diff --git a/libavcodec/mpegvideo.c b/libavcodec/mpegvideo.c
index 23a5797c7a..c603eeb75e 100644
--- a/libavcodec/mpegvideo.c
+++ b/libavcodec/mpegvideo.c
@@ -965,6 +965,77 @@ static int free_context_frame(MpegEncContext *s)
     return 0;
 }
 
+int ff_MPV_common_frame_size_change(MpegEncContext *s)
+{
+    int i, err = 0;
+
+    if (s->slice_context_count > 1) {
+        for (i = 0; i < s->slice_context_count; i++) {
+            free_duplicate_context(s->thread_context[i]);
+        }
+        for (i = 1; i < s->slice_context_count; i++) {
+            av_freep(&s->thread_context[i]);
+        }
+    } else
+        free_duplicate_context(s);
+
+    free_context_frame(s);
+
+    if (s->picture)
+        for (i = 0; i < s->picture_count; i++) {
+                s->picture[i].needs_realloc = 1;
+        }
+
+    s->last_picture_ptr         =
+    s->next_picture_ptr         =
+    s->current_picture_ptr      = NULL;
+
+    // init
+    if (s->codec_id == AV_CODEC_ID_MPEG2VIDEO && !s->progressive_sequence)
+        s->mb_height = (s->height + 31) / 32 * 2;
+    else if (s->codec_id != AV_CODEC_ID_H264)
+        s->mb_height = (s->height + 15) / 16;
+
+    if ((s->width || s->height) &&
+        av_image_check_size(s->width, s->height, 0, s->avctx))
+        return AVERROR_INVALIDDATA;
+
+    if ((err = init_context_frame(s)))
+        goto fail;
+
+    s->thread_context[0]   = s;
+
+    if (s->width && s->height) {
+        int nb_slices = s->slice_context_count;
+        if (nb_slices > 1) {
+            for (i = 1; i < nb_slices; i++) {
+                s->thread_context[i] = av_malloc(sizeof(MpegEncContext));
+                memcpy(s->thread_context[i], s, sizeof(MpegEncContext));
+            }
+
+            for (i = 0; i < nb_slices; i++) {
+                if (init_duplicate_context(s->thread_context[i], s) < 0)
+                    goto fail;
+                    s->thread_context[i]->start_mb_y =
+                        (s->mb_height * (i) + nb_slices / 2) / nb_slices;
+                    s->thread_context[i]->end_mb_y   =
+                        (s->mb_height * (i + 1) + nb_slices / 2) / nb_slices;
+            }
+        } else {
+            if (init_duplicate_context(s, s) < 0)
+                goto fail;
+            s->start_mb_y = 0;
+            s->end_mb_y   = s->mb_height;
+        }
+        s->slice_context_count = nb_slices;
+    }
+
+    return 0;
+ fail:
+    ff_MPV_common_end(s);
+    return err;
+}
+
 /* init common structure for both encoder and decoder */
 void ff_MPV_common_end(MpegEncContext *s)
 {
@@ -1124,7 +1195,17 @@ void ff_release_unused_pictures(MpegEncContext*s, int remove_current)
     }
 }
 
-int ff_find_unused_picture(MpegEncContext *s, int shared)
+static inline int pic_is_unused(MpegEncContext *s, Picture *pic)
+{
+    if (pic->f.data[0] == NULL)
+        return 1;
+    if (pic->needs_realloc)
+        if (!pic->owner2 || pic->owner2 == s)
+            return 1;
+    return 0;
+}
+
+static int find_unused_picture(MpegEncContext *s, int shared)
 {
     int i;
 
@@ -1135,11 +1216,11 @@ int ff_find_unused_picture(MpegEncContext *s, int shared)
         }
     } else {
         for (i = s->picture_range_start; i < s->picture_range_end; i++) {
-            if (s->picture[i].f.data[0] == NULL && s->picture[i].f.type != 0)
+            if (pic_is_unused(s, &s->picture[i]) && s->picture[i].f.type != 0)
                 return i; // FIXME
         }
         for (i = s->picture_range_start; i < s->picture_range_end; i++) {
-            if (s->picture[i].f.data[0] == NULL)
+            if (pic_is_unused(s, &s->picture[i]))
                 return i;
         }
     }
@@ -1161,6 +1242,20 @@ int ff_find_unused_picture(MpegEncContext *s, int shared)
     return -1;
 }
 
