
     int th;
+    VP56mv mv;
 
     if (s->ss_h) {
         // BUG https://code.google.com/p/webm/issues/detail?id=820
-        mx = scale_mv(mv->x, 0) + (scale_mv(x * 16, 0) & ~15) + (scale_mv(x * 32, 0) & 15);
+        mv.x = av_clip(in_mv->x, -(x + bw + 4) << 4, (s->cols * 4 - x + 3) << 4);
+        mx = scale_mv(mv.x, 0) + (scale_mv(x * 16, 0) & ~15) + (scale_mv(x * 32, 0) & 15);
     } else {
-        mx = scale_mv(mv->x << 1, 0) + scale_mv(x * 16, 0);
+        mv.x = av_clip(in_mv->x, -(x + bw + 4) << 3, (s->cols * 8 - x + 3) << 3);
+        mx = scale_mv(mv.x << 1, 0) + scale_mv(x * 16, 0);
     }
     if (s->ss_v) {
         // BUG https://code.google.com/p/webm/issues/detail?id=820
-        my = scale_mv(mv->y, 1) + (scale_mv(y * 16, 1) & ~15) + (scale_mv(y * 32, 1) & 15);
+        mv.y = av_clip(in_mv->y, -(y + bh + 4) << 4, (s->rows * 4 - y + 3) << 4);
+        my = scale_mv(mv.y, 1) + (scale_mv(y * 16, 1) & ~15) + (scale_mv(y * 32, 1) & 15);
     } else {
-        my = scale_mv(mv->y << 1, 1) + scale_mv(y * 16, 1);
+        mv.y = av_clip(in_mv->y, -(y + bh + 4) << 3, (s->rows * 8 - y + 3) << 3);
+        my = scale_mv(mv.y << 1, 1) + scale_mv(y * 16, 1);
     }
 #undef scale_mv
     y = my >> 4;