Large Map Scrolling in GBDK

Written by Jason

drunk-ass@beer.com

http://www.gbdev.org/news/



One of the most common requests for things that people are too lazy 

to write for themselves is an 8-way scrolling engine that can hold 

maps larger than 32x32. In this document, I hope to try and explain 

how to do it.



The way I'm doing it is not the most optimized way of doing it. It

is, however, the easiest way I can explain it without losing too 

many people.



I didn't test this. I'm just writing out the logic behind it. Alot

of the values need to be tweaked here

Making your map, you should use a program like Harry Mulder's Gameboy 

Map Builder. Design your map, and when you export, export to 1 byte

per tile. 



Now when you include it in your GBDK project, try and include it in

a higher bank such as 2+ so that it is not included in bank 0. Bank

0 is 16k and cannot be changed, plus on intense games you need as 

much space there as possible. Data should never be stored in bank

0.



For convenience and making it easier to understand where I got the

numbers from, we'll make some quick defines.

	#define MapSizeX 64	// where 64 is defined by your map

	#define MapSizeY 64	// where 64 is defined by your map



You need a couple of variables to keep track of things.

	UBYTE ScrollXCnt;	// Scrolling X Counter

	UBYTE ScrollYCnt;	// Scrolling Y Counter

	BYTE SCXCnt;		// update info

	BYTE SCYCnt;		// update info

	UBYTE tempa, tempb;	// temp numbers

	UWORD Cnt;		// counter



Load the initial screen

	Cnt = 0;

	wait_vbl_done();

	for(tempa = 0; tempa != 32; tempa++) {

		set_bkg_tiles(0,tempa,32,tempa+1,TileMap[Cnt]);

		Cnt = Cnt + MapSizeX;	// increase Cnt the size

					// of x so that the next

					// column is loaded

	}



Check the joypad for information telling where you need to scroll.

	if(joypad() & J_UP) ScrollUp();

	if(joypad() & J_DOWN) ScrollDown();

	if(joypad() & J_LEFT) ScrollLeft();

	if(joypad() & J_RIGHT) ScrollRight();

You only need 4 because if joypad is being pressed in two directions,

the joypad() & direction1, joypad() & direction2 should return true.



Now make sure the actual scroll routines are in vlblank interrupt.

	wait_vbl_done();

	scroll_bkg(x,y);  // where x and y are positive or negative

			  // numbers depending on the direction

			  // and the numbers depending on the speed



Update the ScrollXCnt and ScrollCntY when you change the info.

	ScrollX++;	or	ScrollX--;

	ScrollY++;	or	ScrollY--;

	SCXCnt++;	or	SCXCnt++;

	SCYCnt++;	or	SCYCnt--;





Check to see if the map needs updated.

	if(SCXCnt == 8) {

		SCXCnt = 0;		// reset the scroll counter

		Cnt = ScrollX / 8;	// find out what tile is on 

					// the left on the screen

		Cnt = Cnt + 20;		// increase the counter the

					// size of the screen

		// repeat this 32 times

		for(tempa = 0; tempa != 32; tempa++) {

			// set the tiles

			set_bkg_tiles(0,tempa,1,tempa+1,TileMap[Cnt]);

			// increase the counter to prevent graphics 

			// loading the wrong tile

			Cnt = Cnt + MapSizeX;

		}		

	}

	if(SCXCnt == -8) {

		SCXCnt = 0;

		SCXCnt = 0;		// reset the scroll counter

		Cnt = ScrollX / 8;	// find out what tile is on 

					// the left on the screen

		Cnt--;			// decrease the counter to

					// prevent bad looking gfx

		// repeat this 32 times

		for(tempa = 0; tempa != 32; tempa++) {

			// set the tiles

			set_bkg_tiles(0,tempa,1,tempa+1,TileMap[Cnt]);

			// increase the counter to prevent graphics 

			// loading the wrong tile

			Cnt = Cnt + MapSizeX;

		}		

	}

	.

	.

	.





Like I said, tweak the code. This is just the logic behind it. I'm

not going to write the code line for line for you, but you should

be able to get an idea from wtf I'm talking about so that you can

do this. Hope you enjoyed it and it was useful. =)