[GRASS5] [mshapiro@ncsa.uiuc.edu: bug in libes/gis/line_dist.c]

[Carl Worth]

On Feb 21, Radim Blazek wrote:
 > On Thursday 21 February 2002 13:45, Markus Neteler wrote:
 > > Hi developers,
 > > I was looking through the grass src code and found a bug in the
 > > G_distance2_point_to_line() function in libes/gis/line_dist.c
 > >
 > > lines 47,48 should be changed from
 > >
 > > 	dx = dx * t + x1;
 > > 	dy = dy * t + y1;
 > > to
 > 
 > I think that (dx * t + x1, dy * t + y1) is point x,y projected to line 
 > x1,y1->x2,y2 (comment in code:  /* go t from x1,y1 towards x2,y2 */)
 > and we want distance between this point and and x,y. That is:
 > dx = dx * t + x1 - x;
 > dy = dy * t + y1 - y;
 > 
 > You should have someone double check me on this, though, just to be sure :)

I've double-checked this. Radim's fix above is correct.

You might even write it in the following (equivalent) way to make a
more clear parallel with the 3 cases above it in the code:

dx = x - (dx * t + x1);
dy = y - (dy * t + y1);

(the change of sign is not significant to the final result since both
dx and dy are squared at the end).

Cheers,

-Carl

-- 
Carl Worth                                        
USC Information Sciences Institute                 cworth at east.isi.edu
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