1al_set_blender(3) Library Functions Manual al_set_blender(3)
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6 al_set_blender
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9 #include <allegro5/allegro.h>
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11 void al_set_blender(int op, int src, int dst)
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14 Sets the function to use for blending for the current thread.
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16 Blending means, the source and destination colors are combined in draw‐
17 ing operations.
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19 Assume the source color (e.g. color of a rectangle to draw, or pixel
20 of a bitmap to draw) is given as its red/green/blue/alpha components
21 (if the bitmap has no alpha it always is assumed to be fully opaque, so
22 255 for 8-bit or 1.0 for floating point): sr, sg, sb, sa. And this
23 color is drawn to a destination, which already has a color: dr, dg, db,
24 da.
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26 The conceptional formula used by Allegro to draw any pixel then depends
27 on the op parameter:
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29 · ALLEGRO_ADD
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31 r = dr * dst + sr * src
32 g = dg * dst + sg * src
33 b = db * dst + sb * src
34 a = da * dst + sa * src
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36 · ALLEGRO_DEST_MINUS_SRC
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38 r = dr * dst - sr * src
39 g = dg * dst - sg * src
40 b = db * dst - sb * src
41 a = da * dst - sa * src
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43 · ALLEGRO_SRC_MINUS_DEST
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45 r = sr * src - dr * dst
46 g = sg * src - dg * dst
47 b = sb * src - db * dst
48 a = sa * src - da * dst
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50 Valid values for src and dst passed to this function are
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52 · ALLEGRO_ZERO
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54 src = 0
55 dst = 0
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57 · ALLEGRO_ONE
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59 src = 1
60 dst = 1
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62 · ALLEGRO_ALPHA
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64 src = sa
65 dst = sa
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67 · ALLEGRO_INVERSE_ALPHA
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69 src = 1 - sa
70 dst = 1 - sa
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72 Blending examples:
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74 So for example, to restore the default of using premultiplied alpha
75 blending, you would use (pseudo code)
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77 al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA)
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79 If you are using non-pre-multiplied alpha, you could use
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81 al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA)
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83 Additive blending would be achieved with
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85 al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE)
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87 Copying the source to the destination (including alpha) unmodified
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89 al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO)
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92 al_set_separate_blender(3), al_get_blender(3)
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96Allegro reference manual al_set_blender(3)