vc1dec: Shuffle field MVs after decoding, not before

This simplifies the code since copying MVs to the reference is not needed
anymore (and maybe something about fixing artifacts).

Also remove the unused mv_f_last.

Fixes a small number of artifacts in black_screen_VC-1.mkv
and several more artifacts in other videos.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1.h

@@ -354,7 +354,6 @@ typedef struct VC1Context{
     int8_t zzi_8x8[64];
     uint8_t *blk_mv_type_base, *blk_mv_type;    ///< 0: frame MV, 1: field MV (interlaced frame)
     uint8_t *mv_f_base, *mv_f[2];               ///< 0: MV obtained from same field, 1: opposite field
-    uint8_t *mv_f_last_base, *mv_f_last[2];
     uint8_t *mv_f_next_base, *mv_f_next[2];
     int field_mode;         ///< 1 for interlaced field pictures
     int fptype;

libavcodec/vc1dec.c

@@ -5419,9 +5419,6 @@ av_cold int ff_vc1_decode_init_alloc_tables(VC1Context *v)
     v->mv_f_base        = av_mallocz(2 * (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2));
     v->mv_f[0]          = v->mv_f_base + s->b8_stride + 1;
     v->mv_f[1]          = v->mv_f[0] + (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2);
-    v->mv_f_last_base   = av_mallocz(2 * (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2));
-    v->mv_f_last[0]     = v->mv_f_last_base + s->b8_stride + 1;
-    v->mv_f_last[1]     = v->mv_f_last[0] + (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2);
     v->mv_f_next_base   = av_mallocz(2 * (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2));
     v->mv_f_next[0]     = v->mv_f_next_base + s->b8_stride + 1;
     v->mv_f_next[1]     = v->mv_f_next[0] + (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2);
@@ -5619,7 +5616,6 @@ av_cold int ff_vc1_decode_end(AVCodecContext *avctx)
     av_freep(&v->mb_type_base);
     av_freep(&v->blk_mv_type_base);
     av_freep(&v->mv_f_base);
-    av_freep(&v->mv_f_last_base);
     av_freep(&v->mv_f_next_base);
     av_freep(&v->block);
     av_freep(&v->cbp_base);
@@ -5889,20 +5885,11 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         v->bits = buf_size * 8;
         v->end_mb_x = s->mb_width;
         if (v->field_mode) {
-            uint8_t *tmp[2];
             s->current_picture.f.linesize[0] <<= 1;
             s->current_picture.f.linesize[1] <<= 1;
             s->current_picture.f.linesize[2] <<= 1;
             s->linesize                      <<= 1;
             s->uvlinesize                    <<= 1;
-            tmp[0]          = v->mv_f_last[0];
-            tmp[1]          = v->mv_f_last[1];
-            v->mv_f_last[0] = v->mv_f_next[0];
-            v->mv_f_last[1] = v->mv_f_next[1];
-            v->mv_f_next[0] = v->mv_f[0];
-            v->mv_f_next[1] = v->mv_f[1];
-            v->mv_f[0] = tmp[0];
-            v->mv_f[1] = tmp[1];
         }
         mb_height = s->mb_height >> v->field_mode;
         for (i = 0; i <= n_slices; i++) {
@@ -5947,15 +5934,15 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         }
         if (v->field_mode) {
             v->second_field = 0;
-            if (s->pict_type == AV_PICTURE_TYPE_B) {
-                memcpy(v->mv_f_base, v->mv_f_next_base,
-                       2 * (s->b8_stride * (s->mb_height * 2 + 1) + s->mb_stride * (s->mb_height + 1) * 2));
-            }
             s->current_picture.f.linesize[0] >>= 1;
             s->current_picture.f.linesize[1] >>= 1;
             s->current_picture.f.linesize[2] >>= 1;
             s->linesize                      >>= 1;
             s->uvlinesize                    >>= 1;
+            if (v->s.pict_type != AV_PICTURE_TYPE_BI && v->s.pict_type != AV_PICTURE_TYPE_B) {
+                FFSWAP(uint8_t *, v->mv_f_next[0], v->mv_f[0]);
+                FFSWAP(uint8_t *, v->mv_f_next[1], v->mv_f[1]);
+            }
         }
         av_dlog(s->avctx, "Consumed %i/%i bits\n",
                 get_bits_count(&s->gb), s->gb.size_in_bits);