Date : Mon, 29 Mar 2004 18:51:05 GMT
From : "Thomas Harte " <thomasharte@...>
Subject: Re: Aspect ratio question
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Sorry if this reply
seems somewhat
short, I've just
lost my original
reply and its put me
in somewhat of a bad
mood.
> With all due
respect, the timing
isn't what matters,
nor is the
> fraction of the
display used.
When we're talking
about how wide the
display is compared
to how high it is,
the question is
exactly and
explicitly
interested in the
fraction of the
display used. The
emulator part was
just my
justification for
asking.
> The fact is that a
Beeb displays
pixels,
> and an emulator
must display
matching pixels if
it's to be accurate.
> How, for example,
are you going to
display a letter "m"
correctly (it
> has a 1-pixel
margin, 2 pixels for
the left leg, then
one blank pixel,
> one pixel for the
centre, one blank,
then two pixels for
the right
> leg)? Something
that turns two
pixels into three
can't display that
> correctly.
Yes it can. Check
the
comp.graphics.algorithms
FAQ for various ways
of reinterpretting
sampled signals that
amount to image
scaling with a much
greater display
quality than you
imply is possible.
See
http://electrem.emuunlim.com/images/AcornUserMode7.gif
for an example of
such a scheme in
action, displaying a
Mode 7 display at
the correct width -
i.e. the same width
as the normal
display, not
significantly
thinner as many of
the older emulators
are forced to.
All that is beside
the point though. As
I said, my emulator
is using an overlay
surface. Which means
a programmable pixel
clock. I get the
correct aspect ratio
without any pixel
scaling. See the
last binary version
of my emulator in
full screen mode as
evidence of this
(http://electrem.emuunlim.com/files/future/ElectrEmFuture07032004Win32.zip
for a Win32 binary,
alt+enter to go full
screen). But, be
warned: there are
still significant
issues with task
switching - don't
switch away then
expect to be able to
switch back, unless
SDL feels like
having a good day.
The ratio is wrong
(its the one in my
original post, not
the one I now think
to be correct), but
it isn't 1:1 with
the underlying
graphics mode (which
is 1024x768, as
obviously overlay
surfaces are
constrained to the
same scanline
counts).
> Well, it does use
square pixels. The
manuals even say
that's a
> square-pixel mode.
Obviously if your
screen is not
correctly adjusted
> it won't *look*
square :-)
Could it be that
they are square when
viewed on a monitor
rather than a TV?
For an 8bit micro, a
surprising number of
BBC owners seem to
have acquired
monitors at some
stage. Otherwise, I
guess the problem
could be Electron vs
BBC, but as they are
based on exactly the
same underlying
clock speeds (don't
believe people who
say the Elk had a
1.7Mhz clock, or
anything like that)
I don't see how that
could be true.
I invite an
explanation of how
my calculations are
incorrect.
-Thomas
Lycos Email has 10 MB of FREE storage space. http://mail.lycos.co.uk
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