DS Programming – Simple Starfield

Posted on by

The third thing I tried was an adaptation of Dev-Scene’s Starfield. It took me a while to figure out how to add multiple button checking, not sure if my solution is ‘the way to go’ but at least it works.

With the following code you generate a starfield and can change its movement in all eight directions.

#include <nds.h>
#include <stdlib.h>
 
#define NUM_STARS 40
#define UPLEFT (KEY_UP | KEY_LEFT)
#define UPRIGHT (KEY_UP | KEY_RIGHT)
#define DOWNLEFT (KEY_DOWN | KEY_LEFT)
#define DOWNRIGHT (KEY_DOWN | KEY_RIGHT)
 
typedef struct 
{
	int x;
	int y;
	int speed;
	unsigned short color;
 
}Star;
 
// Function Prototypes
void setStar(Star* star, int xSpeed, int ySpeed, int xStart, int yStart);
void MoveStar(Star* star);
void ClearScreen(void);
void InitStars(void);
void DrawStar(Star* star);
void EraseStar(Star* star);
 
Star stars[NUM_STARS];
int lastKey;
 
void MoveStar(Star* star)
{
	if((DOWNRIGHT & lastKey) == DOWNRIGHT)
		setStar(star, star->speed, star->speed, rand() % SCREEN_WIDTH, 0);
	else if((DOWNLEFT & lastKey) == DOWNLEFT)
		setStar(star, -star->speed, +star->speed, rand() % SCREEN_WIDTH, 0);
	else if((UPLEFT & lastKey) == UPLEFT)
		setStar(star, -star->speed, -star->speed, rand() % SCREEN_WIDTH, SCREEN_HEIGHT);
	else if((UPRIGHT & lastKey) == UPRIGHT)
		setStar(star, star->speed, -star->speed, rand() % SCREEN_WIDTH, SCREEN_HEIGHT);
	else if(KEY_RIGHT & lastKey)
		setStar(star, star->speed, 0, 0, rand() % SCREEN_HEIGHT);
	else if(KEY_LEFT & lastKey)
		setStar(star, -star->speed, 0, SCREEN_WIDTH, rand() % SCREEN_HEIGHT);
	else if(KEY_UP & lastKey)
		setStar(star, 0, -star->speed, rand() % SCREEN_WIDTH, SCREEN_HEIGHT);
	else if(KEY_DOWN & lastKey)
		setStar(star, 0, star->speed, rand() % SCREEN_WIDTH, 0);
	else
		setStar(star, star->speed, 0, 0, rand() % SCREEN_HEIGHT);
}
 
void setStar(Star* star, int xSpeed, int ySpeed, int xStart, int yStart)
{
	star->x += xSpeed;
	star->y += ySpeed;
 
	if(star->y < 0 || star->y > SCREEN_HEIGHT)
	{
		star->x = xStart;
		star->y = yStart;
		star->speed = rand() % 4 + 1;	
	}
	else if(star->x < 0 || star->x > SCREEN_WIDTH)
	{
		star->x = xStart;
		star->y = yStart;
		star->speed = rand() % 4 + 1;	
	}
}
 
void ClearScreen(void)
{
     int i;    
     for(i = 0; i < 256 * 192; i++)
           VRAM_A[i] = RGB15(0,0,0);
}
 
void InitStars(void)
{
	int i;
	for(i = 0; i < NUM_STARS; i++)
	{
		stars[i].color = rand(); // both color & rand() are a 16bit value
		stars[i].x = rand() % 256;
		stars[i].y = rand() % 192;
		stars[i].speed = rand() % 4 + 1;
	}
}
void DrawStar(Star* star)
{
	VRAM_A[star->x + star->y * SCREEN_WIDTH] = star->color;
}
 
void EraseStar(Star* star)
{
	VRAM_A[star->x + star->y * SCREEN_WIDTH] = RGB15(0,0,0);
}
 
int main(void) 
{
	int i;
 
	irqInit();
	irqEnable(IRQ_VBLANK);
 
	videoSetMode(MODE_FB0);
	vramSetBankA(VRAM_A_LCD);
 
    ClearScreen();
	InitStars();
 
	while(1)
	{
		scanKeys();
		if(keysDown())
			lastKey = keysHeld();
 
		swiWaitForVBlank();
 
		for(i = 0; i < NUM_STARS; i++)
		{
			EraseStar(&stars[i]);
			MoveStar(&stars[i]);
			DrawStar(&stars[i]);
		}
	}
	return 0;
}

Nothing spectacular as I’m just going through the different tutorials I find on the net. I’m posting these examples for people who are new to the whole DS programming scene and would like to see the examples but with a few extras. I’m still learning how all of it works myself so don’t expect the most well written code!