SDL关于ALPHA的设置

int main(int argc, char *argv[])
{
const SDL_VideoInfo *info;
SDL_Surface *screen;
int    w, h;
Uint8  video_bpp;
Uint32 videoflags;
int    i, done;
SDL_Event event;
SDL_Surface *light;
int mouse_pressed;
Uint32 ticks, lastticks;

/* Initialize SDL */
if ( SDL_Init(SDL_INIT_VIDEO) < 0 ) {
  fprintf(stderr, "Couldn't initialize SDL: %s\n",SDL_GetError());
  return(1);
}
/* Alpha blending doesn't work well at 8-bit color */
#ifdef _WIN32_WCE
/* Pocket PC */
w = 240;
h = 320;
#else
w = 640;
h = 480;
#endif
info = SDL_GetVideoInfo();
if ( info->vfmt->BitsPerPixel > 8 ) {
  video_bpp = info->vfmt->BitsPerPixel;
} else {
  video_bpp = 16;
                fprintf(stderr, "forced 16 bpp mode\n");
}
videoflags = SDL_SWSURFACE;
for ( i = 1; argv[ i]; ++i ) {
  if ( strcmp(argv[ i], "-bpp") == 0 ) {
   video_bpp = atoi(argv[++i]);
                        if (video_bpp<=8) {
                            video_bpp=16;
                            fprintf(stderr, "forced 16 bpp mode\n");
                        }
  } else
  if ( strcmp(argv[ i], "-hw") == 0 ) {
   videoflags |= SDL_HWSURFACE;
  } else
  if ( strcmp(argv[ i], "-warp") == 0 ) {
   videoflags |= SDL_HWPALETTE;
  } else
  if ( strcmp(argv[ i], "-width") == 0 && argv[i+1] ) {
   w = atoi(argv[++i]);
  } else
  if ( strcmp(argv[ i], "-height") == 0 && argv[i+1] ) {
   h = atoi(argv[++i]);
  } else
  if ( strcmp(argv[ i], "-resize") == 0 ) {
   videoflags |= SDL_RESIZABLE;
  } else
  if ( strcmp(argv[ i], "-noframe") == 0 ) {
   videoflags |= SDL_NOFRAME;
  } else
  if ( strcmp(argv[ i], "-fullscreen") == 0 ) {
   videoflags |= SDL_FULLSCREEN;
  } else {
   fprintf(stderr,
   "Usage: %s [-width N] [-height N] [-bpp N] [-warp] [-hw] [-fullscreen]\n",
        argv[0]);
   quit(1);
  }
}
/* Set video mode */
if ( (screen=SDL_SetVideoMode(w,h,video_bpp,videoflags)) == NULL ) {
  fprintf(stderr, "Couldn't set %dx%dx%d video mode: %s\n",
      w, h, video_bpp, SDL_GetError());
  quit(2);
}
FillBackground(screen);
/* Create the light */
light = CreateLight(82);
if ( light == NULL ) {
  quit(1);
}
/* Load the sprite */
if ( LoadSprite(screen, "icon.bmp") < 0 ) {
  SDL_FreeSurface(light);
  quit(1);
}
/* Print out information about our surfaces */
printf("Screen is at %d bits per pixel\n",screen->format->BitsPerPixel);
if ( (screen->flags & SDL_HWSURFACE) == SDL_HWSURFACE ) {
  printf("Screen is in video memory\n");
} else {
  printf("Screen is in system memory\n");
}
if ( (screen->flags & SDL_DOUBLEBUF) == SDL_DOUBLEBUF ) {
  printf("Screen has double-buffering enabled\n");
}
if ( (sprite->flags & SDL_HWSURFACE) == SDL_HWSURFACE ) {
  printf("Sprite is in video memory\n");
} else {
  printf("Sprite is in system memory\n");
}
/* Run a sample blit to trigger blit acceleration */
MoveSprite(screen, NULL);
if ( (sprite->flags & SDL_HWACCEL) == SDL_HWACCEL ) {
  printf("Sprite blit uses hardware alpha acceleration\n");
} else {
  printf("Sprite blit dosn't uses hardware alpha acceleration\n");
}
/* Set a clipping rectangle to clip the outside edge of the screen */
{ SDL_Rect clip;
  clip.x = 32;
  clip.y = 32;
  clip.w = screen->w-(2*32);
  clip.h = screen->h-(2*32);
  SDL_SetClipRect(screen, &clip);
}
/* Wait for a keystroke */
lastticks = SDL_GetTicks();
done = 0;
mouse_pressed = 0;
while ( !done ) {
  /* Update the frame -- move the sprite */
  if ( mouse_pressed ) {
   MoveSprite(screen, light);
   mouse_pressed = 0;
  } else {
   MoveSprite(screen, NULL);
  }
  /* Slow down the loop to 30 frames/second */
  ticks = SDL_GetTicks();
  if ( (ticks-lastticks) < FRAME_TICKS ) {
#ifdef CHECK_SLEEP_GRANULARITY
fprintf(stderr, "Sleeping %d ticks\n", FRAME_TICKS-(ticks-lastticks));
#endif
   SDL_Delay(FRAME_TICKS-(ticks-lastticks));
#ifdef CHECK_SLEEP_GRANULARITY
fprintf(stderr, "Slept %d ticks\n", (SDL_GetTicks()-ticks));
#endif
  }
  lastticks = ticks;
  /* Check for events */
  while ( SDL_PollEvent(&event) ) {
   switch (event.type) {
    case SDL_VIDEORESIZE:
     screen = SDL_SetVideoMode(event.resize.w, event.resize.h, video_bpp, videoflags);
     if ( screen ) {
      FillBackground(screen);
     }
     break;
    /* Attract sprite while mouse is held down */
    case SDL_MOUSEMOTION:
     if (event.motion.state != 0) {
      AttractSprite(event.motion.x,
        event.motion.y);
      mouse_pressed = 1;
     }
     break;
    case SDL_MOUSEBUTTONDOWN:
     if ( event.button.button == 1 ) {
      AttractSprite(event.button.x,
                    event.button.y);
      mouse_pressed = 1;
     } else {
      SDL_Rect area;
      area.x = event.button.x-16;
      area.y = event.button.y-16;
      area.w = 32;
      area.h = 32;
      SDL_FillRect(screen, &area, 0);
      SDL_UpdateRects(screen,1,&area);
     }
     break;
    case SDL_KEYDOWN:
#ifdef _WIN32_WCE
     // there is no ESC key at all
     done = 1;
#else
     if ( event.key.keysym.sym == SDLK_ESCAPE ) {
      done = 1;
     } else if (event.key.keysym.sym == SDLK_t) {
      videoflags ^= SDL_FULLSCREEN;
      screen = SDL_SetVideoMode(w, h, video_bpp, videoflags);
      if ( screen == NULL ) {
       fprintf(stderr, "Couldn't toggle video mode: %s\n",
         SDL_GetError());
       quit(2);
      }
      FillBackground(screen);
     }
#endif
     break;
    case SDL_QUIT:
     done = 1;
     break;
    default:
     break;
   }
  }
}
SDL_FreeSurface(light);
SDL_FreeSurface(sprite);
SDL_FreeSurface(backing);
/* Print out some timing information */
if ( flashes > 0 ) {
  printf("%d alpha blits, ~%4.4f ms per blit\n",
   flashes, (float)flashtime/flashes);
}
SDL_Quit();
return(0);

