[mythtv] bobdeint: vertical jitter behaviour

[Bruce Markey]

Jeroen Brosens wrote:
> I found that when using bobdeint the image sometimes starts to go 'out 
> of sync' and the nice fluid motion goes into a horrible vertically 
> jittering one. It manifests erratically and without apparent reason. Can 
> anyone confirm this behaviour? I am using an ASUS Pundit with SiS 
> integrated video, X.org 6.8.1, march=pentium4, MythTV CVS (current).

When you start playback, what does the mythfrontend log say for
"Video timing method: ..."? If it is anything other than OpenGL
or DRM, what you describe is a known limitation.

With OpenGL or DRM, the system can get the frame timing from the
card and update the frames in sync with the hardware retraces.
Otherwise, the frames are updated every frame interval time apart.
When these updates fall somewhere in between the (unknown) hardware
retraces, you get a new frame for every refresh and it is as
smooth as possible. However, if the updates are almost on top of
the hardware retraces, some frame updates will happen a couple
milliseconds early, others a couple milliseconds late. This causes
a rapid fire studdering effect that can last for seconds. Ironically,
the more accurate the timing, the longer these events last. When
this happens with bob turned on, you will see the vertical 'bob'bing
as you see the alternating line doubled half fields.

To avoid this, first see if you can get the OpenGL or at least DRM
timing methods to work. I know nothing about SiS cards or drivers
 so I don't know how or even if this is possible. With an nVidia
card, it is simple to use the OpenGL method.

--  bjm