Path: chuka.playstation.co.uk!scea!greg_labrec@interactive.sony.com
From: jamin1@psu.edu (Jamin Frederick)
Newsgroups: scea.yaroze.programming.2d_graphics
Subject: Re: Beginner in atmosphere!
Date: Wed, 09 Jul 1997 19:03:21 GMT
Organization: SCEA Net Yaroze News
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On Tue, 08 Jul 1997 23:19:11 -0700, "Linda J. Hodge"
<hodgke@earthlink.net> wrote:

>Can anybody explain how hidden page scrolling works for the yaroze?

	You can do it using the GsBG structure that Sony provides, but
that's sort of limited because you have to use 16x16 pixel tiles.
Anyway, you just load up some TIM images, say 5 different tiles you
want to use for your background and then you'd make 5 GsCELL structs
that reference these images -- cells are the elements that make up the
GsMAP struct.  In fact, in the GsMAP struct there's a pointer to an 
array of GsCELL structs, in this case its size would be 5, which are
all the different image tiles your world will have.  It also has a
pointer to an (m x n) array of ushorts, called 'index', where m is the
width in cells of your world, and n is the height in cells, which are
just indices into the corresponding GsCELL array.  So this is the map
that corresponds to your game world.   
When you do GsSortFixBg16(), the psx takes care of where the screen is
positioned in the world by means of the scrollx and scrolly elements
of the GsBG struct.  if you specify (0,0), you'll get the upper-left
corner of your world.  If you specify (2, 3), the screen will be 2
pixels right and 3 pixels down.  If you specify (-2, -4), you'll be 2
pixels left and 4 pixels up from the bottom right of your game world
(it wraps).  Take a look at my tiles1.zip and tiles2.zip, I use this
method to scroll.
	Alternatively, you can use your own sprites as game tiles and
forget about the BG tiling thing.  What you do is give your sprites a
world position, and the screen a world position.  The sprites will be
positioned on the screen according to the translation

SpriteScreenPosition = SpriteWorldPosition - ScreenWorldPosition

so if your screen is at (0,0), the sprite's screen position will just
be its world position.  However, if your user decides to scroll the
screen down and right, then the ScreenWorldPosition will increase both
x and y directions, and consequently, the SpriteScreenPosition will
decrease.  So it looks like your sprite is moving up and left, get it?
I've done this too, with logos.zip.
	The only thing that you should worry about after this, though,
is when to start "clipping" your sprites, or not stick them in the
Ot to be sorted, since they are off-screen and are not seen anyway.
If there's say 100 sprites you're sorting, but only 2 on the screen,
then it's sort of a waste to make the hardware clip these with the OT,
but who knows, maybe the hardware clips the extra sprites just as fast
as I would *not sort* them into the OT anyway.  What does everyone
else think?


Jamin
http://www.cse.psu.edu/~frederic/home.htm