}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "SDL.h"

#define FRAME_TICKS        (1000/30)                /* 30 frames/second */

/* Call this instead of exit(), so we can clean up SDL: atexit() is evil. */
static void quit(int rc)
{
        SDL_Quit();
        exit(rc);
}

/* Fill the screen with a gradient */
static void FillBackground(SDL_Surface *screen)
{
        Uint8 *buffer;
        Uint16 *buffer16;
        Uint16 color;
        Uint8  gradient;
        int    i, k;

        /* Set the surface pixels and refresh! */
        if ( SDL_LockSurface(screen) < 0 ) {
                fprintf(stderr, "Couldn't lock the display surface: %s\n",
                                                        SDL_GetError());
                quit(2);
        }
        buffer=(Uint8 *)screen->pixels;
        if (screen->format->BytesPerPixel!=2) {
                for ( i=0; i<screen->h; ++i ) {
                        memset(buffer,(i*255)/screen->h, screen->w*screen->format->BytesPerPixel);
                        buffer += screen->pitch;
                }
        }
        else
        {
                for ( i=0; i<screen->h; ++i ) {
                        gradient=((i*255)/screen->h);
                        color = (Uint16)SDL_MapRGB(screen->format, gradient, gradient, gradient);
                        buffer16=(Uint16*)buffer;
                        for (k=0; k<screen->w; k++)
                        {
                            *(buffer16+k)=color;
                        }
                        buffer += screen->pitch;
                }
        }

