Ubuntu Manpage: al_set_new_bitmap_flags

NAME

       al_set_new_bitmap_flags - Allegro 5 API

SYNOPSIS

              #include <allegro5/allegro.h>

              void al_set_new_bitmap_flags(int flags)

DESCRIPTION

Sets the flags to use for newly created bitmaps. Valid flags are:

ALLEGRO_VIDEO_BITMAP
Creates a bitmap that resides in the video card memory. These types of bitmaps receive the
greatest benefit from hardware acceleration. al_set_new_bitmap_flags(3alleg5) will implicitly set
this flag unless ALLEGRO_MEMORY_BITMAP is present.

ALLEGRO_MEMORY_BITMAP
Create a bitmap residing in system memory. Operations on, and with, memory bitmaps will not be
hardware accelerated. However, direct pixel access can be relatively quick compared to video
bitmaps, which depend on the display driver in use.

Note: Allegro's software rendering routines are currently very unoptimised.

ALLEGRO_KEEP_BITMAP_FORMAT
Only used when loading bitmaps from disk files, forces the resulting ALLEGRO_BITMAP(3alleg5) to
use the same format as the file.

This is not yet honoured.

ALLEGRO_FORCE_LOCKING
When drawing to a bitmap with this flag set, always use pixel locking and draw to it using
Allegro's software drawing primitives. This should never be used if you plan to draw to the
bitmap using Allegro's graphics primitives as it would cause severe performance penalties.
However if you know that the bitmap will only ever be accessed by locking it, no unneeded FBOs
will be created for it in the OpenGL drivers.

ALLEGRO_NO_PRESERVE_TEXTURE
Normally, every effort is taken to preserve the contents of bitmaps, since Direct3D may forget
them. This can take extra processing time. If you know it doesn't matter if a bitmap keeps its
pixel data, for example its a temporary buffer, use this flag to tell Allegro not to attempt to
preserve its contents. This can increase performance of your game or application, but there is a
catch. See ALLEGRO_EVENT_DISPLAY_LOST for further information.

ALLEGRO_ALPHA_TEST
This is a driver hint only. It tells the graphics driver to do alpha testing instead of alpha
blending on bitmaps created with this flag. Alpha testing is usually faster and preferred if your
bitmaps have only one level of alpha (0). This flag is currently not widely implemented (i.e.,
only for memory bitmaps).

ALLEGRO_MIN_LINEAR
When drawing a scaled down version of the bitmap, use linear filtering. This usually looks
better. You can also combine it with the MIPMAP flag for even better quality.

ALLEGRO_MAG_LINEAR
When drawing a magnified version of a bitmap, use linear filtering. This will cause the picture
to get blurry instead of creating a big rectangle for each pixel. It depends on how you want
things to look like whether you want to use this or not.

ALLEGRO_MIPMAP
This can only be used for bitmaps whose width and height is a power of two. In that case, it will
generate mipmaps and use them when drawing scaled down versions. For example if the bitmap is
64x64, then extra bitmaps of sizes 32x32, 16x16, 8x8, 4x4, 2x2 and 1x1 will be created always
containing a scaled down version of the original.

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.

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