NAME

       al_load_bitmap_flags - Allegro 5 API

SYNOPSIS

              #include <allegro5/allegro.h>

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

DESCRIPTION

       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:

       ALLEGRO_NO_PREMULTIPLIED_ALPHA
              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, ALLEGRO_INVERSE_ALPHA).

              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
                     al_set_new_bitmap_format(ALLEGRO_PIXEL_FORMAT_ANY_32_WITH_ALPHA);
                     // 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_set_target_bitmap(tmp_buffer);
                     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_set_target_bitmap(al_get_backbuffer(display));
                     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.

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

       ALLEGRO_KEEP_BITMAP_FORMAT
              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.

SINCE

       5.1.0

SEE ALSO

       al_load_bitmap(3alleg5)