Re: [stella] Mortal Kurling!

Subject: Re: [stella] Mortal Kurling!
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Sat, 4 Jan 2003 14:14:26 +0100
I'm talking about a real shearing effect as you move the "ship" (which is
the ball) vertically. I'm running z26 in -v9 (Display #scanlines - a true
godsend!) and now switching modes, it doesn't exhibit this behaviour. Only
in -v9 mode. It kinda looks like it's moving every 2nd bit every other frame
(ie: Moving bit 0,2,4 on frame A and bit 1,3,5 on frame B). It only occurs
with vertical movement, only in -v9 and (maybe) only the ball (not 100% sure
on that last one, I'd have to test but I haven't seen it on anything

This happens because the video memory for mode 9 is set up in such
a way that every 4th pixel shares the same memory bank. Since this
mode is so large, we can't do double buffering for the video data
here. So on a slower computer it might be that the frame buffer gets
copied into the VGA RAM while the picture is already displayed. In
this case the data for some of the VGA memory banks might already
contain the data for the new frame, while others still have the data
for the old frame. When something is moving vertically by one line,
it can result in the effect that you describe.

Try to add the -e option to your z26 command line. This enables
faster 32-bit copying routines which usually are able to move
the whole frame during the vertical blank period even on my slow
P166MMX. We didn't want to make them default, because some older
VGA cards seem to have problems with 32-bit accesses. Therefore
you have to enable them manually.

Ciao, Eckhard Stolberg

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