Date   : Mon, 29 Mar 2004 00:22:21 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Aspect ratio question

On Mar 28, 13:37, Thomas Harte  wrote:
>
> 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.

With all due respect, the timing isn't what matters, nor is the
fraction of the display used.  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.

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

Actually, you are correct there, the addressable part of the mode 3
screen doesn't extend down quite as far.  Sorry, I miscounted and
didn't double-check.

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

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 :-)

-- 
Peter Turnbull
<postmaster@...                 >