[mythtv-users] Unable to skip forwards during DVD playback

[Richard Hulme]

On 13 April 2014 19:50:51 CEST, Thomas Boehm <mythtv-users at lists.boehmi.net> wrote:
>On 13/04/14 17:56, Jan Ceuleers wrote:
>
>> I just tried watching a DVD, but not from the start. I'd watched the
>> first part on another machine (nothing to do with Myth), then wanted
>to
>> watch the rest on my frontend.
>> 
>> So I fast-forwarded to the part I wanted to watch, but when I pressed
>> play it began playing the movie back from the beginning, not from the
>> part I had fast-forwarded to.
>> 
>> Does anyone know what's going on?
>
>You seem to have just discovered UOP ;-)
>
>http://en.wikipedia.org/wiki/User_operation_prohibition
>
>But I'm not sure whether Myth (or Linux in general) obeys UOP. I ripped
>my DVDs long time ago...
>
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Hi,

UOPs are ignored by MythTV. They're a double-edged sword, really.  Most of the time they're a pain in the neck because they're used to force you to sit through all the crud that comes before you get to the main menu.

On the other hand, DVDs are built on the basis of a virtual machine. Along with the video, audio and sub-picture streams, the DVD author can add instructions to read and write variables and jump somewhere conditionally. So, for example, when a DVD starts by asking you which language you want, it stores that in a variable and uses it to ensure it always jumps to the right version of each menu or sub-menu.

These 'programs' can get quite complicated with lots of different variables being used to note different states.  Sometimes UOPs are used to ensure that the user cannot skip a certain point because doing so would cause the variables to get out of sync. For that reason, not enforcing UOPs opens up the possibility of weird behaviour that then make the player look bad.

This is one of the problems that players sometimes have that try to jump straight to the main feature and not 'bother' the user with menus. The DVD VM is not set up the way the DVD expected it to be and playback fails either entirely or at unexpected points.

DVD playback improved dramatically in MythTV when I made changes to stop the code trying to second-guess things and just start at the beginning.

Richard
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