al_set_blender
void al_set_blender(int op, int src, int dst)
Introduced in 5.0.0
Sets the function to use for blending for the current thread.
Blending means, the source and destination colors are combined in drawing operations.
Assume the source color (e.g. color of a rectangle to draw, or pixel of a bitmap to draw) is given as its red/green/blue/alpha components (if the bitmap has no alpha it always is assumed to be fully opaque, so 255 for 8-bit or 1.0 for floating point): sr, sg, sb, sa. And this color is drawn to a destination, which already has a color: dr, dg, db, da.
The conceptional formula used by Allegro to draw any pixel then depends on the op
parameter:
ALLEGRO_ADD
r = dr * dst + sr * src g = dg * dst + sg * src b = db * dst + sb * src a = da * dst + sa * src
ALLEGRO_DEST_MINUS_SRC
r = dr * dst - sr * src g = dg * dst - sg * src b = db * dst - sb * src a = da * dst - sa * src
ALLEGRO_SRC_MINUS_DEST
r = sr * src - dr * dst g = sg * src - dg * dst b = sb * src - db * dst a = sa * src - da * dst
Valid values for src
and dst
passed to this function are
ALLEGRO_ZERO
src = 0 dst = 0
ALLEGRO_ONE
src = 1 dst = 1
ALLEGRO_ALPHA
src = sa dst = sa
ALLEGRO_INVERSE_ALPHA
src = 1 - sa dst = 1 - sa
Blending examples:
So for example, to restore the default of using premultiplied alpha blending, you would use (pseudo code)
al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA)
If you are using non-pre-multiplied alpha, you could use
al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA)
Additive blending would be achieved with
al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE)
Copying the source to the destination (including alpha) unmodified
al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO)
See also: al_set_separate_blender, al_get_blender
Examples: ex_bitmap_flip, ex_bitmap_target, ex_blend, ex_blend2, ex_blend_bench, ex_blend_test, ex_blit, ex_clip, ex_display_options, ex_draw, ex_draw_bitmap, ex_expose, ex_haiku, ex_lines, ex_logo, ex_membmp, ex_mixer_pp, ex_mouse_cursor, ex_mouse_events, ex_native_filechooser, ex_pixelformat, ex_premulalpha, ex_prim, ex_rotate, ex_scale, ex_stream_seek, ex_threads, ex_threads2, ex_transform, ex_windows
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