--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Xcode-iPhoneOS/Demos/src/accelerometer.c	Sat Sep 19 07:32:36 2009 +0000
@@ -0,0 +1,251 @@
+/*
+ *	accelerometer.c
+ *	written by Holmes Futrell
+ *	use however you want
+ */
+
+#include "SDL.h"
+#include "math.h"
+#include "common.h"
+
+#define MILLESECONDS_PER_FRAME 16       /* about 60 frames per second */
+#define DAMPING 0.5f;           /* after bouncing off a wall, damping coefficient determines final speed */
+#define FRICTION 0.0008f        /* coefficient of acceleration that opposes direction of motion */
+#define GRAVITY_CONSTANT 0.004f /* how sensitive the ship is to the accelerometer */
+
+/*	If we aren't on an iPhone, then this definition ought to yield reasonable behavior */
+#ifndef SDL_IPHONE_MAX_GFORCE
+#define SDL_IPHONE_MAX_GFORCE 5.0f
+#endif
+
+static SDL_Joystick *accelerometer;     /* used for controlling the ship */
+
+static struct
+{
+    float x, y;                 /* position of ship */
+    float vx, vy;               /* velocity of ship (in pixels per millesecond) */
+    SDL_Rect rect;              /* (drawn) position and size of ship */
+} ship;
+
+static SDL_TextureID shipID = 0;        /* texture for spaceship */
+static SDL_TextureID spaceID = 0;       /* texture for space (background */
+
+void
+render(void)
+{
+
+
+    /* get joystick (accelerometer) axis values and normalize them */
+    float ax = SDL_JoystickGetAxis(accelerometer, 0);
+    float ay = -SDL_JoystickGetAxis(accelerometer, 1);
+
+    /* ship screen constraints */
+    Uint32 minx = 0.0f;
+    Uint32 maxx = SCREEN_WIDTH - ship.rect.w;
+    Uint32 miny = 0.0f;
+    Uint32 maxy = SCREEN_HEIGHT - ship.rect.h;
+
+#define SINT16_MAX ((float)(0x7FFF))
+
+    /* update velocity from accelerometer
+       the factor SDL_IPHONE_MAX_G_FORCE / SINT16_MAX converts between 
+       SDL's units reported from the joytick, and units of g-force, as reported by the accelerometer
+     */
+    ship.vx +=
+        ax * SDL_IPHONE_MAX_GFORCE / SINT16_MAX * GRAVITY_CONSTANT *
+        MILLESECONDS_PER_FRAME;
+    ship.vy +=
+        ay * SDL_IPHONE_MAX_GFORCE / SINT16_MAX * GRAVITY_CONSTANT *
+        MILLESECONDS_PER_FRAME;
+
+    float speed = sqrt(ship.vx * ship.vx + ship.vy * ship.vy);
+
+    if (speed > 0) {
+        /* compensate for friction */
+        float dirx = ship.vx / speed;   /* normalized x velocity */
+        float diry = ship.vy / speed;   /* normalized y velocity */
+
+        /* update velocity due to friction */
+        if (speed - FRICTION * MILLESECONDS_PER_FRAME > 0) {
+            /* apply friction */
+            ship.vx -= dirx * FRICTION * MILLESECONDS_PER_FRAME;
+            ship.vy -= diry * FRICTION * MILLESECONDS_PER_FRAME;
+        } else {
+            /* applying friction would MORE than stop the ship, so just stop the ship */
+            ship.vx = 0.0f;
+            ship.vy = 0.0f;
+        }
+    }
+
+    /* update ship location */
+    ship.x += ship.vx * MILLESECONDS_PER_FRAME;
+    ship.y += ship.vy * MILLESECONDS_PER_FRAME;
+
+    if (ship.x > maxx) {
+        ship.x = maxx;
+        ship.vx = -ship.vx * DAMPING;
+    } else if (ship.x < minx) {
+        ship.x = minx;
+        ship.vx = -ship.vx * DAMPING;
+    }
+    if (ship.y > maxy) {
+        ship.y = maxy;
+        ship.vy = -ship.vy * DAMPING;
+    } else if (ship.y < miny) {
+        ship.y = miny;
+        ship.vy = -ship.vy * DAMPING;
+    }
+
+    /* draw the background */
+    SDL_RenderCopy(spaceID, NULL, NULL);
+
+    /* draw the ship */
+    ship.rect.x = ship.x;
+    ship.rect.y = ship.y;
+
+    SDL_RenderCopy(shipID, NULL, &ship.rect);
+
+    /* update screen */
+    SDL_RenderPresent();
+
+}
+
+void
+initializeTextures()
+{
+
+    SDL_Surface *bmp_surface;
+    SDL_Surface *bmp_surface_rgba;
+    int format = SDL_PIXELFORMAT_ABGR8888;      /* desired texture format */
+    Uint32 Rmask, Gmask, Bmask, Amask;  /* masks for desired format */
+    int bpp;                    /* bits per pixel for desired format */
+
+    /* load the ship */
+    bmp_surface = SDL_LoadBMP("ship.bmp");
+    if (bmp_surface == NULL) {
+        fatalError("could not ship.bmp");
+    }
+    /* set blue to transparent on the ship */
+    SDL_SetColorKey(bmp_surface, 1,
+                    SDL_MapRGB(bmp_surface->format, 0, 0, 255));
+    SDL_PixelFormatEnumToMasks(format, &bpp, &Rmask, &Gmask, &Bmask, &Amask);
+    /*
+       create a new RGBA surface and blit the bmp to it
+       this is an extra step, but it seems to be necessary for the color key to work
+
+       does the fact that this is necessary indicate a bug in SDL?
