[Coco] GIME Scrolling Questions

[Chris Spry]

Hey guys,

Well I've decided to go in a bit of a different direction with MiniHog and
will have a quick scrolling game but the gameplay is going to be different
than originally planned.  I was messing around with the GIME scrolling
registers quite a bit on Sunday and could not quite figure them out.  I was
using a standard 320x192x16 screen (HSCREEN 2). I could get the screen to
scroll the contents from left to right by poking FF9E and FF9F rather
seperately or together in different steps.  Usually steps of 1.  However
when using FF9F (Horizontal Offset Register) it would scroll the screen by
four pixels per step (Poking with a value of 0-127 sets the horizontal
offset address in a different location at a video location x2 of the value
being inputed).  It would work up to a point.  First off I noticed the
oncoming screen would be shifted up by one line but would scroll fine until
the value going into FF9F went past 48.  Once it went past there it would
lower the line back down and start showing a different location.  I found
that out by actually drawing a slope across the screen and then add to this
slope by 4 horizontal boxes all the way to the left, adding to it to the
right after the scroll looking like it's ascending (since the screen shifts
over by 4 per step).  Then after the 48 value it was as if it wasn't taking
my boxes anymore and just showing some of the original stuff from before.  I
tried FF9E and it seems to work alright, except that it scrolls too much too
fast for what I need.  I tried combining the two poking different values and
getting odd results depending on what I did.  I tried to find the formula to
make a smooth side scrolling effect adding oncoming new screen data, but
can't figure out what this might be if there is one.  I image that there is
one, especially after seeing Sock Master's scrolling/game demo.  Can anybody
possible help me out with this, I'd really like to get this game going in
production again.

-Chris