How to use al_calc_elliptical_arc?

no-reply@allegro.cc (maxdev) — Sun, 04 Oct 2015 22:27:27 +0000

Hello, I want to calculate the points of an elliptical arc, but I do not really now, how to use

void al_calculate_arc(float* dest, int stride, float cx, float cy,
float rx, float ry, float start_theta, float delta_theta, float thickness,
int num_segments)

Could you give me an example how to store the values of the arc, if cx, cy, rx, ry, start_theta, delta_theta and thickness are given?

no-reply@allegro.cc (Mark Oates) — Sun, 04 Oct 2015 23:55:13 +0000

[edit] removed; posted in the wrong place

no-reply@allegro.cc (maxdev) — Mon, 05 Oct 2015 22:00:16 +0000

Nice game, but that doesn't help me

no-reply@allegro.cc (Mark Oates) — Tue, 06 Oct 2015 01:38:22 +0000

Woah. , I was wondering where my post went. I was like "I swear I posted it! "

no-reply@allegro.cc (Edgar Reynaldo) — Tue, 06 Oct 2015 02:03:48 +0000

void al_calculate_arc(float* dest, int stride, float cx, float cy,
float rx, float ry, float start_theta, float delta_theta, float thickness,
int num_segments)

The key is 'num_segments'. You need to have room in 'dest' (in an array or dynamically allocated memory) for at least num_segments*2 x and y values, and if it's a packed array then stride should be 2*sizeof(float). It will store the coordinates of the arc in the 'dest' array. If thickness is greater than zero you need twice as many elements in the array.

For a quick code example here :

#define NUM_PTS 10
float fltarray[2*NUM_PTS];
float* fltarray2 = new float[2*NUM_PTS];
float* fltarray3 = new float[2*2*NUM_PTS];

al_calculate_arc(fltarray , 2*sizeof(float) , 0 , 0 , 100 , 50 , M_PI/2.0f , M_PI/2.0f , 0 , NUM_PTS);
al_calculate_arc(fltarray2 , 2*sizeof(float) , 0 , 0 , 100 , 50 , M_PI/2.0f , M_PI/2.0f , 0 , NUM_PTS);
al_calculate_arc(fltarray3 , 4*sizeof(float) , 0 , 0 , 100 , 50 , M_PI/2.0f , M_PI/2.0f , 10.0f , NUM_PTS);

no-reply@allegro.cc (Mark Oates) — Tue, 06 Oct 2015 02:19:23 +0000

Here's my example:

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#SelectExpand
  1#include 
  2#include 
  3#include 
  4#include 
  5
  6
  7
  8struct xy_points
  9{
 10  float x;
 11  float y;
 12};
 13
 14void my_draw_calc_arc(float x, float y)
 15{
 16  xy_points points[16];
 17
 18  al_calculate_arc(&points[0].x, sizeof(xy_points),
 19    x, y, 220, 140,
 20    20.0 * ALLEGRO_PI / 180.0, 60.0 * ALLEGRO_PI / 180.0, // start theta and delta theta (in radians)
 21    20.0, 8);
 22
 23  for (unsigned i=0; i<16; i++)
 24  {
 25    al_draw_circle(points[i].x, points[i].y, 5, al_color_name("dodgerblue"), 2.0);
 26    std::cout << "(" << points[i].x << ", " << points[i].y << ")" << std::endl;
 27  }
 28}
 29
 30
 31
 32int main(int argc, char* argv[])
 33{
 34  al_init();
 35  al_init_primitives_addon();
 36
 37  ALLEGRO_DISPLAY *display = al_create_display(960, 600);
 38
 39  my_draw_calc_arc(al_get_display_width(display)/2, al_get_display_height(display)/2);
 40
 41  al_flip_display();
 42
 43  al_rest(2);
 44}

This example program shows for 2 seconds then closes.