[vlc-devel] Video Time does not match Reported Time

Eric Montellese emontellese at gmail.com
Sun May 12 03:02:28 CEST 2013


The project is "Datavyu" (http://datavyu.com/), which is a "data coding
tool" for scientific research.

An overview of "coding":
In psychology research, videos are often used in order to collect accurate
After the experiment is completed, folks in the lab "code" the videos.
 "Coding" refers to a process of watching the videos and recording the
exact moments when certain actions occur.  (For example, when the subject
crosses a particular threshold, or selects a particular object, etc).

The tricky bit is that this coding must be frame accurate.  Some of these
experiments are very precise, and being off by even a frame or two may skew
the results (some experiments even use high-framerate video).  For this
reason, the coders will often watch the videos at a very slow rate, pausing
and jogging frame-by-frame often at points of interest in order to get the
necessary accuracy.

However, during development, we determined that there is an issue with
VLC's time-accuracy.  The original bug was that coming out of pause into a
slow-forward mode would take a very long time.  It was determined that the
root cause of this issue is that the time of the video onscreen was not in
sync with the time reported by VLC.

This was confirmed by pausing and then calling:
which shows visual jump in the video.

What we are looking to determine is whether or not VLC can be a viable
solution as the video player embedded into datavyu, or whether we would be
better served to look at other projects.

For reference, Datavyu is a Java program; and we are using the "VLCJ"
project, which uses libvlc.  I have already investigated the code in VLCJ
and determined that in this case it is just a pass-through to libvlc.

Any information you can provide would be appreciated,


On Sat, May 11, 2013 at 7:50 PM, Jean-Baptiste Kempf <jb at videolan.org>wrote:

> On 09 May, Eric Montellese wrote :
> > We are currently determining whether we can continue trying to work with
> > VLC, or whether we need to jump ship to another video player.
> You need to explain a bit more what you are trying to achieve, to be
> honest.
> Best regards,
> --
> Jean-Baptiste Kempf
> http://www.jbkempf.com/ - +33 672 704 734
> Sent from my Electronic Device
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