[mythtv-users] Preview generation in 0.24-fixes

Michael T. Dean mtdean at thirdcontact.com
Mon Jan 31 23:54:19 UTC 2011

  On 01/31/2011 06:12 PM, John Pilkington wrote:
> Recently the previews shown in the Watch Recordings screen seem to be 
> based on frames much further into the (edited) recording than they 
> used to be;  I have two recordings (around 10 minutes) where the 
> previews are almost at the end, and several of around an hour where 
> they are about halfway through, but it doesn't seem consistent.  I 
> thought the preview location used to be specified by the user, but my 
> hunt through the menus yesterday didn't find a setting, and I don't 
> see anything that looks relevant on the list.  Anyone else seeing this?

There is no longer a setting to allow users to specify the one place in 
every single recording they currently have--and will ever create in the 
future--which is ideal for creation of a preview pixmap.  :)

MythTV now calculates a position for the default preview pixmap based on 
several factors, including program duration, actual recording start time 
(adjusting for recording-rule-specified start early and "Time to record 
before start of show (secs)" values to ensure the preview is within the 
scheduled broadcast), as well as the commercial flag list and the cut list.

In testing the new algorithm, I found that the old setting allowing us 
to specify an offset was a complete and total placebo.  Commercial 
placement, show intros, and early credits were consistently 
inconsistent.  In order to consistently specify a location that was 
within the actual show, we would need to take into account far too many 
factors--including which show it is (is it one that uses the new-style 
6-breaks division or the old-style 4-breaks division, and--for this 
particular show--how are those breaks distributed), from which channel 
the show was recorded, whether it's a first-run or repeat or syndicated 
episode, and even, to some extent, which input was used to record it (as 
some inputs impose a delay of up to a couple seconds).  Even after 
taking into account these factors, it still requires considerable 
guesswork to find a good place.  Because of this, and because the new 
algorithm was significantly more accurate than the old "one place for 
every recording" approach, I removed the setting.

Users wanting to specify a location for a preview should do so by 
setting a bookmark.  Upcoming changes to bookmark handling will make 
this even easier/more user-friendly.

If your recordings have consistently-broken previews, it likely means 
that you have broken seek tables.

As far as the short recordings having previews consistently at the end 
of the recording, this would happen if you have a ludicrously-large 
value for "Time to record before start of show (secs)" setting and you 
a) cut recordings (to make them smaller than they originally were) or b) 
used to have a small value for the setting, but recently changed it to a 
huge value.  There is no information stored about whether a recording 
used the "Time to record before start of show (secs)" value or what that 
value was when the recording was made.  Therefore, and because it's 
meant to be a small value that allows you to spin up hard drives or let 
your slow-tuning-capture devices (such as HD-PVR) settle, it's assumed 
to have been used for all recordings.  So, if you had a value of 10 
minutes, all your 10-minute recordings would consistently have previews 
from the end of the recording.  (Note, also, that the old approach was 
to blindly add the "Time to record before start of show (secs)" value on 
top of your specified offset, which would result in the exact same issue 
described here--so this part hasn't changed.)


More information about the mythtv-users mailing list