PostGIS  2.3.8dev-r@@SVN_REVISION@@

◆ lw_arc_length()

double lw_arc_length ( const POINT2D A1,
const POINT2D A2,
const POINT2D A3 
)

Returns the length of a circular arc segment.

Definition at line 118 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.

119 {
120  POINT2D C;
121  double radius_A, circumference_A;
122  int a2_side, clockwise;
123  double a1, a3;
124  double angle;
125 
126  if ( lw_arc_is_pt(A1, A2, A3) )
127  return 0.0;
128 
129  radius_A = lw_arc_center(A1, A2, A3, &C);
130 
131  /* Co-linear! Return linear distance! */
132  if ( radius_A < 0 )
133  {
134  double dx = A1->x - A3->x;
135  double dy = A1->y - A3->y;
136  return sqrt(dx*dx + dy*dy);
137  }
138 
139  /* Closed circle! Return the circumference! */
140  circumference_A = M_PI * 2 * radius_A;
141  if ( p2d_same(A1, A3) )
142  return circumference_A;
143 
144  /* Determine the orientation of the arc */
145  a2_side = lw_segment_side(A1, A3, A2);
146 
147  /* The side of the A1/A3 line that A2 falls on dictates the sweep
148  direction from A1 to A3. */
149  if ( a2_side == -1 )
150  clockwise = LW_TRUE;
151  else
152  clockwise = LW_FALSE;
153 
154  /* Angles of each point that defines the arc section */
155  a1 = atan2(A1->y - C.y, A1->x - C.x);
156  a3 = atan2(A3->y - C.y, A3->x - C.x);
157 
158  /* What's the sweep from A1 to A3? */
159  if ( clockwise )
160  {
161  if ( a1 > a3 )
162  angle = a1 - a3;
163  else
164  angle = 2*M_PI + a1 - a3;
165  }
166  else
167  {
168  if ( a3 > a1 )
169  angle = a3 - a1;
170  else
171  angle = 2*M_PI + a3 - a1;
172  }
173 
174  /* Length as proportion of circumference */
175  return circumference_A * (angle / (2*M_PI));
176 }
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:105
double x
Definition: liblwgeom.h:327
#define LW_FALSE
Definition: liblwgeom.h:76
#define LW_TRUE
Return types for functions with status returns.
Definition: liblwgeom.h:75
double y
Definition: liblwgeom.h:327
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:227
int p2d_same(const POINT2D *p1, const POINT2D *p2)
Definition: lwalgorithm.c:49
int lw_segment_side(const POINT2D *p1, const POINT2D *p2, const POINT2D *q)
lw_segment_side()
Definition: lwalgorithm.c:64
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