[FFmpeg-devel] [PATCH v4 2/3] aadec: add chapters and seeking

[Karsten Otto]

> Am 08.07.2018 um 11:12 schrieb Michael Niedermayer <michael at niedermayer.cc>:
> 
> Signierter PGP-Teil
> On Sat, Jul 07, 2018 at 07:41:28PM +0200, Karsten Otto wrote:
>> read_packet reads content in chunks. Thus seek must be clamped to valid
>> chunk positions in the file, which in turn are relative to chapter start
>> positions.
>> 
>> So in read_header, scan for chapter headers once by skipping through the
>> content. Set stream time_base based on bitrate in bytes/s, for easy
>> timestamp to position conversion.
>> 
>> Then in read_seek, find the chapter containing the seek position, calculate
>> the nearest chunk position, and reinit the read_seek state accordingly.
>> ---
>> libavformat/aadec.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>> 1 file changed, 83 insertions(+), 4 deletions(-)
>> 
>> diff --git a/libavformat/aadec.c b/libavformat/aadec.c
>> index 4db71b1939..e3c03bc522 100644
>> --- a/libavformat/aadec.c
>> +++ b/libavformat/aadec.c
>> @@ -35,6 +35,8 @@
>> #define MAX_TOC_ENTRIES 16
>> #define MAX_DICTIONARY_ENTRIES 128
>> #define TEA_BLOCK_SIZE 8
>> +#define CHAPTER_HEADER_SIZE 8
>> +#define TIMEPREC 1000
>> 
>> typedef struct AADemuxContext {
>>     AVClass *class;
>> @@ -46,6 +48,7 @@ typedef struct AADemuxContext {
>>     struct AVTEA *tea_ctx;
>>     uint8_t file_key[16];
>>     int64_t current_chapter_size;
>> +    int64_t content_start;
>>     int64_t content_end;
>> } AADemuxContext;
>> 
>> @@ -70,7 +73,7 @@ static int aa_read_header(AVFormatContext *s)
>>     uint32_t nkey, nval, toc_size, npairs, header_seed = 0, start;
>>     char key[128], val[128], codec_name[64] = {0};
>>     uint8_t output[24], dst[8], src[8];
>> -    int64_t largest_size = -1, current_size = -1;
>> +    int64_t largest_size = -1, current_size = -1, chapter_pos;
>>     struct toc_entry {
>>         uint32_t offset;
>>         uint32_t size;
>> @@ -173,19 +176,23 @@ static int aa_read_header(AVFormatContext *s)
>>         st->codecpar->codec_id = AV_CODEC_ID_MP3;
>>         st->codecpar->sample_rate = 22050;
>>         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
>> -        st->start_time = 0;
>> +        avpriv_set_pts_info(st, 64, 8, 32000 * TIMEPREC);
>>     } else if (!strcmp(codec_name, "acelp85")) {
>>         st->codecpar->codec_id = AV_CODEC_ID_SIPR;
>>         st->codecpar->block_align = 19;
>>         st->codecpar->channels = 1;
>>         st->codecpar->sample_rate = 8500;
>> +        st->codecpar->bit_rate = 8500;
>>         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
>> +        avpriv_set_pts_info(st, 64, 8, 8500 * TIMEPREC);
>>     } else if (!strcmp(codec_name, "acelp16")) {
>>         st->codecpar->codec_id = AV_CODEC_ID_SIPR;
>>         st->codecpar->block_align = 20;
>>         st->codecpar->channels = 1;
>>         st->codecpar->sample_rate = 16000;
>> +        st->codecpar->bit_rate = 16000;
>>         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
>> +        avpriv_set_pts_info(st, 64, 8, 16000 * TIMEPREC);
>>     }
>> 
>>     /* determine, and jump to audio start offset */
>> @@ -198,7 +205,30 @@ static int aa_read_header(AVFormatContext *s)
>>     }
>>     start = TOC[largest_idx].offset;
>>     avio_seek(pb, start, SEEK_SET);
>> +
>> +    // extract chapter positions. since all formats have constant bit rate, use it
>> +    // as time base in bytes/s, for easy stream position <-> timestamp conversion
>> +    st->start_time = 0;
>> +    c->content_start = start;
>>     c->content_end = start + largest_size;
>> +
> 
>> +    while ((chapter_pos = avio_tell(pb)) >= 0 && chapter_pos < c->content_end) {
>> +        int chapter_idx, chapter_size;
>> +        chapter_idx = s->nb_chapters;
>> +        chapter_pos -= start + CHAPTER_HEADER_SIZE * chapter_idx;
>> +        chapter_size = avio_rb32(pb);
>> +        if (chapter_size == 0) break;
>> +        avio_skip(pb, 4);
>> +        avio_skip(pb, chapter_size);
> 
> I think this can end in an infinite loop if chapter_size is negative
> 
Nice catch, and my bad, chapter_size is supposed to be uint32_t. And I notice I should 
combine the avio_skip() which is probably more efficient. Will send a patch update.

Cheers, Karsten