        SDL_UnlockSurface(screen);
        SDL_UpdateRect(screen, 0, 0, 0, 0);
}

/* Create a "light" -- a yellowish surface with variable alpha */
SDL_Surface *CreateLight(int radius)
{
        Uint8  trans, alphamask;
        int    range, addition;
        int    xdist, ydist;
        Uint16 x, y;
        Uint16 skip;
        Uint32 pixel;
        SDL_Surface *light;

#ifdef LIGHT_16BIT
        Uint16 *buf;

        /* Create a 16 (4/4/4/4) bpp square with a full 4-bit alpha channel */
        /* Note: this isn't any faster than a 32 bit alpha surface */
        alphamask = 0x0000000F;
        light = SDL_CreateRGBSurface(SDL_SWSURFACE, 2*radius, 2*radius, 16,
                        0x0000F000, 0x00000F00, 0x000000F0, alphamask);
#else
        Uint32 *buf;

        /* Create a 32 (8/8/8/8) bpp square with a full 8-bit alpha channel */
        alphamask = 0x000000FF;
        light = SDL_CreateRGBSurface(SDL_SWSURFACE, 2*radius, 2*radius, 32,
                        0xFF000000, 0x00FF0000, 0x0000FF00, alphamask);
        if ( light == NULL ) {
                fprintf(stderr, "Couldn't create light: %s\n", SDL_GetError());
                return(NULL);
        }
#endif

        /* Fill with a light yellow-orange color */
        skip = light->pitch-(light->w*light->format->BytesPerPixel);
#ifdef LIGHT_16BIT
        buf = (Uint16 *)light->pixels;
#else
        buf = (Uint32 *)light->pixels;
#endif
        /* Get a tranparent pixel value - we'll add alpha later */
        pixel = SDL_MapRGBA(light->format, 0xFF, 0xDD, 0x88, 0);
        for ( y=0; y<light->h; ++y ) {
                for ( x=0; x<light->w; ++x ) {
                        *buf++ = pixel;
                }
                buf += skip;        /* Almost always 0, but just in case... */
        }

        /* Calculate alpha values for the surface. */
#ifdef LIGHT_16BIT
        buf = (Uint16 *)light->pixels;
#else
        buf = (Uint32 *)light->pixels;
#endif
        for ( y=0; y<light->h; ++y ) {
                for ( x=0; x<light->w; ++x ) {
                        /* Slow distance formula (from center of light) */
                        xdist = x-(light->w/2);
                        ydist = y-(light->h/2);
                        range = (int)sqrt(xdist*xdist+ydist*ydist);

                        /* Scale distance to range of transparency (0-255) */
                        if ( range > radius ) {
                                trans = alphamask;
                        } else {
                                /* Increasing transparency with distance */
                                trans = (Uint8)((range*alphamask)/radius);

                                /* Lights are very transparent */
                                addition = (alphamask+1)/8;
                                if ( (int)trans+addition > alphamask ) {
                                        trans = alphamask;
                                } else {
                                        trans += addition;
                                }
                        }
                        /* We set the alpha component as the right N bits */
                        *buf++ |= (255-trans);
                }
                buf += skip;        /* Almost always 0, but just in case... */
        }
        /* Enable RLE acceleration of this alpha surface */
        SDL_SetAlpha(light, SDL_SRCALPHA|SDL_RLEACCEL, 0);

        /* We're done! */
        return(light);
}

static Uint32 flashes = 0;
static Uint32 flashtime = 0;

void FlashLight(SDL_Surface *screen, SDL_Surface *light, int x, int y)
{
        SDL_Rect position;
        Uint32   ticks1;
        Uint32   ticks2;

        /* Easy, center light */
        position.x = x-(light->w/2);
        position.y = y-(light->h/2);
        position.w = light->w;
        position.h = light->h;
        ticks1 = SDL_GetTicks();
        SDL_BlitSurface(light, NULL, screen, &position);
        ticks2 = SDL_GetTicks();
        SDL_UpdateRects(screen, 1, &position);
        ++flashes;

        /* Update time spend doing alpha blitting */
        flashtime += (ticks2-ticks1);
}

static int sprite_visible = 0;
static SDL_Surface *sprite;
static SDL_Surface *backing;
static SDL_Rect    position;
static int         x_vel, y_vel;
static int           alpha_vel;

int LoadSprite(SDL_Surface *screen, char *file)
{
        SDL_Surface *converted;

