| Message ID | 57D72C08.4020808@impactstudiopro.com |
|---|---|
| State | Superseded |
| Headers | show |
On Mon, Sep 12, 2016 at 03:28:24PM -0700, Jonathan Campbell wrote: > These patches fix up the DVD caption handling in mpeg12dec.c to better handle odd cases. > It's based on code I've written elsewhere to handle captions. > While it's common for these packets to contain 15 frames worth and start on the odd field there are also DVDs that start on even field or even encode extra fields and switch starting fields. > Part of the patch is to document comprehensively the format of the DVD caption packet. > > Jonathan Campbell > mpeg12dec.c | 27 ++++++++++++++++++++++++++- > 1 file changed, 26 insertions(+), 1 deletion(-) > a839a0d0e9000ab140f6aef9dee9577f242462bf 0001-add-comments-documenting-the-format-of-the-DVD-CC-us.patch > From 9213012c7d8ceef2af43fe3c218b1b50728e8f80 Mon Sep 17 00:00:00 2001 > From: Jonathan Campbell <jonathan@castus.tv> > Date: Mon, 12 Sep 2016 12:34:48 -0700 > Subject: [PATCH 1/2] add comments documenting the format of the DVD CC > user-data packet. this is to aid development and maintenance of that code. > > --- > libavcodec/mpeg12dec.c | 27 ++++++++++++++++++++++++++- > 1 file changed, 26 insertions(+), 1 deletion(-) > > diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c > index 204a578..522621a 100644 > --- a/libavcodec/mpeg12dec.c > +++ b/libavcodec/mpeg12dec.c > @@ -2262,7 +2262,32 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx, > return 1; > } else if (buf_size >= 11 && > p[0] == 'C' && p[1] == 'C' && p[2] == 0x01 && p[3] == 0xf8) { > - /* extract DVD CC data */ > + /* extract DVD CC data > + * for more information see: [https://en.wikipedia.org/wiki/EIA-608#DVD_GOP_User_Data_Insertion] wikipedia is not a good reference, in fact its not even a constant reference without a revission. wikipedia can massivly change and may at times, especially with niche areas be just wrong, the link itself also wont always work possibly Please use the specifications itself, H.262 is public, its the 4th link when searching for H.262 with google for example (wikipedia refers to H.262 IIUC) [...]
diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c index 204a578..522621a 100644 --- a/libavcodec/mpeg12dec.c +++ b/libavcodec/mpeg12dec.c @@ -2262,7 +2262,32 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx, return 1; } else if (buf_size >= 11 && p[0] == 'C' && p[1] == 'C' && p[2] == 0x01 && p[3] == 0xf8) { - /* extract DVD CC data */ + /* extract DVD CC data + * for more information see: [https://en.wikipedia.org/wiki/EIA-608#DVD_GOP_User_Data_Insertion] + * + * uint32_t user_data_start_code 0x000001B2 (big endian) + * uint16_t user_identifier 0x4343 "CC" + * uint8_t user_data_type_code 0x01 + * uint8_t caption_block_size 0xF8 + * uint8_t + * bit 7 caption_odd_field_first 1=odd field (CC1/CC2) first 0=even field (CC3/CC4) first + * bit 6 caption_filler 0 + * bit 5:1 caption_block_count number of caption blocks (pairs of caption words = frames). Most DVDs use 15 per start of GOP. + * bit 0 caption_extra_field_added 1=one additional caption word + * + * struct caption_field_block { + * uint8_t + * bit 7:1 caption_filler 0x7F (all 1s) + * bit 0 caption_field_odd 1=odd field (this is CC1/CC2) 0=even field (this is CC3/CC4) + * uint8_t caption_first_byte + * uint8_t caption_second_byte + * } caption_block[(caption_block_count * 2) + caption_extra_field_added]; + * + * Some DVDs encode caption data for both fields with caption_field_odd=1. The only way to decode the fields + * correctly is to start on the field indicated by caption_odd_field_first and count between odd/even fields. + * Don't assume that the first caption word is the odd field. There do exist MPEG files in the wild that start + * on the even field. There also exist DVDs in the wild that encode an odd field count and the + * caption_extra_field_added/caption_odd_field_first bits change per packet to allow that. */ int cc_count = 0; int i; // There is a caption count field in the data, but it is often
These patches fix up the DVD caption handling in mpeg12dec.c to better handle odd cases. It's based on code I've written elsewhere to handle captions. While it's common for these packets to contain 15 frames worth and start on the odd field there are also DVDs that start on even field or even encode extra fields and switch starting fields. Part of the patch is to document comprehensively the format of the DVD caption packet. Jonathan Campbell From 9213012c7d8ceef2af43fe3c218b1b50728e8f80 Mon Sep 17 00:00:00 2001 From: Jonathan Campbell <jonathan@castus.tv> Date: Mon, 12 Sep 2016 12:34:48 -0700 Subject: [PATCH 1/2] add comments documenting the format of the DVD CC user-data packet. this is to aid development and maintenance of that code. --- libavcodec/mpeg12dec.c | 27 ++++++++++++++++++++++++++- 1 file changed, 26 insertions(+), 1 deletion(-)