bitmap_mask_color(3)             Allegro manual             bitmap_mask_color(3)

       bitmap_mask_color - Returns the mask color of the specified bitmap.
       Allegro game programming library.

       #include <allegro.h>

       int bitmap_mask_color(BITMAP *bmp);

       Returns the mask color for the specified bitmap (the value which is
       skipped when drawing sprites). For 256-color bitmaps this is zero, and
       for truecolor bitmaps it is bright pink (maximum red and blue, zero
       green). A frequent use of this function is to clear a bitmap with the
       mask color so you can later use this bitmap with masked_blit() or
       draw_sprite() after drawing other stuff on it. Example:

          /* Replace mask color with another color. */
          for (y = 0; y h; y++)
             for (x = 0; x w; x++)
                if (getpixel(bmp, x, y) == bitmap_mask_color(bmp))
                   putpixel(bmp, x, y, another_color);

       MASK_COLOR_8(3), set_color_depth(3), bitmap_color_depth(3), ex3d(3),
       exmouse(3), expat(3)

Allegro                           version 4.4.3             bitmap_mask_color(3)