+     */
+    bmp_surface_rgba =
+        SDL_CreateRGBSurface(0, bmp_surface->w, bmp_surface->h, bpp, Rmask,
+                             Gmask, Bmask, Amask);
+    SDL_BlitSurface(bmp_surface, NULL, bmp_surface_rgba, NULL);
+
+    /* create ship texture from surface */
+    shipID = SDL_CreateTextureFromSurface(format, bmp_surface_rgba);
+    if (shipID == 0) {
+        fatalError("could not create ship texture");
+    }
+    SDL_SetTextureBlendMode(shipID, SDL_BLENDMODE_BLEND);
+
+    /* set the width and height of the ship from the surface dimensions */
+    ship.rect.w = bmp_surface->w;
+    ship.rect.h = bmp_surface->h;
+
+    SDL_FreeSurface(bmp_surface_rgba);
+    SDL_FreeSurface(bmp_surface);
+
+    /* load the space background */
+    bmp_surface = SDL_LoadBMP("space.bmp");
+    if (bmp_surface == NULL) {
+        fatalError("could not load space.bmp");
+    }
+    /* create space texture from surface */
+    spaceID = SDL_CreateTextureFromSurface(format, bmp_surface);
+    if (spaceID == 0) {
+        fatalError("could not create space texture");
+    }
+    SDL_FreeSurface(bmp_surface);
+
+}
+
+
+
+int
+main(int argc, char *argv[])
+{
+
+    SDL_WindowID windowID;      /* ID of main window */
+    Uint32 startFrame;          /* time frame began to process */
+    Uint32 endFrame;            /* time frame ended processing */
+    Uint32 delay;               /* time to pause waiting to draw next frame */
+    int done;                   /* should we clean up and exit? */
+
+    /* initialize SDL */
+    if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_JOYSTICK) < 0) {
+        fatalError("Could not initialize SDL");
+    }
+
+    /* create main window and renderer */
+    windowID = SDL_CreateWindow(NULL, 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT,
+                                SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN |
+                                SDL_WINDOW_BORDERLESS);
+    SDL_CreateRenderer(windowID, 0, 0);
+
+    /* print out some info about joysticks and try to open accelerometer for use */
+    printf("There are %d joysticks available\n", SDL_NumJoysticks());
+    printf("Default joystick (index 0) is %s\n", SDL_JoystickName(0));
+    accelerometer = SDL_JoystickOpen(0);
+    if (accelerometer == NULL) {
+        fatalError("Could not open joystick (accelerometer)");
+    }
+    printf("joystick number of axis = %d\n",
+           SDL_JoystickNumAxes(accelerometer));
+    printf("joystick number of hats = %d\n",
+           SDL_JoystickNumHats(accelerometer));
+    printf("joystick number of balls = %d\n",
+           SDL_JoystickNumBalls(accelerometer));
+    printf("joystick number of buttons = %d\n",
+           SDL_JoystickNumButtons(accelerometer));
+
+    /* load graphics */
+    initializeTextures();
+
+    /* setup ship */
+    ship.x = (SCREEN_WIDTH - ship.rect.w) / 2;
+    ship.y = (SCREEN_HEIGHT - ship.rect.h) / 2;
+    ship.vx = 0.0f;
+    ship.vy = 0.0f;
+
+    done = 0;
+    /* enter main loop */
+    while (!done) {
+        startFrame = SDL_GetTicks();
+        SDL_Event event;
+        while (SDL_PollEvent(&event)) {
+            if (event.type == SDL_QUIT) {
+                done = 1;
+            }
+        }
+        render();
+        endFrame = SDL_GetTicks();
+
+        /* figure out how much time we have left, and then sleep */
+        delay = MILLESECONDS_PER_FRAME - (endFrame - startFrame);
+        if (delay < 0) {
+            delay = 0;
+        } else if (delay > MILLESECONDS_PER_FRAME) {
+            delay = MILLESECONDS_PER_FRAME;
+        }
+        SDL_Delay(delay);
+    }
+
+    /* delete textures */
+    SDL_DestroyTexture(shipID);
+    SDL_DestroyTexture(spaceID);
+
+    /* shutdown SDL */
+    SDL_Quit();
+
+    return 0;
+
+}