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 ellipti‐
       cal  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 thick‐
       ness / 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  hold‐
       ing  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)

Allegro reference manual                                                               al_calculate_arc(3alleg5)