[FFmpeg-devel] Problem with seeking in avi file.

[Neil Brown]

I have another interesting movie file that ffmpeg doesn't quite work
properly with.

This one is an avi file containing dvvideo extracted off a digital
camcorder (I can find out what program was used to exact it if that
might be of interest).

Anyway, the problem is that the first time that I seek into the file,
if I aim for beyond 4:57, I alway arrive at 4:57.  Subsequent seeks
work as expected.
e.g.

  ffmpeg -ss 400 filename.avi

starts at 4:57, as does any number of seconds greater than 300.\

The file starts with RIFF with a size of 1069618820.
This covers about 1/4 of the file (5 minutes of about 20).
There are 3 more RIFF headers of 1069611496, then
one of 63795560 which completes the file.

The file contains a 'dmlh' tag, so is_odml is set.
The file contains a 'LIST' tag with a 'movi' subtag and a size of
1069467476.  This just covers the first of 5 RIFF sections.

So ->movi_end gets set to 1/4 of the way through the file - the end of
the first section.

In avi_read_packet, after a seek further into the file,
'i' is the current position in the file, so this code:

        if (i >= avi->movi_end) {
            if (avi->is_odml) {
                url_fskip(pb, avi->riff_end - i);
                avi->riff_end = avi->movi_end = url_fsize(pb);
            } else
                break;
        }

seeks back to the end of the first RIFF, then sets movi_end to the
correct end of the file.
It seems to me that if 'odml' allows as to set movi_end to fsize at
this point, it should allow movi_end to be set to fsize right at the
start.

So I changed avi_read_header so that instead of:

                if(size) avi->movi_end = avi->movi_list + size + (size & 1);
                else     avi->movi_end = url_fsize(pb);

it does

                if(size && ! avi->is_odml)
			 avi->movi_end = avi->movi_list + size + (size & 1);
                else     avi->movi_end = url_fsize(pb);

As the 'dmlh' header precedes the LIST header (in my example file),
this works and I can seek correctly.

If the dmlh header is allowed to come after LIST, maybe the handler
for dmlh should set movi_end to url_fsize ??

Any help with further understanding, or otherwise getting this issue
fixed would be most appreciated.

If it helps, I can zero out bits of the file that are not read during
testing, compress it, and put it somewhere for download.

Thanks,
NeilBrown