Impress video glitch - comments and a workaround

[Mike Scott]

Hi all. I've raised this issue on the LO users list; they suggested 
transferring it here for better comment. So, I'm totally new here and to 
LO's source code and its quirks (as well as very out of date with my C++ 
skills!). Apologies if I transgress any mail-list rules.

There has been (even IIRC since the OOo days) a glitch when playing an 
embedded video clip into an Impress slide show. I believe the issue has 
been reported by various people over the years, with at best 
unsatisfactory work-arounds suggested.

Jumping straight in, when embedding a clip into a slide, a place-holder 
frame is picked. This appears to be done by a method grabFrame() defined 
in the file mediawindow.cxx. This file picks a frame as near as possible 
to 3 seconds into the clip -- a position that is compiled in. As far as 
the user is concerned, it's effectively a random frame.

Unfortunately, while that is adequate for a static placeholder at design 
time, the exact same frame is displayed during the 'start slide' 
transition before the video starts to play, and also left visible when 
the video finishes. This typically gives rise to a flash at the start if 
there's no slide transition or a wrong frame displaying if the slide 
transition is slow (the video only starts when the transition is complete).


There is, however the simple and obvious workaround of setting the time 
offset for that frame to zero. At the very least, a video will normally 
fade in from and out to the same frame (or can be contrived to do so).


I've tried this out on the current HEAD sources (6.something), with the 
expected effect on the start of the video - no inappropriate frame 
displayed, and the video starts smoothly.

However, for reasons unknown, I've found that the /true/ last frame 
remains displayed at the video end (I've been testing with a video each 
of whose frames contains just the frame number). (A bonus, but the 
reason really should be understood.)

A minor snag is that if a video starts with a black frame, that's what 
will be seen when the slide is being laid out.

Is it reasonable to suggest for now a single-character alteration to the 
source code? At line 37 in
libreoffice/avmedia/source/viewer/mediawindow.cxx

change
#define AVMEDIA_FRAMEGRABBER_DEFAULTFRAME_MEDIATIME 3.0
to
#define AVMEDIA_FRAMEGRABBER_DEFAULTFRAME_MEDIATIME 0.0



(I'm using linux Mint, but I don't believe the LO issue is OS-specific. 
The version I've edited locally shows as 6.0...alpha1+, but it looks as 
though this code has been unchanged for a long time.)


-- 
Mike Scott (unet2 <at> [deletethis] scottsonline.org.uk)
Harlow Essex England
"The only way is Brexit" -- anon.