Alpha Blending problems

Alpha Blending problems

thehuggableone

Member #7,625

August 2006

Ok, I'm working on a platform shooter, and the edges of my graphics are a little pixelated, so, as I did in a backgammon game I wrote not too long ago, I'm using a function that creates an "anti-aliased" sprite by merging the real sprite with a separate alpha channel (same shape, but black and white with blurred edges). Below is almost exactly the same function I used for my backgammon game, however this one keeps giving me my sprite rendered as a black box.

I don't know what's going on. Masked draw is simply a call to masked blit. Is there something I'm missing? I've scoured my backgammon code and can't figure out what's going on. It would help if I actually understood what's going on with the transparency stuff, but it's not a necessity I don't guess.

I tried making a call to set_alpha_blender() before printing and even calling draw_trans_sprite() but that didn't work either. Any help would be wonderful, either in fixing my problem or suggesting a better way to do what I'm trying to do. I AM fairly new to game programming and could use all the help I can get.

Thanks

--Wes

1 
2void main()
3{
  /*MAIN STUFF*/
  daggy[0] = makeMasked("data/images/daggy_sprite.bmp","data/images/daggy_alpha.bmp", 128);
  daggy[1] = makeMasked("data/images/daggy_sprite2.bmp","data/images/daggy_alpha2.bmp", 128);
7 
  CSprite playerCharacter(daggy,1,0,0,0,0,0,100,100,0);
  FixPlatformCollision(playerCharacter,background);
  playerCharacter.update();
  FixBoundsCollisions(playerCharacter);
  
  playerCharacter.masked_draw(buffer);
14 
  blit(buffer,screen,0,0,0,0,SCREENW,SCREENH);
  while(!key[KEY_ESC}){}
  /*MAIN STUFF*/
18}
19END_OF_MAIN();
20 
21BITMAP* makeMasked(char* image_file, char* alpha_file, int alpha)
22{
  BITMAP *tempPiece;
  BITMAP *sprite;
  int c, a;
  
  tempPiece = load_bitmap(image_file,NULL);
  sprite = load_bitmap(alpha_file,NULL);
    
  drawing_mode(DRAW_MODE_TRANS, NULL, 0, 0);
  set_write_alpha_blender();
    
  for (int y=0; y<sprite->h; y++) {
    for (int x=0; x<sprite->w; x++) {
      c = getpixel(sprite, x, y);
      a = getr(c) + getg(c) + getb(c);
      a = MID(0, a/2-alpha, 255);
      putpixel(tempPiece, x, y, a);
    }
  }
  
  drawing_mode(DRAW_MODE_SOLID, NULL, 0, 0);
  
  destroy_bitmap(sprite);
  return tempPiece;
46}

Arthur Kalliokoski

Second in Command

February 2005

I'm just taking a wild guess here, but stick in a couple lines in the program to save the masked bitmaps to .tga files, then make a little allegro utility to display these and call getpixel() on mouse clicks to display the rgba values.

They all watch too much MSNBC... they get ideas.