[Bug 663750] dvdspu: dvdsubs aren't correctly scaled/positioned when corresponding video isn't DVD resolution

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=663750

Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> changed:

           What    |Removed                     |Added
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                 CC|                            |nicolas.dufresne at collabora.
                   |                            |co.uk

--- Comment #14 from Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> ---
>From bug #685282 by Jan:
> On a real disc, the video would actually be encoded at 1920x1080, so the 
> placement would be a non-issue.

I do remember a random question on IRC and me replying strictly that overlay
composition must be inside the video. Sorry, if I didn't get the point here. So
from what I now understand, while on DVD there is black bars (so no issues),
those are stripped off when put on file. Though the overlay remain placed in
what use to be a black bar so they don't hide the content.

I think we will all agree that "adding" black bars by copying the stream is not
exactly where we want to go. I have few ideas, they might not all make sense
(or be doable in sinks, we need to study that).

1) Do nothing here, assume that sinks will figure-out by calculating the
bounding box of the video and the overlay and make sure it's all right by
adding black bars (or moving the overlay). Not sure this one is realist, could
mean that bars will change in size every time a new text is sent. That means
sink need to stop clipping.

2) We are aware of the display size. We can add the display aspect ratio, and
with that we'd have complete information to figure-out best adjustement
placement for the text. It does not matter if sink scales or not, the goal
would be to make sure the text falls inside the display. That means, if the
sink keep the aspect ratio, and it's using one of TV standard resolution, the
text will endup where we expect it to be (though will vary slightly between
screen aspect ratio). That requires sink to clip on display size instead of
video size.

3) Add and negotiate a new meta, so signal the sink that black borders are
required. Hopefully most sink can do something cheaper, but have a fallback
where we copy the frame into a larger frame. This one will give the same
placement regardless of the display size. It is I think the most correct, as
you get that same placement regardless of your setup. Though is really
expensive if you have a sink that does not support overlay composition and also
that feature.

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