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Re: Instability in rectangular cursor

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  Re: Instability in rectangular cursor Richard Ashbery <basura@invalid.addr.uk> - 2020-11-15 13:20 +0000

#6224 — Re: Instability in rectangular cursor

On 13 Nov in comp.sys.acorn.programmer, Steve Fryatt
<news@stevefryatt.org.uk> wrote:
> > You make a fair point and although WAIT does work in this example
> > it dramatically slows down the program that this code has been
> > ripped from.

> It will do, because it seems to rely on replotting the screen many
> times a frame, which is what causes the tearing effect.

> WAIT simply pauses the program until the start of the next video
> frame, when there's a small window of time in which to update the
> screen while the video flyback is happening. If the code is
> expecting to execute the REPEAT loop many more per second than the
> monitor refresh rate, then the game will slow down as a result.

> Unfortunately, you can't change the video refresh rate because
> that's set by your monitor, so to fix the tearing you will need to
> decouple the game timing from the screen update.

Thanks Steve for the explanation. It looks as though a complete rewrite
is necessary. Steve D indicated that triple buffering may be required -
I briefly had a look at this but understanding the process is
challenging - maybe it's time for a revisit.

My simplified version does away with the mouse cursor and relies on the
wash colour to erase the verticals - it seems to work just as well but
doesn't look so pretty.

Richard

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