Date   : Sun, 28 Mar 2004 13:37:27 GMT 
From   : "Thomas Harte " <thomasharte@...>
Subject: Re: Aspect ratio question

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> How do you work
that out?  It should
be 960 x 768, but
that will be
> splitting pixels
in modes 0 and 3.  

Splitting pixels
isn't an issue in
the slightest, since
the emulator is
using an overlay
surface. I
calculated it from
PAL timings. As I
understand it, the
visible PAL area is
(give or take) 288
lines and each
scanline is 52 µs of
visible time, of
which the Acorn
machines use 40 µs.

You therefore have a
display that uses
8/9ths of the
vertical screen area
and 10/13ths of the
horizontal. If we
scale both numbers
by 9/8 so that the
image is scaled to
full the whole
vertical area, we
see that 45/52ths of
the horizontal area
is used - actually
about 886 pixels in
a 1024 across video
mode.

So, in conclusion,
I've no idea how I
got 932 before. But
this is the route
from which I
calculated the
result.

> The Beeb displays
in modes 0 to 6
> are all the same
size (mode 3 has
gaps between the
character lines so
> it uses less
memory and has fewer
addressable pixels
than mode 0, but
> it's exactly the
same screen size) 

With all due
respect,  twenty
five 10 pixel
character lines
makes a 250 line
display, compared to
256 in the other
modes.

> and are equivalent
to 5:4 or 640 x
> 256 pixels in mode
0 -- a ratio of 1.25
(the pixels are
exactly twice
> as high as they
are wide).  

So in Mode 4
(320x256) you have
exactly square
pixels? It just
doesn't look like
that to me.

-Thomas

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