help: how to use al_calculate_arc()

no-reply@allegro.cc (Pho75_) — Wed, 04 Nov 2020 01:30:25 +0000

Hi all,
I'm trying to figure out how to use al_calculate_arc() to
get an array of points that I can manually draw
but I don't understand from the documentation how to use the output.
It's not drawing correctly.
What am I doing wrong?

Thanks.

#SelectExpand
  1#include 
  2#include 
  3
  4int main()
  5{
  6    al_init();
  7    al_init_primitives_addon();
  8    al_set_new_display_flags(ALLEGRO_RESIZABLE);
  9    al_create_display(1280, 720);
 10    al_clear_to_color(al_map_rgb(0,0,0));
 11
 12    {
 13        const int num_points = 16;
 14        float points[num_points*2][2];
 15
 16        // calculate circle points
 17    //            al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 100, 100, 0, ALLEGRO_PI / 2, 2, num_points);
 18        al_calculate_arc(&points[0][0], 2 * sizeof(float), 100, 100, 100, 100, 0, ALLEGRO_PI * 2, 2, num_points);
 19
 20        // BROKEN
 21        for (int i=0; i+3 <= num_points; i++)
 22        {
 23//            al_draw_filled_polygon(&points[i][0], 4, al_map_rgb(255,255,255));
 24            al_draw_filled_triangle(
 25                    points[i*3][0],     points[i*3][1],
 26                    points[i*3+1][0], points[i*3+1][1],
 27                    points[i*3+2][0], points[i*3+2][1],
 28                    al_map_rgb(255,255,255));
 29        }
 30
 31//        BROKEN
 32//        for (int i=0; i+4 <= num_points; i+=2)
 33//        {
 34//            al_draw_filled_polygon(&points[i][0], 4, al_map_rgb(255,255,255));
 35//        }
 36    }
 37
 38    al_flip_display();
 39
 40    getchar();
 41
 42    return 0;
 43}

no-reply@allegro.cc (Edgar Reynaldo) — Wed, 04 Nov 2020 01:52:00 +0000

https://liballeg.org/a5docs/5.2.6/primitives.html#al_calculate_arc

There are two sets of points with thickness > 0. The outer points and the inner points. Draw your triangle between the two.

no-reply@allegro.cc (Pho75_) — Wed, 04 Nov 2020 05:17:39 +0000

Ok, I figured it out.
Thanks for the help.
Here is the code so maybe it will help someone else.

#SelectExpand
  1#include 
  2#include 
  3#include 
  4
  5int main()
  6{
  7    al_init();
  8    al_init_primitives_addon();
  9    al_set_new_display_flags(ALLEGRO_RESIZABLE);
 10    al_create_display(1280, 720);
 11    al_clear_to_color(al_map_rgb(0,0,0));
 12
 13    {
 14        const int num_points = 16;
 15        float points[num_points*2][2];
 16        al_calculate_arc(&points[0][0], 2 * sizeof(float), 100, 100, 100, 100, 0, ALLEGRO_PI * 2, 2, num_points);
 17
 18        for (int i=0; i+4 <= num_points*2; i+=2)
 19        {
 20            al_draw_filled_triangle(
 21                    points[i][0],   points[i][1],
 22                    points[i+1][0], points[i+1][1],
 23                    points[i+2][0], points[i+2][1],
 24                    al_map_rgb(255,255,255));
 25            al_draw_filled_triangle(
 26                    points[i+2][0], points[i+2][1],
 27                    points[i+3][0], points[i+3][1],
 28                    points[i+1][0], points[i+1][1],
 29                    al_map_rgb(255,255,255));
 30        }
 31    }
 32
 33    al_flip_display();
 34
 35    getchar();
 36
 37    return 0;
 38}

no-reply@allegro.cc (Edgar Reynaldo) — Thu, 05 Nov 2020 02:00:58 +0000

I didn't see it the first time, but yes, the outer and inner points are interleaved. That's probably what caused the confusion. Thank you for the example.