[mythtv-users] LCD and traditional CRT TVs, overscan and UI size and widget placement

Brian J. Murrell brian at interlinx.bc.ca
Wed Nov 11 13:53:58 UTC 2009

Hey everyone,

I have a traditional CRT TV (am I the only one left with one of
these?  :-).  I am driving that with an nvidia 5200 via S-Video.

As everyone knows, these TVs have an "overscan" feature where they only
make a portion of the original broadcasted picture visible to the
viewer.  Historically, this is done because the outer edges of the
broadcast signal are rough and therefore hidden in this "overscan" area.

But the TV broadcasters know that TVs do this and they compensate by not
putting anything they want visible too close to the edge of the signal
they are broadcasting.  This becomes obvious if you watch a TV station
that does have something at the edge of the visible picture, like CNN's
ticker at the bottom.  If you look at CNN on a device that does not hide
the overscan (i.e. like mythfrontend in a window on your computer) you
can see how much picture is available between the bottom edge of the
ticker and the bottom edge of your window.

Anyway, that's a bit about overscan.

The problem with this overscan though is that unless you adjust it, the
MythUI will draw outside the visible area and thus, you won't see some
of the widgets.  But MythUI has adjustments for this, allowing you to
bring the UI back inside the viewable area.  The problem is that it's
not leaving the UI the same size and just drawing the widgets closer to
center (i.e. to bring them into the viewable area on all sides), but
rather it's just reducing the size of the UI.

The problem with this approach is that it's futile trying to make the
edge of the UI "canvas" meet the visible portion of the picture as the
borders of this visible portion of the picture are never straight an
square.  Frequently they are curved and/or have wows in them.

See this picture I took of my TV:

You can see a "wow" on the top half of the left border.  But this is
just how CRT TVs are.  This is exactly why overscan exists.

The solution, as I see it is rather than scaling the UI canvas down to
fit inside the visible window, the canvas needs to stay the same size
and the positioning of the widgets on the canvas needs to be offset to
fit them inside the visible area.

This effect is not limited to CRT TVs though.

I have an LCD TV that has some overscan as well.  It's not as much as
the TV, but it's still about 20 pixels per side.

On this LDC TV, I run a gnome desktop and the effect of the overscan is
that the panels at the top and bottom are only half visible on the
screen.  When I start a mythfrontend on that screen, with it's
dimensions the full size of the screen, gnome takes this hint and puts
the window on top of the panels.

This same problem of the UI widgets being drawn outside the visible area
exists though, and naturally I want to use the adjustment tool to bring
them back inside the visible area.  But instead of leaving the UI window
the same size as the screen (1280x720) and just moving the widgets in
tighter, this adjustment actually makes the mythfrontend window smaller
and them blam, gnome notices that the window is no longer the same
dimension as the screen and puts the panels on top of the mythfrontend
window again.

So another case to be made for leaving the canvas size alone and simply
moving the UI widgets so that they are on the visible portion of the



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