NAME

       al_calculate_arc - Allegro 5 API

SYNOPSIS

              #include <allegro5/allegro_primitives.h>

              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_points)

DESCRIPTION

       When  thickness  <=  0  this  function  computes  positions  of  num_points regularly spaced points on an
       elliptical arc.  When thickness > 0 this function computes two sets of points, obtained as  follows:  the
       first  set  is  obtained  by  taking  the points computed in the thickness <= 0 case and shifting them by
       thickness / 2 outward, in a direction perpendicular to the arc curve.  The second set is  the  same,  but
       shifted  thickness  /  2  inward  relative  to  the  arc.   The two sets of points are interleaved in the
       destination buffer (i.e. the first pair of points will be collinear with the arc center, the first  point
       of  the pair will be farther from the center than the second point; the next pair will also be collinear,
       but at a different angle and so on).

       The destination buffer dest is interpreted as a set of  regularly  spaced  pairs  of  floats,  each  pair
       holding  the coordinates of the corresponding point on the arc.  The two floats in the pair are adjacent,
       and the distance (in bytes) between the addresses of the first float in two successive pairs  is  stride.
       For  example, if you have a tightly packed array of floats with no spaces between pairs, then stride will
       be exactly 2 * sizeof(float).

       Example with thickness <= 0:

              const int num_points = 4;
              float points[num_points][2];
              al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 0, num_points);

              assert((int)points[0][0] == 10);
              assert((int)points[0][1] == 0);

              assert((int)points[num_points - 1][0] == 0);
              assert((int)points[num_points - 1][1] == 10);

       Example with thickness > 0:

              const int num_points = 4;
              float points[num_points * 2][2];
              al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 2, num_points);

              assert((int)points[0][0] == 11);
              assert((int)points[0][1] == 0);
              assert((int)points[1][0] == 9);
              assert((int)points[1][1] == 0);

              assert((int)points[(num_points - 1) * 2][0] == 0);
              assert((int)points[(num_points - 1) * 2][1] == 11);
              assert((int)points[(num_points - 1) * 2 + 1][0] == 0);
              assert((int)points[(num_points - 1) * 2 + 1][1] == 9);

       Parameters:

       • dest - The destination buffer

       • stride - Distance (in bytes) between starts of successive pairs of points

       • cx, cy - Center of the arc

       • rx, ry - Radii of the arc

       • start_theta - The initial angle from which the arc is calculated in radians

       • delta_theta - Angular span of the arc in radians (pass a negative number to switch direction)

       • thickness - Thickness of the arc

       • num_points - The number of points to calculate

SEE ALSO

       al_draw_arc(3alleg5), al_calculate_spline(3alleg5), al_calculate_ribbon(3alleg5)