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

       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);

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

MASK_COLOR_8(3alleg4),     set_color_depth(3alleg4),     bitmap_color_depth(3alleg4),      ex3d(3alleg4),
       exmouse(3alleg4), expat(3alleg4)

Allegro                                           version 4.4.3                       bitmap_mask_color(3alleg4)

#include<allegro.h>intbitmap_mask_color(BITMAP*bmp);