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

Rémi Denis-Courmont remi at remlab.net
Tue Apr 24 19:06:41 CEST 2012

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

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