[gameprogrammer] Whaley's OpenGL question

On Wed, 2004-04-28 at 00:48, Alexander Whaley wrote:
> By the way, I have a small OpenGL question:
> Is there an easy way to set a colour to be transparent when loading a 
> bitmap. I mean, if I have a set of pictures and say that magenta 
> indicates a transparent colour, can I set it as a default somewhere or 
> must I convert each "magenta pixel" as I load the bitmap into memory to 
> become a transparent pixel. Something in the lines of: 
> "set_transparent_color(RGB)"??

Run through the image and set the alpha for each pixel with your
transparent color so that the alpha makes it transparent. Here is a
block of code that does that using SDL to load the image and to make the
colorkey pixels transparent.

//----------------------------------------------------------

//  Create a texture from a surface. Set the alpha according
//  to the color key. Pixels that match the color key get an
//  alpha of zero while all other pixels get an alpha of
//  one.

GLuint loadTextureColorKey(SDL_Surface *surface, 
                           GLfloat *texcoord,
                           int ckr, 
                           int ckg, 
                           int ckb)
{
  GLuint texture;
  int w, h;
  SDL_Surface *image;
  SDL_Rect area;
  Uint32 colorkey;

  // Use the surface width and height expanded to powers of 2 

  w = powerOfTwo(surface->w);
  h = powerOfTwo(surface->h);
  texcoord[0] = 0.0f;                    // Min X 
  texcoord[1] = 0.0f;                    // Min Y 
  texcoord[2] = (GLfloat)surface->w / w; // Max X 
  texcoord[3] = (GLfloat)surface->h / h; // Max Y 

  image = SDL_CreateRGBSurface(
                               SDL_SWSURFACE,
                               w, h,
                               32,
#if SDL_BYTEORDER == SDL_LIL_ENDIAN // OpenGL RGBA masks 
                               0x000000FF, 
                               0x0000FF00, 
                               0x00FF0000, 
                               0xFF000000
#else
                               0xFF000000,
                               0x00FF0000, 
                               0x0000FF00, 
                               0x000000FF
#endif
                               );
  if (image == NULL)
  {
    return 0;
  }

  // Set up so that colorkey pixels become transparent 

  colorkey = SDL_MapRGBA(image->format, ckr, ckg, ckb, 0);
  SDL_FillRect(image, NULL, colorkey);

  colorkey = SDL_MapRGBA(surface->format, ckr, ckg, ckb, 0);
  SDL_SetColorKey(surface, SDL_SRCCOLORKEY, colorkey);

  // Copy the surface into the GL texture image 
  area.x = 0;
  area.y = 0;
  area.w = surface->w;
  area.h = surface->h;
  SDL_BlitSurface(surface, &area, image, &area);

  // Create an OpenGL texture for the image 

  glGenTextures(1, &texture);
  glBindTexture(GL_TEXTURE_2D, texture);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
  glTexImage2D(GL_TEXTURE_2D,
               0,
               GL_RGBA,
               w, h,
               0,
               GL_RGBA,
               GL_UNSIGNED_BYTE,
               image->pixels);

  SDL_FreeSurface(image); // No longer needed 

  return texture;
}


-- 
+--------------------------------------+
+ Bob Pendleton: writer and programmer +
+ email: Bob@xxxxxxxxxxxxx             +
+ blog:  www.Stonewolf.net             +
+ web:   www.GameProgrammer.com        +
+--------------------------------------+

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