        /* Load the sprite image */
        sprite = SDL_LoadBMP(file);
        if ( sprite == NULL ) {
                fprintf(stderr, "Couldn't load %s: %s", file, SDL_GetError());
                return(-1);
        }

        /* Set transparent pixel as the pixel at (0,0) */
        if ( sprite->format->palette ) {
                SDL_SetColorKey(sprite, SDL_SRCCOLORKEY,
                                                *(Uint8 *)sprite->pixels);
        }

        /* Convert sprite to video format */
        converted = SDL_DisplayFormat(sprite);
        SDL_FreeSurface(sprite);
        if ( converted == NULL ) {
                fprintf(stderr, "Couldn't convert background: %s\n",
                                                        SDL_GetError());
                return(-1);
        }
        sprite = converted;

        /* Create the background */
        backing = SDL_CreateRGBSurface(SDL_SWSURFACE, sprite->w, sprite->h, 8,
                                                                0, 0, 0, 0);
        if ( backing == NULL ) {
                fprintf(stderr, "Couldn't create background: %s\n",
                                                        SDL_GetError());
                SDL_FreeSurface(sprite);
                return(-1);
        }

        /* Convert background to video format */
        converted = SDL_DisplayFormat(backing);
        SDL_FreeSurface(backing);
        if ( converted == NULL ) {
                fprintf(stderr, "Couldn't convert background: %s\n",
                                                        SDL_GetError());
                SDL_FreeSurface(sprite);
                return(-1);
        }
        backing = converted;

        /* Set the initial position of the sprite */
        position.x = (screen->w-sprite->w)/2;
        position.y = (screen->h-sprite->h)/2;
        position.w = sprite->w;
        position.h = sprite->h;
        x_vel = 0; y_vel = 0;
        alpha_vel = 1;

        /* We're ready to roll. :) */
        return(0);
}

void AttractSprite(Uint16 x, Uint16 y)
{
        x_vel = ((int)x-position.x)/10;
        y_vel = ((int)y-position.y)/10;
}

void MoveSprite(SDL_Surface *screen, SDL_Surface *light)
{
        SDL_Rect updates[2];
        int alpha;

        /* Erase the sprite if it was visible */
        if ( sprite_visible ) {
                updates[0] = position;
                SDL_BlitSurface(backing, NULL, screen, &updates[0]);
        } else {
                updates[0].x = 0; updates[0].y = 0;
                updates[0].w = 0; updates[0].h = 0;
                sprite_visible = 1;
        }

        /* Since the sprite is off the screen, we can do other drawing
           without being overwritten by the saved area behind the sprite.
         */
        if ( light != NULL ) {
                int x, y;

                SDL_GetMouseState(&x, &y);
                FlashLight(screen, light, x, y);
        }
          
        /* Move the sprite, bounce at the wall */
        position.x += x_vel;
        if ( (position.x < 0) || (position.x >= screen->w) ) {
                x_vel = -x_vel;
                position.x += x_vel;
        }
        position.y += y_vel;
        if ( (position.y < 0) || (position.y >= screen->h) ) {
                y_vel = -y_vel;
                position.y += y_vel;
        }

        /* Update transparency (fade in and out) */
        alpha = sprite->format->alpha;
        if ( (alpha+alpha_vel) < 0 ) {
                alpha_vel = -alpha_vel;
        } else
        if ( (alpha+alpha_vel) > 255 ) {
                alpha_vel = -alpha_vel;
        }
        SDL_SetAlpha(sprite, SDL_SRCALPHA, (Uint8)(alpha+alpha_vel));

        /* Save the area behind the sprite */
        updates[1] = position;
        SDL_BlitSurface(screen, &updates[1], backing, NULL);
       
        /* Blit the sprite onto the screen */
        updates[1] = position;
        SDL_BlitSurface(sprite, NULL, screen, &updates[1]);

        /* Make it so! */
        SDL_UpdateRects(screen, 2, updates);
}

void WarpSprite(SDL_Surface *screen, int x, int y)
{
        SDL_Rect updates[2];

        /* Erase, move, Draw, update */
        updates[0] = position;
        SDL_BlitSurface(backing, NULL, screen, &updates[0]);
        position.x = x-sprite->w/2;        /* Center about X */
        position.y = y-sprite->h/2;        /* Center about Y */
        updates[1] = position;
        SDL_BlitSurface(screen, &updates[1], backing, NULL);
        updates[1] = position;
        SDL_BlitSurface(sprite, NULL, screen, &updates[1]);
        SDL_UpdateRects(screen, 2, updates);
}