[theora] oggz reading & seeking
conrad at metadecks.org
Thu Aug 27 15:05:09 PDT 2009
2009/8/28 Chris Pearce <chris at pearce.org.nz>:
> On 8/27/2009 9:43 AM, Manolache Adrian wrote:
> > I'm wondering if i am missing something obvious.
> I'd guess you're missing something non-obvious!
> However, your code after oggz_seek_units() is non-optimal. It's reading
> whatever packet is 1/4 of the way behind the read cursor, and if the
> first packet at that point isn't a keyframe, it jumps back 1/4 of the
> way to the original byteOffset. You need a more reasoned approach -
> you'll likely skip past previous keyframe packets with this appraoch.
> Also oggz_tell_granulepos() returns the gp of the last page read, which
> is the gp of the last packet on that page. If there's an interframe
> packet on the page after the keyframe, that page's gp won't have a 0
> offset, despite containing a keyframe. So you could be pass over a
> keyframe packet without knowing it. The page with offset-0 gp is also
> where the keyframe ends, it may start on a previous page!
I'm working on a new (and backwards compatible) interface for
oggz_read, where the oggz_packet provided to a read callback includes
the byte offset of the page the packet started on:
This is used in the oggz_packet and seek-rewrite branches, so if
you're working with byte offsets then that interface might be a good
way to go -- simply change the interface of your read callbacks from
ogg_packet to oggz_packet. (The oggz_packet branch contains only this
change, on top of master).
> You should use th_packet_iskeyframe() to determine if a theora *packet*
> is a keyframe. You also don't need to decode the packets (using
> th_decode_packetin()) until you find the keyframe.
> Chris P.
> On 8/27/2009 9:43 AM, Manolache Adrian wrote:
>> Thanks a lot you guys! I am using liboggz and i need this for playback
>> from disk. I tried patching my liboggz with the patches mentioned by
>> Chris but haven't had any luck(tried skipping the files that we're not
>> there but always got a malformed patch file error when no more skipping
>> could be done).
>> So after reading the URL's you posted i tried using the not so fast
>> method described here
>> http://lists.xiph.org/pipermail/theora/2009-July/002544.html. I've been
>> successuful in eliminating the artefacts but it is rather slow.
>> I tried implementing the following approach then:
>> 1.0 seek to time t, retrieve current byteOffset (with oggz_tell) and
>> 2.0 repeat until the offset of the current frame is not 0(beginning of
>> the frame)
>> -> read data until a full packet is formed
>> -> extract frame and offset from this packet, if frame=currentFrame and
>> offset is 0
>> then stop, else decide in which direction to seek(forward if
>> frame<currentFrame, or backward if offset>0 and frame=currentFrame)
>> Every time curretByteOffset is incremented(with a value that gets halfed
>> now, if we seeked before currentFrame and/) or decremented so this
>> process is reasonably fast.
>> I do get some artefacts though, even if this method is faster. I'm
>> wondering if i am missing something obvious. Here is some code if that
>> would help better understand the approach i am using:
>> The seek routine(the numbers are just for testing).
>> and everytime a video packet arrives i call this function:
>> --- On *Mon, 8/24/09, Chris Pearce /<chris at pearce.org.nz>/* wrote:
>> From: Chris Pearce <chris at pearce.org.nz>
>> Subject: Re: [theora] oggz reading & seeking
>> To: "Manolache Adrian" <prog_ady at yahoo.com>
>> Cc: theora at xiph.org
>> Date: Monday, August 24, 2009, 6:23 PM
>> Your decode will be encountering only interframes after the seek. It
>> will return garbage until it encounters a keyframe. As ogg.k.ogg.k
>> says, you need to seek back to the previous keyframe and decode
>> forwards from there.
>> I fixed this in Firefox's HTML video element implementation. There's
>> a patch in this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=463358
>> which you may find useful. This uses the double-bisect method ogg.k
>> mentioned. It still fails in some cases, due to these reasons:
>> Seeking is hard.
>> Another solution is here, but it's not as fast on local files:
>> Chris P.
>> On 8/24/2009 7:06 AM, Manolache Adrian wrote:
>> > I've rewritten my theora player using liboggz. I've stumbled upon a
>> > few issues though. First of all liboggz exposes oggz_read and
>> > but how can i read only sound data, or only video data using liboggz,
>> > say something like this: oggz_read_sound(1024),
>> > Second of all i tried using liboggz's seek function. Immediately
>> > the seek i get some "garbage" frames: how can i address this issue?
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