libgeda/src/m_line.c

   1 /* gEDA - GPL Electronic Design Automation
   2  * libgeda - gEDA's library
   3  * Copyright (C) 1998-2010 Ales Hvezda
   4  * Copyright (C) 1998-2010 gEDA Contributors (see ChangeLog for details)
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  19  */
  20
  21 /*! \file m_line.c
  22  *
  23  *  \brief Low-level mathmatical functions for lines
  24  */
  25
  26 #include <config.h>
  27 #include <math.h>
  28 #include <stdio.h>
  29
  30 #include "libgeda_priv.h"
  31
  32 #ifdef HAVE_LIBDMALLOC
  33 #include <dmalloc.h>
  34 #endif
  35
  36
  37 /*! \brief Calculates the distance between the given point and the closest
  38  *  point on the given line segment.
  39  *
  40  *  If the closest point on the line resides beyond the line segment's
  41  *  end point, this function returns the distance from the given point to the
  42  *  closest end point.
  43  *
  44  *  If the line represents a single point (the endpoints are the same), this
  45  *  function calcualtes the distance to that point.
  46  *
  47  *  \param [in] line  The LINE object.
  48  *  \param [in] x     The x coordinate of the given point.
  49  *  \param [in] y     The y coordinate of the given point.
  50  *  \return The shortest distance from the object to the point. With an
  51  *  invalid parameter, this function returns G_MAXDOUBLE.
  52  */
  53 double m_line_shortest_distance (LINE *line, int x, int y)
  54 {
  55   double cx, cy;
  56   double dx, dy;
  57   double dx0, dy0;
  58   double lx0, ly0;
  59   double ldx, ldy;
  60   double t;
  61
  62   g_return_val_if_fail (line != NULL, G_MAXDOUBLE);
  63
  64   lx0 = (double)line->x[0];
  65   ly0 = (double)line->y[0];
  66   ldx = (double)(line->x[1] - line->x[0]);
  67   ldy = (double)(line->y[1] - line->y[0]);
  68
  69   if (ldx == 0 && ldy == 0) {
  70     /* if line is a point, just calculate distance to the point */
  71     dx = x - lx0;
  72     dy = y - ly0;
  73
  74   } else {
  75     /* calculate parametric value of perpendicular intersection */
  76     dx0 = ldx * (x - lx0);
  77     dy0 = ldy * (y - ly0);
  78
  79     t = (dx0 + dy0) / (ldx * ldx + ldy * ldy);
  80
  81     /* constrain the parametric value to a point on the line */
  82     t = max (t, 0);
  83     t = min (t, 1);
  84
  85     /* calculate closest point on the line */
  86     cx = t * ldx + lx0;
  87     cy = t * ldy + ly0;
  88
  89     /* calculate distance to closest point */
  90     dx = x - cx;
  91     dy = y - cy;
  92   }
  93
  94   return sqrt ((dx * dx) + (dy * dy));
  95 }