I’m happy to report that callbacks seem to be working with DirectShow in
Windows in LC 8.1 RC2.
Basic:
Callbacks are a property of a player. They are basically are a list of numbers,
each line followed by a message name. The number corresponds to a time in a
movie/audio file loaded into a player. When the player reaches that time, it
sends the message. This is useful to have things happen in time synced to the
progress of a movie.
Advanced:
I already have these callback lists prepared while I was working with .mov
files on Mac and Windows. With the deprecation on QT in Windows, I’m converting
those files to wma/wmv.
Integral to the use of callbacks is the timeScale property of a file. The
timeScale is a number of intervals or blocks per second in a movie. The files I
had already created with the QuickTime player all have a timeScale of 600. So
when a player plays, these are the numbers that correspond to the progress of
time.
After converting a MOV file to WMV, I’ve found that the timeScale went from 600
to 10,000,000.
So 10,000,000/600 yields a constant of 16,667. That's the number I use to
multiply each callback number I already have to play in Windows. And it seems
to work.
Panos suggested:
> Now, regarding your question, I am not 100% sure but I think that
> the Windows player reports always longer duration compared to OSX, but this
> multiplier is not constant, it differs depending on the audio/video file.
> This is why I suggest setting the callbacks as a fraction of the duration,
> since this will work cross platform. i.e.
>
> (duration/8),callback1
> (duration/6),callback2
> (duration/2),callback3
> ...
I did not have to do anything with the duration. The cross-platform conversion
where I know the original timeScale in QT, and the original callback in QT:
(timeScale as WMV / timeScale as MOV) * existing callback in QT = callback in
DirectShow
To get the timeScale of a file, you have to first load it into a player and
then issue:
put the timeScale of player “Player” into tVar
However, the player in an existing stack reported a timeScale of not 10
million, but 10,000. So there is variation in the timeScale which affects the
duration correspondingly. So the algorithm above has to be used.
That existing stack also crashes in the runtime at the point in the script
where the callback is set. It works properly in the IDE. I haven’t tracked down
why, but the stack is part of a large project. If I run that stack in isolation
in the runtime environment, it works properly, and now has a timeScale again of
10 million. Perhaps when the player reports a timeScale of 10 thousand there is
something wrong and it then causes a crash.
Otherwise… having callbacks working in native Windows is a huge deal for my
multimedia application, Music In the Air, and I’m very grateful to Panos and
the LC team for getting it working in LC8!
I’m planning to move the app in the future to HTML5 and would love to see a
player with callbacks working there… :)
Peter Bogdanoff
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