[FFmpeg-devel] [PATCH v2] mpegvideo_parser: check picture_structure for field order

[Tom Yan]

the top_field_first bit is only used to indicate the field order
when the picture is a frame picture (which consists of two fields)
but not when it is a field picture (which consists of one single
top or bottom field).

also removing the unnecessary progressive_sequence check (the bit
is mandated to be 0 if progressive_frame is 0 on any picture in the
sequence).

Signed-off-by: Tom Yan <tom.ty89 at gmail.com>
---
 libavcodec/mpegvideo_parser.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/libavcodec/mpegvideo_parser.c b/libavcodec/mpegvideo_parser.c
index c5dc867d24..004ff602f6 100644
--- a/libavcodec/mpegvideo_parser.c
+++ b/libavcodec/mpegvideo_parser.c
@@ -181,6 +181,7 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
                     break;
                 case 0x8: /* picture coding extension */
                     if (bytes_left >= 5) {
+                        s->picture_structure = buf[2] & 0x3;
                         top_field_first = buf[3] & (1 << 7);
                         repeat_first_field = buf[3] & (1 << 1);
                         progressive_frame = buf[4] & (1 << 7);
@@ -198,13 +199,15 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
                             }
                         }
 
-                        if (!pc->progressive_sequence && !progressive_frame) {
-                            if (top_field_first)
-                                s->field_order = AV_FIELD_TT;
-                            else
-                                s->field_order = AV_FIELD_BB;
-                        } else
+                        if (progressive_frame)
                             s->field_order = AV_FIELD_PROGRESSIVE;
+                        else if (top_field_first ||
+                                 /* top_field_first is mandated to be 0 when
+                                    the picture is not a frame picture) */
+                                 s->picture_structure == AV_PICTURE_STRUCTURE_TOP_FIELD)
+                            s->field_order = AV_FIELD_TT;
+                        else
+                            s->field_order = AV_FIELD_BB;
                     }
                     break;
                 }
-- 
2.35.1