background.c

Linux游戏编程源码

#include <time.h>
#include "gamedefs.h"
#include "background.h"
#include "resources.h"

/* Two-dimensional arrays for storing the world's tiles (by index). */
static int front_tiles[PARALLAX_GRID_WIDTH][PARALLAX_GRID_HEIGHT];
static int back_tiles[PARALLAX_GRID_WIDTH][PARALLAX_GRID_HEIGHT];

/* This is a simple custom pseudorandom number generator. It's not a very
   good one, but it's sufficient for our purposes. Never trust the rand()
   included with the C library. Its quality varies between implementations,
   and it's easy to run into patterns within the generated numbers. At least
   this one is somewhat consistent. */
static Sint32 seed = 0;

static void initrandom()
{
    seed = time(NULL);
}

static unsigned int getrandom()
{
    Sint32 p1 = 1103515245;
    Sint32 p2 = 12345;
    seed = (seed * p1 + p2) % 2147483647;
    return (unsigned) seed / 3;
}

/* Sets up the starry background by assigning random tiles.
   This should be called after LoadGameData(). */
void InitBackground()
{
    int x, y;

    initrandom();
    for (x = 0; x < PARALLAX_GRID_WIDTH; x++) {
	for (y = 0; y < PARALLAX_GRID_HEIGHT; y++) {
	    front_tiles[x][y] = getrandom() % num_star_tiles;
	    back_tiles[x][y] = getrandom() % num_star_tiles;
	}
    }
}

/* Draws the background on the screen, with respect to the global
   "camera" position. The camera marks the 640x480 section of the
   world that we can see at any given time. This is usually in the
   vicinity of the player's ship. */

void DrawBackground(SDL_Surface * dest, int camera_x, int camera_y)
{
    int draw_x, draw_y;		/* drawing position on the screen */
    int start_draw_x, start_draw_y;

    int tile_x, tile_y;		/* indices in the back_tiles[][] array */
    int start_tile_x, start_tile_y;

    /* Map the camera position into tile indices. */
    start_tile_x = ((camera_x / PARALLAX_BACK_FACTOR) /
		    TILE_WIDTH) % PARALLAX_GRID_WIDTH;
    start_tile_y = ((camera_y / PARALLAX_BACK_FACTOR) /
		    TILE_HEIGHT) % PARALLAX_GRID_HEIGHT;

    start_draw_x = -((camera_x / PARALLAX_BACK_FACTOR) % TILE_WIDTH);
    start_draw_y = -((camera_y / PARALLAX_BACK_FACTOR) % TILE_HEIGHT);

    /* Use nested loops to scan down the screen, drawing rows of tiles. */
    tile_y = start_tile_y;
    draw_y = start_draw_y;
    while (draw_y < SCREEN_HEIGHT) {
	tile_x = start_tile_x;
	draw_x = start_draw_x;
	while (draw_x < SCREEN_WIDTH) {
	    SDL_Rect srcrect, destrect;

	    srcrect.x = TILE_WIDTH * back_tiles[tile_x][tile_y];
	    srcrect.y = 0;
	    srcrect.w = TILE_WIDTH;
	    srcrect.h = TILE_HEIGHT;
	    destrect.x = draw_x;
	    destrect.y = draw_y;
	    destrect.w = TILE_WIDTH;
	    destrect.h = TILE_HEIGHT;

	    SDL_BlitSurface(back_star_tiles, &srcrect, dest, &destrect);

	    tile_x++;
	    tile_x %= PARALLAX_GRID_WIDTH;
	    draw_x += TILE_WIDTH;
	}
	tile_y++;
	tile_y %= PARALLAX_GRID_HEIGHT;
	draw_y += TILE_HEIGHT;
    }
}

void DrawParallax(SDL_Surface * dest, int camera_x, int camera_y)
{
    int draw_x, draw_y;		/* drawing position on the screen */
    int start_draw_x, start_draw_y;

    int tile_x, tile_y;		/* indices in the back_tiles[][] array */
    int start_tile_x, start_tile_y;

    /* Map the camera position into tile indices. */
    start_tile_x = ((camera_x / PARALLAX_FRONT_FACTOR) /
		    TILE_WIDTH) % PARALLAX_GRID_WIDTH;
    start_tile_y = ((camera_y / PARALLAX_FRONT_FACTOR) /
		    TILE_HEIGHT) % PARALLAX_GRID_HEIGHT;

    start_draw_x = -((camera_x / PARALLAX_FRONT_FACTOR) % TILE_WIDTH);
    start_draw_y = -((camera_y / PARALLAX_FRONT_FACTOR) % TILE_HEIGHT);

    /* Use nested loops to scan down the screen, drawing rows of tiles. */
    tile_y = start_tile_y;
    draw_y = start_draw_y;
    while (draw_y < SCREEN_HEIGHT) {
	tile_x = start_tile_x;
	draw_x = start_draw_x;
	while (draw_x < SCREEN_WIDTH) {
	    SDL_Rect srcrect, destrect;

	    srcrect.x = TILE_WIDTH * front_tiles[tile_x][tile_y];
	    srcrect.y = 0;
	    srcrect.w = TILE_WIDTH;
	    srcrect.h = TILE_HEIGHT;
	    destrect.x = draw_x;
	    destrect.y = draw_y;
	    destrect.w = TILE_WIDTH;
	    destrect.h = TILE_HEIGHT;

	    SDL_BlitSurface(front_star_tiles, &srcrect, dest, &destrect);

	    tile_x++;
	    tile_x %= PARALLAX_GRID_WIDTH;
	    draw_x += TILE_WIDTH;
	}
	tile_y++;
	tile_y %= PARALLAX_GRID_HEIGHT;
	draw_y += TILE_HEIGHT;
    }
}