avcodec/vc1: support multi-slice field interlaced pictures with hwaccel

When using hardware accelerated decoding for multi-slice field interlaced pictures,
only the first slice was decoded. This patch adds the neccesary looping over the
remaining slices that may exist in field interlaced pictures. Additionally, we align
the calculation of mby_start for the second field with the method given in VC-1 spec.
Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>

libavcodec/vc1dec.c

@@ -698,9 +698,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                                                     slices[n_slices].buf);
                     init_get_bits(&slices[n_slices].gb, slices[n_slices].buf,
                                   buf_size3 << 3);
-                    /* assuming that the field marker is at the exact middle,
-                       hope it's correct */
-                    slices[n_slices].mby_start = s->mb_height + 1 >> 1;
+                    slices[n_slices].mby_start = avctx->coded_height + 31 >> 5;
                     slices[n_slices].rawbuf = start;
                     slices[n_slices].raw_size = size + 4;
                     n_slices1 = n_slices - 1; // index of the last slice of the first field
@@ -903,13 +901,41 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
             s->picture_structure = PICT_BOTTOM_FIELD - v->tff;
             if ((ret = avctx->hwaccel->start_frame(avctx, buf_start, buf_start_second_field - buf_start)) < 0)
                 goto err;
-            if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start, buf_start_second_field - buf_start)) < 0)
-                goto err;
+
+            if (n_slices1 == -1) {
+                // no slices, decode the field as-is
+                if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start, buf_start_second_field - buf_start)) < 0)
+                    goto err;
+            } else {
+                if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start, slices[0].rawbuf - buf_start)) < 0)
+                    goto err;
+
+                for (i = 0 ; i < n_slices1 + 1; i++) {
+                    s->gb = slices[i].gb;
+                    s->mb_y = slices[i].mby_start;
+
+                    v->pic_header_flag = get_bits1(&s->gb);
+                    if (v->pic_header_flag) {
+                        if (ff_vc1_parse_frame_header_adv(v, &s->gb) < 0) {
+                            av_log(v->s.avctx, AV_LOG_ERROR, "Slice header damaged\n");
+                            ret = AVERROR_INVALIDDATA;
+                            if (avctx->err_recognition & AV_EF_EXPLODE)
+                                goto err;
+                            continue;
+                        }
+                    }
+
+                    if ((ret = avctx->hwaccel->decode_slice(avctx, slices[i].rawbuf, slices[i].raw_size)) < 0)
+                        goto err;
+                }
+            }
+
             if ((ret = avctx->hwaccel->end_frame(avctx)) < 0)
                 goto err;
 
             // decode second field
             s->gb = slices[n_slices1 + 1].gb;
+            s->mb_y = slices[n_slices1 + 1].mby_start;
             s->picture_structure = PICT_TOP_FIELD + v->tff;
             v->second_field = 1;
             v->pic_header_flag = 0;
@@ -922,8 +948,35 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
 
             if ((ret = avctx->hwaccel->start_frame(avctx, buf_start_second_field, (buf + buf_size) - buf_start_second_field)) < 0)
                 goto err;
-            if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start_second_field, (buf + buf_size) - buf_start_second_field)) < 0)
-                goto err;
+
+            if (n_slices - n_slices1 == 2) {
+                // no slices, decode the field as-is
+                if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start_second_field, (buf + buf_size) - buf_start_second_field)) < 0)
+                    goto err;
+            } else {
+                if ((ret = avctx->hwaccel->decode_slice(avctx, buf_start_second_field, slices[n_slices1 + 2].rawbuf - buf_start_second_field)) < 0)
+                    goto err;
+
+                for (i = n_slices1 + 2; i < n_slices; i++) {
+                    s->gb = slices[i].gb;
+                    s->mb_y = slices[i].mby_start;
+
+                    v->pic_header_flag = get_bits1(&s->gb);
+                    if (v->pic_header_flag) {
+                        if (ff_vc1_parse_frame_header_adv(v, &s->gb) < 0) {
+                            av_log(v->s.avctx, AV_LOG_ERROR, "Slice header damaged\n");
+                            ret = AVERROR_INVALIDDATA;
+                            if (avctx->err_recognition & AV_EF_EXPLODE)
+                                goto err;
+                            continue;
+                        }
+                    }
+
+                    if ((ret = avctx->hwaccel->decode_slice(avctx, slices[i].rawbuf, slices[i].raw_size)) < 0)
+                        goto err;
+                }
+            }
+
             if ((ret = avctx->hwaccel->end_frame(avctx)) < 0)
                 goto err;
         } else {