Path: utzoo!attcan!uunet!wuarchive!mailrus!jarvis.csri.toronto.edu!eecg.toronto.edu!leblanc
From: leblanc@eecg.toronto.edu (Marcel LeBlanc)
Newsgroups: comp.sys.cbm
Subject: Re: Machine Language Smooth Scrolling Problem
Summary: Easier with scroll UP
Keywords: machine language, scrolling, C64, squid, gratification
Message-ID: <1989Oct29.134328.26475@jarvis.csri.toronto.edu>
Date: 29 Oct 89 18:43:28 GMT
References: <2317@jarthur.Claremont.EDU> <136@cbmtor.UUCP> <2823@husc6.harvard.edu> <1989Oct11.151724.18728@jarvis.csri.toronto.edu> <2911@husc6.harvard.edu> <1274@utkcs2.cs.utk.edu>
Organization: EECG, University of Toronto
Lines: 47

In article <1274@utkcs2.cs.utk.edu> mccaslan@cetus1a.cs.utk.edu (Donald McCasland) writes:
>>Regarding scrolling down.  This algorithm does it, starting from the top of
>>the screen and working its way down: 
>>
>>1. Copy present character row into buffer B.
>>2. Copy buffer A into present character row.
>>3. Copy buffer B into buffer A.
>>4. Go to next lower character row, if there is one; if not, end.
>>5. Go to step 1.
>>
>>But the big question: is it fast enough to get to the bottom of the screen
>>before the raster, released at the previous bottom edge, does?  I don't
>>think so, after Marcel's analysis.  It should take about 3 times as long as
>>Marcel's routine, or about 1/20 of a second.  Dang it, not fast enough!
>>However, if it were possible to do this much computation in 1/60 of a
>>second, and your normal down-scroll algorithm wasn't fast enough to beat
>>the raster to the top of the screen, this one would do it.
>
>Yes, I am an extremely unpracticed and unknowledgeable c64 user.
>However, (this may seem stupid) has anyone thought of waiting for the
>raster to be past the first 8 or so (far enough past so that the information
>writing wouldn't catch up to the raster) character rows on the screen, and
>then applying the aforementioned (I bet that's spelled wrong) buffering
>algorithm.  In another words, what I'm trying to say is, would it be
>possible to draw, smoothly, the screen while the raster is on another
>part of the screen, rather than off the screen?

This is exactly what my original posting on this subject addressed.  With
the scroll _UP_ algorithm that I presented, I does indeed seem possible to
start the scroll at the top of the screen once the raster has reached the
2nd character row, and have it complete before the raster starts the next
frame.  The problem is that this doesn't work with the scroll _DOWN_
algorithm.  The simple algorithm that I presented started from the bottom of
the screen, moving up.  This doesn't allow enough time to complete the
scroll before the raster starts the next frame.  The scroll _DOWN_ algorithm
quoted at the beginning of this posting starts at the top of the screen and
moves down.  Because of this, it has more time to complete the scroll.
However, it still won't be able to scroll smoothly without double buffering
because it performs about 3 times as many byte moves as the simpler
algorithm, which requires about 3 times as many CPU cycles.

>					Don McCasland

Marcel A. LeBlanc	   | University of Toronto -- Toronto, Canada
"leblanc@eecg.toronto.edu" | and: LMS Technologies Ltd, Fredericton, NB, Canada
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