Xref: utzoo comp.sys.amiga.tech:4218 comp.sys.amiga:30788
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From: w-colinp@microsoft.UUCP (Colin Plumb)
Newsgroups: comp.sys.amiga.tech,comp.sys.amiga
Subject: Re: Fast 2-D Scrolling Method Needed (kinda long)
Message-ID: <12367@microsoft.UUCP>
Date: 18 Mar 89 04:52:58 GMT
References: <112@dg.dg.com> <10871@well.UUCP> <113@dg.dg.com> <10921@well.UUCP> <736@cord.UUCP>
Reply-To: w-colinp@microsoft.uucp (Colin Plumb)
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I'm pretty sure this is feasable.  Handling multiple bitplanes is
tricky (you have to change them all in one scan line), but I think
it can be done with some ingenuity...

Don't bother with the slices.  Each bitplane is as tall as the screen,
plus some slop for off-screen rendering.  Initially, you start the
display at row 0 and it goes to row n-1.  Rows n through m-1 are
off-screen.  Then, as you scroll down, rows 0, 1, 2, 3... fall off the
top, and rows n, n+1, n+2, n+3... appear on the bottom.  When
row m would be needed, stick a copper instruction into the display
list to reset the memory pointer to row 0.  Then this instruction moves
up the copper list until it gets to the top of the screen and we're
back where we started.

For multiple bitplanes, stagger them so the copper instructions are between
different scan lines.  Thus, only two registers (BPLxPTH and BPLxPTL,
the high and low halves of the pointer) have to be reset in one scan
line.  Seems doable.

To render into this bitplane, you can write your own graphics routines
that understand the need to wrap around, or do nasty things to the
layers.library and let it worry about the discontinuity in the
circular buffer.

You can also use this to scroll horizontally, although you have to be more
careful - I doubt you can jump from end to beginning of your circular
buffer in mid-scan-line.

Maybe someone more into graphics hacking than I (Leo?  Have you finished
Onion yet?) can write some sample code.
-- 
	-Colin (uunet!microsoft!w-colinp)

"Don't listen to me.  I never do." - The Doctor