+int ff_find_unused_picture(MpegEncContext *s, int shared)
+{
+    int ret = find_unused_picture(s, shared);
+
+    if (ret >= 0 && ret < s->picture_range_end) {
+        if (s->picture[ret].needs_realloc) {
+            s->picture[ret].needs_realloc = 0;
+            free_picture(s, &s->picture[ret]);
+            avcodec_get_frame_defaults(&s->picture[ret].f);
+        }
+    }
+    return ret;
+}
+
 static void update_noise_reduction(MpegEncContext *s)
 {
     int intra, i;
@@ -1213,7 +1308,7 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
                 if (s->picture[i].owner2 == s && s->picture[i].f.data[0] &&
                     &s->picture[i] != s->last_picture_ptr &&
                     &s->picture[i] != s->next_picture_ptr &&
-                    s->picture[i].f.reference) {
+                    s->picture[i].f.reference && !s->picture[i].needs_realloc) {
                     if (!(avctx->active_thread_type & FF_THREAD_FRAME))
                         av_log(avctx, AV_LOG_ERROR,
                                "releasing zombie picture\n");
diff --git a/libavcodec/mpegvideo.h b/libavcodec/mpegvideo.h
index 39f23222d3..1b8791cbce 100644
--- a/libavcodec/mpegvideo.h
+++ b/libavcodec/mpegvideo.h
@@ -141,6 +141,7 @@ typedef struct Picture{
     int32_t *mb_cmp_score;      ///< Table for MB cmp scores, for mb decision FIXME remove
     int b_frame_score;          /* */
     struct MpegEncContext *owner2; ///< pointer to the MpegEncContext that allocated this picture
+    int needs_realloc;          ///< Picture needs to be reallocated (eg due to a frame size change)
 } Picture;
 
 /**
@@ -758,6 +759,7 @@ void ff_MPV_common_defaults(MpegEncContext *s);
 
 void ff_MPV_decode_defaults(MpegEncContext *s);
 int ff_MPV_common_init(MpegEncContext *s);
+int ff_MPV_common_frame_size_change(MpegEncContext *s);
 void ff_MPV_common_end(MpegEncContext *s);
 void ff_MPV_decode_mb(MpegEncContext *s, DCTELEM block[12][64]);
 int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx);
diff --git a/libavcodec/rv34.c b/libavcodec/rv34.c
index 5c19735375..4ca134e208 100644
--- a/libavcodec/rv34.c
+++ b/libavcodec/rv34.c
@@ -1537,10 +1537,9 @@ int ff_rv34_decode_update_thread_context(AVCodecContext *dst, const AVCodecConte
         return 0;
 
     if (s->height != s1->height || s->width != s1->width) {
-        ff_MPV_common_end(s);
         s->height = s1->height;
         s->width  = s1->width;
-        if ((err = ff_MPV_common_init(s)) < 0)
+        if ((err = ff_MPV_common_frame_size_change(s)) < 0)
             return err;
         if ((err = rv34_decoder_realloc(r)) < 0)
             return err;
@@ -1659,20 +1658,13 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
         if (s->width != si.width || s->height != si.height) {
             int err;
 
-            if (HAVE_THREADS &&
-                (s->avctx->active_thread_type & FF_THREAD_FRAME)) {
-                av_log_missing_feature(s->avctx, "Width/height changing with "
-                                       "frame threading is", 0);
-                return AVERROR_PATCHWELCOME;
-            }
-
             av_log(s->avctx, AV_LOG_WARNING, "Changing dimensions to %dx%d\n",
                    si.width, si.height);
-            ff_MPV_common_end(s);
+
             s->width  = si.width;
             s->height = si.height;
             avcodec_set_dimensions(s->avctx, s->width, s->height);
-            if ((err = ff_MPV_common_init(s)) < 0)
+            if ((err = ff_MPV_common_frame_size_change(s)) < 0)
                 return err;
             if ((err = rv34_decoder_realloc(r)) < 0)
                 return err;