[vlc-devel] Why is my subpicture getting displayed for more time than specified?

[Kaarlo Räihä]

24. huhtikuuta 2012 21.07 Peter Tap <ptrtap at yahoo.com> kirjoitti:

> Hi Remi,
>
> Thank you for your help.
>
> The issue we have is that operators in some of our movie theaters do not
> monitor if someone is video recording the movie off the screen. If we
> get our hands on such a pirated movie, we would like to be able to
> identify the theater the movie was recorded from.
>
> The idea was to implement a
> watermarking scheme that is invisible to the human eye but caught by the
> video camera. My thought was that if we show the hardware id for a very
> small time, perhaps we could achieve this.
>
> If we show the text at the same location, pirates can use a video editing
> software and wipe that region off. We have to find a way to discourage
> pirates from editing the movie. If we show the id at random location on the
> screen, frame by frame, it would become very hard for the pirates to edit
> the movie.
>
>
> Is this logic flawed? Is this achievable within VLC?.
>

I think it would be easier to write a plugin for frame based video editor
(like AVIdemux or Virtualdub) and re-encode the video with the filter. That
way you could be sure that watermark is only shown for one frame.


>
> If this is achievable, perhaps the way I am trying to achieve it is not
> right.
>
> I have written a "video filter" module. When VLC calls my "Filter" method,
> the only parameters I can control are the start and the stop time of the
> created subpicture.The start time is usually the time value that VLC
> supplies as a parameter. The stop time is based on the computed timeout
> value.
>
> If I set my timeout value to, say, 1ms, and create a new subpicture at
> every 40.6ms, will it work?
>
> Or, is there a better way to do it instead of writing a video filter?
>
> Appreciate your help.
>
> Regards,
> Peter
>
>
>
>
> ----- Original Message -----
> From: Rémi Denis-Courmont <remi at remlab.net>
> To: vlc-devel at videolan.org
> Cc:
> Sent: Tuesday, April 24, 2012 10:06 AM
> Subject: Re: [vlc-devel] Why is my subpicture getting displayed for more
> time than specified?
>
> Le mardi 24 avril 2012 11:31:18 Peter Tap, vous avez écrit :
> > Honestly, I don't know what you are talking about.
>
> I really cannot see too many ways to put the question:
> How many frames actually exhibit the watermark on the physical display?
>
> > The display time is based on the assumption that most commercial movies
> > (which is what I am dealing with) contains 24 frames per second. If
> there is
> > a flaw in my logic, I would appreciate it if you could present a better
> > idea.
>
> I think you can get the theoretical frame rate in the input video format
> passed to the filter.
>
> *Obviously* though actual rendering must obey the display frame rate. Most
> computer flat panels run at 60Hz nowadays. Only high-end ones support
> 120Hz.
> This is also known as vertical synchronization. As is not a multiple of
> 24Hz,
> frames cannot physically be shown the exact correct time. Ideally, every
> second frame would be displayed for 3 intervals (50 ms) or and 2 (37ms).
>
> 50 ms is surely longer than enough for the human eye to notice the
> artifact,
> especially if it is is stark contrast with the rest of the picture.
>
> Add scheduling jitter in the operating system, and some frames will be
> shown
> for even longer. Only if some frames show for a lot longer than they should
> would I start worry that there is a bug or performance problem within VLC.
>
> --
> Rémi Denis-Courmont
> http://www.remlab.net/
> http://fi.linkedin.com/in/remidenis
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