ptarray_raycast_intersections()

int ptarray_raycast_intersections	(	const POINTARRAY *	pa,
		const POINT2D *	p,
		int *	on_boundary
	)

Definition at line 818 of file ptarray.c.

{
        // A valid linestring must have at least 2 point
        if (pa->npoints < 2)
                lwerror("%s called on invalid linestring", __func__);
 
        int intersections = 0;
        double px = p->x;
        double py = p->y;
 
        // Iterate through each edge of the polygon
        for (uint32_t i = 0; i < pa->npoints-1; ++i)
        {
                const POINT2D* p1 = getPoint2d_cp(pa, i);
                const POINT2D* p2 = getPoint2d_cp(pa, i+1);
 
                /* Skip zero-length edges */
                if (p2d_same(p1, p2))
                        continue;
 
                /* --- Step 1: Check if the point is ON the boundary edge --- */
                if (lw_pt_on_segment(p1, p2, p))
                {
                        *on_boundary = LW_TRUE;
                        return 0;
                }
 
                /* --- Step 2: Perform the Ray Casting intersection test --- */
 
                /*
                 * Check if the horizontal ray from p intersects the edge (p1, p2).
                 * This is the core condition for handling vertices correctly:
                 *   - One vertex must be strictly above the ray (py < vertex.y)
                 *   - The other must be on or below the ray (py >= vertex.y)
                 */
                if (((p1->y <= py) && (py < p2->y)) || ((p2->y <= py) && (py < p1->y)))
                {
                        /*
                         * Calculate the x-coordinate where the edge intersects the ray's horizontal line.
                         * Formula: x = x1 + (y - y1) * (x2 - x1) / (y2 - y1)
                         */
                        double x_intersection = p1->x + (py - p1->y) * (p2->x - p1->x) / (p2->y - p1->y);
 
                        /*
                         * If the intersection point is to the right of our test point,
                         * it's a valid "crossing".
                         */
                        if (x_intersection > px)
                        {
                                intersections++;
                        }
                }
        }
 
        return intersections;
}

References getPoint2d_cp(), lw_pt_on_segment(), LW_TRUE, lwerror(), POINTARRAY::npoints, p2d_same(), POINT2D::x, POINT2D::y, and pixval::y.

Referenced by lwcompound_contains_point().

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