PostGIS  2.1.10dev-r@@SVN_REVISION@@
double lw_arc_length ( const POINT2D A1,
const POINT2D A2,
const POINT2D A3 
)

Returns the length of a circular arc segment.

Definition at line 119 of file lwalgorithm.c.

References lw_arc_center(), lw_arc_is_pt(), LW_FALSE, lw_segment_side(), LW_TRUE, p2d_same(), POINT2D::x, and POINT2D::y.

Referenced by ptarray_arc_length_2d(), and test_lw_arc_length().

120 {
121  POINT2D C;
122  double radius_A, circumference_A;
123  int a2_side, clockwise;
124  double a1, a3;
125  double angle;
126 
127  if ( lw_arc_is_pt(A1, A2, A3) )
128  return 0.0;
129 
130  radius_A = lw_arc_center(A1, A2, A3, &C);
131 
132  /* Co-linear! Return linear distance! */
133  if ( radius_A < 0 )
134  {
135  double dx = A1->x - A3->x;
136  double dy = A1->y - A3->y;
137  return sqrt(dx*dx + dy*dy);
138  }
139 
140  /* Closed circle! Return the circumference! */
141  circumference_A = M_PI * 2 * radius_A;
142  if ( p2d_same(A1, A3) )
143  return circumference_A;
144 
145  /* Determine the orientation of the arc */
146  a2_side = lw_segment_side(A1, A3, A2);
147 
148  /* The side of the A1/A3 line that A2 falls on dictates the sweep
149  direction from A1 to A3. */
150  if ( a2_side == -1 )
151  clockwise = LW_TRUE;
152  else
153  clockwise = LW_FALSE;
154 
155  /* Angles of each point that defines the arc section */
156  a1 = atan2(A1->y - C.y, A1->x - C.x);
157  a3 = atan2(A3->y - C.y, A3->x - C.x);
158 
159  /* What's the sweep from A1 to A3? */
160  if ( clockwise )
161  {
162  if ( a1 > a3 )
163  angle = a1 - a3;
164  else
165  angle = 2*M_PI + a1 - a3;
166  }
167  else
168  {
169  if ( a3 > a1 )
170  angle = a3 - a1;
171  else
172  angle = 2*M_PI + a3 - a1;
173  }
174 
175  /* Length as proportion of circumference */
176  return circumference_A * (angle / (2*M_PI));
177 }
int lw_arc_is_pt(const POINT2D *A1, const POINT2D *A2, const POINT2D *A3)
Returns true if arc A is actually a point (all vertices are the same) .
Definition: lwalgorithm.c:106
double x
Definition: liblwgeom.h:284
#define LW_FALSE
Definition: liblwgeom.h:52
#define LW_TRUE
Return types for functions with status returns.
Definition: liblwgeom.h:51
double y
Definition: liblwgeom.h:284
double lw_arc_center(const POINT2D *p1, const POINT2D *p2, const POINT2D *p3, POINT2D *result)
Determines the center of the circle defined by the three given points.
Definition: lwalgorithm.c:228
int p2d_same(const POINT2D *p1, const POINT2D *p2)
Definition: lwalgorithm.c:47
int lw_segment_side(const POINT2D *p1, const POINT2D *p2, const POINT2D *q)
lw_segment_side()
Definition: lwalgorithm.c:62

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