[FFmpeg-devel,PATCHv2,4/4] avcodec/svq1dec: detect buggy FFmpeg encoder and apply correction to interframe mean symbols

diff --git a/libavcodec/svq1dec.c b/libavcodec/svq1dec.c
index 828b38b93d..c7269456e2 100644
--- a/libavcodec/svq1dec.c
+++ b/libavcodec/svq1dec.c
@@ -73,6 +73,8 @@  typedef struct SVQ1Context {
     int height;
     int frame_code;
     int nonref;         // 1 if the current frame won't be referenced
+
+    int last_tempref;
 } SVQ1Context;
 
 static const uint8_t string_table[256] = {
@@ -229,7 +231,7 @@  static int svq1_decode_block_intra(GetBitContext *bitbuf, uint8_t *pixels,
 }
 
 static int svq1_decode_block_non_intra(GetBitContext *bitbuf, uint8_t *pixels,
-                                       ptrdiff_t pitch)
+                                       ptrdiff_t pitch, int buggy)
 {
     uint32_t bit_cache;
     uint8_t *list[63];
@@ -270,6 +272,13 @@  static int svq1_decode_block_non_intra(GetBitContext *bitbuf, uint8_t *pixels,
 
         mean = get_vlc2(bitbuf, svq1_inter_mean.table, 9, 3) - 256;
 
+        if (buggy) {
+            if (mean == -128)
+                mean = 128;
+            else if (mean == 128)
+                mean = -128;
+        }
+
         SVQ1_CALC_CODEBOOK_ENTRIES(ff_svq1_inter_codebooks);
 
         for (y = 0; y < height; y++) {
@@ -455,7 +464,7 @@  static int svq1_decode_delta_block(AVCodecContext *avctx, HpelDSPContext *hdsp,
                                    GetBitContext *bitbuf,
                                    uint8_t *current, uint8_t *previous,
                                    ptrdiff_t pitch, svq1_pmv *motion, int x, int y,
-                                   int width, int height)
+                                   int width, int height, int buggy)
 {
     uint32_t block_type;
     int result = 0;
@@ -487,7 +496,7 @@  static int svq1_decode_delta_block(AVCodecContext *avctx, HpelDSPContext *hdsp,
             ff_dlog(avctx, "Error in svq1_motion_inter_block %i\n", result);
             break;
         }
-        result = svq1_decode_block_non_intra(bitbuf, current, pitch);
+        result = svq1_decode_block_non_intra(bitbuf, current, pitch, buggy);
         break;
 
     case SVQ1_BLOCK_INTER_4V:
@@ -498,7 +507,7 @@  static int svq1_decode_delta_block(AVCodecContext *avctx, HpelDSPContext *hdsp,
             ff_dlog(avctx, "Error in svq1_motion_inter_4v_block %i\n", result);
             break;
         }
-        result = svq1_decode_block_non_intra(bitbuf, current, pitch);
+        result = svq1_decode_block_non_intra(bitbuf, current, pitch, buggy);
         break;
 
     case SVQ1_BLOCK_INTRA:
@@ -524,15 +533,18 @@  static void svq1_parse_string(GetBitContext *bitbuf, uint8_t out[257])
     out[i] = 0;
 }
 
-static int svq1_decode_frame_header(AVCodecContext *avctx, AVFrame *frame)
+static int svq1_decode_frame_header(AVCodecContext *avctx, AVFrame *frame, int * buggy)
 {
     SVQ1Context *s = avctx->priv_data;
     GetBitContext *bitbuf = &s->gb;
     int frame_size_code;
     int width  = s->width;
     int height = s->height;
+    int tempref;
 
-    skip_bits(bitbuf, 8); /* temporal_reference */
+    tempref = get_bits(bitbuf, 8); /* temporal_reference */
+    *buggy = tempref == 0 && s->last_tempref == 0 && avctx->extradata_size == 0;
+    s->last_tempref = tempref;
 
     /* frame type */
     s->nonref = 0;
@@ -624,7 +636,7 @@  static int svq1_decode_frame(AVCodecContext *avctx, AVFrame *cur,
     int buf_size       = avpkt->size;
     SVQ1Context     *s = avctx->priv_data;
     uint8_t *current;
-    int result, i, x, y, width, height;
+    int result, i, x, y, width, height, buggy;
     int ret;
 
     /* initialize bit buffer */
@@ -664,7 +676,7 @@  static int svq1_decode_frame(AVCodecContext *avctx, AVFrame *cur,
             src[i] = ((src[i] << 16) | (src[i] >> 16)) ^ src[7 - i];
     }
 
-    result = svq1_decode_frame_header(avctx, cur);
+    result = svq1_decode_frame_header(avctx, cur, &buggy);
     if (result != 0) {
         ff_dlog(avctx, "Error in svq1_decode_frame_header %i\n", result);
         return result;
@@ -734,7 +746,7 @@  static int svq1_decode_frame(AVCodecContext *avctx, AVFrame *cur,
                     result = svq1_decode_delta_block(avctx, &s->hdsp,
                                                      &s->gb, &current[x],
                                                      previous, linesize,
-                                                     s->pmv, x, y, width, height);
+                                                     s->pmv, x, y, width, height, buggy);
                     if (result != 0) {
                         ff_dlog(avctx,
                                 "Error in svq1_decode_delta_block %i\n",
@@ -820,6 +832,8 @@  static av_cold int svq1_decode_init(AVCodecContext *avctx)
 
     ff_thread_once(&init_static_once, svq1_static_init);
 
+    s->last_tempref = 0xFF;
+
     return 0;
 }