Path: chuka.playstation.co.uk!news
From: Andrew Partington <andrew.partington@harbinger.com>
Newsgroups: scee.yaroze.freetalk.english
Subject: negative scrolling on homebrew map routine - need help!!
Date: Wed, 14 Mar 2001 14:09:54 +0000
Organization: PlayStation Net Yaroze (SCEE)
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Message-ID: <3AAF7BB2.B427AED3@harbinger.com>
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Hi everyone

I have a 98% working routine for drawing my own maps (I don't want to
use the GsBG stuff supplied because I want zoomrotation in my
project!).  It zoomrotates fine, and seems to work 100% when scrolling
in a positive direction (i.e. down and to the right).  However, my maths
fails on the following range of negative numbers...

The problem lies in the following formula, which calculates the block in
the map to start drawing from:

offsetX = (X scroll position (in pixels) / block size X (also in pixels)
) % background size X (number of blocks going across the map, in this
case 80);

I'm using the mod operator in order to wrap the values around to the
other side of a 2D array containing map data if the scrollX/scrollY
values go out of range.  For any positive numbers I am asking to scroll
the map by, this approach works fine...

However, If the scroll value is in the range of -blocksize to
(-blocksize * 16) (i.e. in the range of -16 to -256), the offset of the
map comes out in the range of positive 15 - 0!  (I need it to be in the
range of 79 to 64).  Any negative scroll numbers NOT in this range work
fine!!!

This is mathematically correct (well according to Windows calculator
anyway ;) , but its a very annoying bug - when you scroll in a negative
direction and you hit that particular range of scroll values, it looks
like some of the old cartoon backgrounds where you notice the wraparound
:)

Is there any way I can make this approach work for all negative numbers
without having to reset my scroll positions for the map, and without
having to resort to a large number of complex, error-prone IF
statements?  I kinda like the simplicity of the maths as it seems to
work for most cases...  (if I reset my scroll positions, that would work
for that particular map, but will knacker any parallax effects I have -
and I don't think you have to resort to any similar kludges when using
the GsBG16 stuff!)

Sorry if this message isn't clear, i'm on my dinner break at work and I
don't have my routine/CW project in front of me to post it, i'll do that
when I get home...

I hope someone can help out with this problem, it's been holding me back
for about 3 months, and I wanna get back into the swing of things!!!

Cheers

Andy