Path: chuka.playstation.co.uk!news
From: Nick Slaven <NickSlaven@compuserve.com>
Newsgroups: scee.yaroze.programming.2d_graphics
Subject: Re: Scrolling backgrounds
Date: Wed, 08 Jul 1998 22:35:54 +0100
Organization: PlayStation Net Yaroze (SCEE)
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Mark Green wrote:
> 
>   I am trying to create a 2D scrolling cloud-type background that is
> automatically generated by an algorithm.  I have the algorithm to generate
> the next line to be scrolled on given the previous line, but I am having
> some problem making the scrolling itself happen.
>   It would seem to make sense that I should hold the background somewhere
> in video RAM, scroll and update it there then copy it to the frame buffer.
> Unfortunately this seems to be very inefficient (requiring two full-screen
> blits per frame, one to scroll the buffer and another to copy it to the frame
> buffer) and will not work with the non-blocking operations anyway since the
> scroll must be assured complete before the copy can take place.
>   I am sure that there is a more efficient way of doing this type of thing
> but it is eluding me at the moment :) Can anyoone give me any pointers?

Have you considered using the BG16 stuff, I've got this working with a scrolling 
background image (which just happens to be some clouds hence the name sky!). 
heres the code

// BG16 uses its own work area, gpupacket area not used (this equation is on
// page 142 of the green book
#define worksize (((BG_FRAME_X/16+1)*(BG_FRAME_X/16+1+1)*6+4)*2+2)

GsBG	BGSky;
GsMAP	mapSky;
GsCELL	cellSky[128];
u_short	indexSky[16][32];
u_long	workSky[worksize];

void LoadSky(void)
{
u_short tpage,cba;
GsIMAGE	Im;
int	i,j,y,x;

tpage=LoadTexture(TX_SKY,&Im);

cba=GetClut(Im.cx,Im.cy);
// chop the loaded tim into 16 pixel chunks
for(i=0,y=0;y<128;y+=16)
	for(x=0;x<256;x+=16,i++)
		{
		cellSky[i].u = x;
		cellSky[i].v = y;
		cellSky[i].cba  = cba;
		cellSky[i].tpage= tpage;
		}

for(y=0,i=0,j=64;y<16;y++)		
	for(x=0;x<32;x++)
		{
		if(y<4)
			{
			if(x<16)	indexSky[y][x]=i++;
			else		indexSky[y][x]=j++;
			}
		else
			indexSky[y][x]=0xffff;
		}
		
mapSky.cellw = 16;
mapSky.cellh = 16;
mapSky.ncellw= 32;
mapSky.ncellh= 16;
mapSky.base  = cellSky;
mapSky.index = &indexSky[0][0];

BGSky.attribute=1<<27|1<<24;
BGSky.x=BG_ORG_X;
BGSky.y=BG_ORG_Y;
BGSky.w=BG_FRAME_X;
BGSky.h=BG_FRAME_X;
BGSky.scrollx=0;
BGSky.scrolly=0;
BGSky.r=127;
BGSky.g=127;
BGSky.b=127;
BGSky.map=&mapSky;
BGSky.scalex=ONE;
BGSky.scaley=ONE;
BGSky.rotate=0;

GsInitFixBg16(&BGSky,workSky);
}


void DrawSky(void)
{
BGSky.scrollx++;
GsSortFixBg16(&BGSky,workSky,&WorldOT[activeBuff],(1<<OT_LENGTH)-1);
}

You have to define the backgound for the entire screen area, but its ok to put
in
blank tiles, hence the 0xffff bit, also my tim is stored as two strips one above 
the other. To use, LoadSky initialises all the stuff for BG16, DrawSKy does
exactly
what it says, incidently you can alter the y scroll as well as any of the other
parms in the GsBG struct.

Cheers


Nick (S)