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Re: MF implementation with python-gst

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Wed, 12 Aug 2009 07:57:50 +1000
Message-ID: <2c0e02830908111457i2232a8e6u7c95bbcbf0511bb2@mail.gmail.com>
To: Guillaume Olivrin <golivrin@meraka.org.za>
Cc: Media Fragment <public-media-fragment@w3.org>
Hi Guillaume,

You should really talk to Edward Hervey, the main developer of PiTiVi. Why?

GStreamer is an excellent media framework to decode, encode,
manipulate, and play back video. However, it doesn't easily do much of
the type of video (and audio) manipulation that we require directly in
the compressed domain for media fragments.

Edward has, however, implemented a python library that extends
GStreamer to do these kind of things. It means that you can do, for
example, time offsets in Ogg without needing to decode it.

I can introduce you to Ed - but you might want to read a bit about the
available documentation. It may be a bit outdated, though, I fear. I
spoke with Ed at OVC and he attended FOMS in January where he
explained what he has done / is still planning to do with PiTiVi.

Cheers,
Silvia.

On Wed, Aug 12, 2009 at 1:15 AM, Guillaume
Olivrin<golivrin@meraka.org.za> wrote:
> Dear Fragmenters,
>
> I have looked into the python-gst library to see if it was feasible to
> easily create Media Fragments with Gstreamer .
>
> I considered 3 levels of implementation :
>
> 1. High level - using direct GST pipeline elements
> 2. Middle level - using GST programmatically (python-gst)
> 3. Low level - implementing new plugins for GST
>
> The principal features that make Gstreamer attractive are:
> * Handles great many media formats
> * Can handle URI
> * Possibility of integration with Jack's MF syntax parser implemented
> with python-url
> * Programmable interface with Python
> * Most importantly modularity : healthy separation between
> Media/Protocol access, Demuxing, Decoding etc ...
>
> --
> 1. Using available GST elements
>
> There are already existing GST plugins to Crop a video or to Seek a
> specific Start and End position in a Audio or Video media :
>
> * videocrop:  aspectratiocrop: aspectratiocrop and  videocrop: Crop
> * debug:  navseek: Seek based on left-right arrows
>
> The problem is that, as far as I know, these two plugins are only usable
> behind a decoder, i.e. using raw YUV or RGB video and PCM audio.
>
> We want to be able to do these operations directly on the media stream
> without decoding and re-encoding it. To do that, we need to place
> ourselves directly behind Demuxers elements. Demuxers know about
> specific Audio or Video files and can parse the structure of the
> internal compressed media stream, providing information about TIME-BYTE
> offsets. There are two other things we can do : send events to the
> pipeline programmatically (2) or create new GST plugins that fit behind
> demuxers.
>
>
> --
> 2. Programmatically with Python
>
> Media Fragment along the Time Axis.
>
> Depending on the plugin involved in the GST pipeline, it is possible to
> perform SEEK operations on the stream using the following unit formats :
>
>        'undefined' / 'GST_FORMAT_UNDEFINED', 'default' /
>        'GST_FORMAT_DEFAULT', 'bytes' / 'GST_FORMAT_BYTES', 'time' /
>        'GST_FORMAT_TIME', 'buffers' / 'GST_FORMAT_BUFFERS', 'percent' /
>        'GST_FORMAT_PERCENT'
>
> Also, there are different SeekType and SeekFlags to change the seeking
> techniques, mode and accuracy. More info at
> http://gtk2-perl.sourceforge.net/doc/pod/GStreamer/Event/Seek.html
> It is implemented through the following function:
>
>        event = gst.event_new_seek(Rate, Units,
>                                           Flags,
>                                           gst.SEEK_TYPE_SET, ClipBegin,
>                                           gst.SEEK_TYPE_SET, ClipEnd)
>        res = self.player.send_event(event)
>        self.player.set_state(gst.STATE_PLAYING).
>
> OR
>        gst_element_seek(
>        pipeline,
>        Rate,
>        GST_FORMAT_TIME,
>        Flags,
>        GST_SEEK_TYPE_SET, pos,
>        GST_SEEK_TYPE_SET, dur);
>
>
> Both commands will send the SEEK event to the whole pipeline and some
> GST elements will be able to handle it. But we might want to be more
> precise and know exactly which elements can handle seek and what are
> their capabilities.
>
> For example, can SEEK events be used at the level of DEMUXERs ?
> source | DEMUXER | sink
>            ^
>           SEEK event
>
> E.G. Consider the following GST chain for OGG :
>
> filesrc | oggdemux |
> filesrc | oggdemux |
>
>
> The questions that must be further investigated are:
>
> * Which GST elements can handle seek events?
> * What unit formats (time ns (nano seconds), frames, bytes, percents,
> buffers) are supported by each GST elements?
> * Can all encapsulation specific demuxers handle time and bytes?
> * Can SEEK events be translated higher up in the chain into BYTES on the
> filesrc SOURCE? Then we could still decode the media, find the actual
> part of the stream required, make sure a filesrc or uridecodebin in
> random access can point to the fragment of the media we need, and SINK
> that MF into a filesink.
>
>
> Until now I haven't been successful in implementing the GST SEEK events
> on a variety of media types ;  neither directly in C or in Python) with
> gst.event_new_seek(..) or gst_element_seek(..).
>
> --
> 3. Writing and Compiling new GST plugins
>
> For Video Cropping, filters at BYTE/STREAM levels behind demuxers ?
>
> It is likely that to perform crop operations on a video stream without
> touching it, we will need specific pluginS to put behind demuxers for
> each type of video streams. This certainly represents quite a bit of
> work.
>
> A possibility to investigate : could there be again a pipeline PULL
> action that request only these bits required for the cropped up video to
> be pulled and sunk back into a file / pipe ?
>
> Best Regards,
> Guillaume
>
>
>
>
Received on Tuesday, 11 August 2009 21:58:46 GMT

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