I would do it like this:
1#include <iostream>
2#include <vector>
3
4
5class LightMask
6{
7private:
8 class point2d
9 {
10 public:
11 float x;
12 float y;
13 point2d(float x, float y) : x(x), y(y) {}
14 };
15
16 ALLEGRO_BITMAP *surface;
17 ALLEGRO_BITMAP *light_bitmap;
18 std::vector<point2d> points;
19
20 void update_surface()
21 {
22 ALLEGRO_STATE previous_state;
23 al_store_state(&previous_state, ALLEGRO_STATE_TARGET_BITMAP | ALLEGRO_STATE_BLENDER);
24 al_set_target_bitmap(surface);
25 al_clear_to_color(al_map_rgb(0, 0, 0));
26 al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE);
27
28 for (auto &point : points)
29 {
30 float half_w = al_get_bitmap_width(light_bitmap) * 0.5;
31 float half_h = al_get_bitmap_height(light_bitmap) * 0.5;
32 al_draw_bitmap(light_bitmap, point.x - half_w, point.y - half_h, 0);
33 }
34
35 al_restore_state(&previous_state);
36 }
37
38public:
39 LightMask(int width, int height)
40 : surface(al_create_bitmap(width, height))
41 , light_bitmap(al_load_bitmap("data/bitmaps/gradient.jpg"))
42 {
43 if (!light_bitmap) std::cout << "There was an error loading the light bitmap" << std::endl;
44 }
45
46 ~LightMask()
47 {
48 al_destroy_bitmap(surface);
49 al_destroy_bitmap(light_bitmap);
50 }
51
52 void clear_points()
53 {
54 points.clear();
55 }
56
57 void add_point(float x, float y)
58 {
59 points.push_back(point2d(x, y));
60 }
61
62 void draw()
63 {
64 update_surface();
65
66 ALLEGRO_STATE previous_state;
67 al_store_state(&previous_state, ALLEGRO_STATE_BLENDER);
68 al_set_blender(ALLEGRO_ADD, ALLEGRO_DST_COLOR, ALLEGRO_ZERO);
69
70 al_draw_bitmap(surface, 0, 0, 0);
71
72 al_restore_state(&previous_state);
73 }
74};
This object has a very simple API:
LightMask(int w, int h) - create a new lightmask.
void add_point(int x, int y) - adds a light point to your mask.
void clear_points() - removes all the light points from the mask.
void draw() - draws the mask.
Here is the light mask being drawn on a green background. It has two static points, and another point that follows the mouse (click for gif):
{"name":"610432","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/4\/6\/4685ce30f58324f61f7186f933af907d.gif","w":656,"h":392,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/4\/6\/4685ce30f58324f61f7186f933af907d"}
(paper-clip icon)
