Provided by: allegro5-doc_5.2.3.0-1_all bug


       al_load_bitmap_flags - Allegro 5 API


              #include <allegro5/allegro.h>

              ALLEGRO_BITMAP *al_load_bitmap_flags(const char *filename, int flags)


       Loads  an  image  file into a new ALLEGRO_BITMAP(3alleg5).  The file type is determined by
       the   extension,   except   if   the   file   has   no    extension    in    which    case
       al_identify_bitmap(3alleg5) is used instead.

       Returns NULL on error.

       The flags parameter may be a combination of the following constants:

              By  default,  Allegro  pre-multiplies the alpha channel of an image with the images
              color data when it loads it.  Typically that would look something like this:

                     r = get_float_byte();
                     g = get_float_byte();
                     b = get_float_byte();
                     a = get_float_byte();

                     r = r * a;
                     g = g * a;
                     b = b * a;

                     set_image_pixel(x, y, r, g, b, a);

              The reason for this can be seen in the Allegro example  ex_premulalpha,  ie,  using
              pre-multiplied alpha gives more accurate color results in some cases.  To use alpha
              blending with images loaded with pre-multiplied alpha, you would  use  the  default
              blending   mode,   which   is  set  with  al_set_blender(ALLEGRO_ADD,  ALLEGRO_ONE,

              The ALLEGRO_NO_PREMULTIPLIED_ALPHA flag being set will ensure that images  are  not
              loaded  with alpha pre-multiplied, but are loaded with color values direct from the
              image.  That looks like this:

                     r = get_float_byte();
                     g = get_float_byte();
                     b = get_float_byte();
                     a = get_float_byte();

                     set_image_pixel(x, y, r, g, b, a);

              To  draw  such  an  image   using   regular   alpha   blending,   you   would   use
              al_set_blender(ALLEGRO_ADD,   ALLEGRO_ALPHA,   ALLEGRO_INVERSE_ALPHA)  to  set  the
              correct blender.  This has some caveats.  First, as mentioned above,  drawing  such
              an  image  can  result  in less accurate color blending (when drawing an image with
              linear filtering on, the edges will be darker than they should  be).   Second,  the
              behaviour is somewhat confusing, which is explained in the example below.

                     // Load and create bitmaps with an alpha channel
                     // Load some bitmap with alpha in it
                     bmp = al_load_bitmap("some_alpha_bitmap.png");
                     // We will draw to this buffer and then draw this buffer to the screen
                     tmp_buffer = al_create_bitmap(SCREEN_W, SCREEN_H);
                     // Set the buffer as the target and clear it
                     al_clear_to_color(al_map_rgba_f(0, 0, 0, 1));
                     // Draw the bitmap to the temporary buffer
                     al_draw_bitmap(bmp, 0, 0, 0);
                     // Finally, draw the buffer to the screen
                     // The output will look incorrect (may take close inspection
                     // depending on the bitmap -- it may also be very obvious)
                     al_draw_bitmap(tmp_buffer, 0, 0, 0);

              To  explain  further,  if  you  have  a  pixel  with  0.5  alpha,  and you're using
              (ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA) for blending, the formula is:

                     a = da * dst + sa * src

              Expands to:

                     result_a = dst_a * (1-0.5) + 0.5 * 0.5

              So if you draw the image to the temporary buffer, it is blended once  resulting  in
              0.75 alpha, then drawn again to the screen, blended in the same way, resulting in a
              pixel has 0.1875 as an alpha value.

              Load the palette indices of 8-bit .bmp and .pcx files instead of  the  rgb  colors.
              Since 5.1.0.

              Force the resulting ALLEGRO_BITMAP(3alleg5) to use the same format as the file.

              This is not yet honoured.

              Note:  the core Allegro library does not support any image file formats by default.
              You must use the allegro_image addon, or register your own format handler.