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 127 of file lwalgorithm.c.

{
        POINT2D C;
        double radius_A, circumference_A;
        int a2_side, clockwise;
        double a1, a3;
        double angle;
 
        if ( lw_arc_is_pt(A1, A2, A3) )
                return 0.0;
 
        radius_A = lw_arc_center(A1, A2, A3, &C);
 
        /* Co-linear! Return linear distance! */
        if ( radius_A < 0 )
        {
        double dx = A1->x - A3->x;
        double dy = A1->y - A3->y;
                return sqrt(dx*dx + dy*dy);
        }
 
        /* Closed circle! Return the circumference! */
        circumference_A = M_PI * 2 * radius_A;
        if ( p2d_same(A1, A3) )
                return circumference_A;
 
        /* Determine the orientation of the arc */
        a2_side = lw_segment_side(A1, A3, A2);
 
        /* The side of the A1/A3 line that A2 falls on dictates the sweep
           direction from A1 to A3. */
        if ( a2_side == -1 )
                clockwise = LW_TRUE;
        else
                clockwise = LW_FALSE;
 
        /* Angles of each point that defines the arc section */
        a1 = atan2(A1->y - C.y, A1->x - C.x);
        a3 = atan2(A3->y - C.y, A3->x - C.x);
 
        /* What's the sweep from A1 to A3? */
        if ( clockwise )
        {
                if ( a1 > a3 )
                        angle = a1 - a3;
                else
                        angle = 2*M_PI + a1 - a3;
        }
        else
        {
                if ( a3 > a1 )
                        angle = a3 - a1;
                else
                        angle = 2*M_PI + a3 - a1;
        }
 
        /* Length as proportion of circumference */
        return circumference_A * (angle / (2*M_PI));
}

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().

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