# al_calculate_arc man page

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(3), al_calculate_spline(3), al_calculate_ribbon(3)

## Referenced By

al_calculate_ribbon(3), al_calculate_spline(3), al_draw_arc(3), al_draw_elliptical_arc(3).