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

[Markus Neteler]

--huq684BweRXVnRxX
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

Hi,

a fwd from Michael...

Greetings,

 Markus

--huq684BweRXVnRxX
Content-Type: message/rfc822
Content-Disposition: inline

Return-Path: <mshapiro@ncsa.uiuc.edu>
Received: from camelot.itc.it (camelot [195.223.171.5])
	by artemide.itc.it (8.11.3/8.11.3) with ESMTP id g1LImoF25781
	for <neteler@itc.it>; Thu, 21 Feb 2002 19:48:50 +0100 (MET)
Received: from ex1.ncsa.uiuc.edu (ex1.ncsa.uiuc.edu [141.142.2.9])
	by camelot.itc.it (8.11.3/8.11.3) with ESMTP id g1LImnA28333
	for <neteler@itc.it>; Thu, 21 Feb 2002 19:48:49 +0100 (MET)
Received: from mx1.ncsa.uiuc.edu (mx1.ncsa.uiuc.edu [141.142.2.8])
	by ex1.ncsa.uiuc.edu (8.11.6/8.11.6) with ESMTP id g1LImr406655
	for <neteler@itc.it>; Thu, 21 Feb 2002 12:48:54 -0600 (CST)
Received: from pecos.ncsa.uiuc.edu (pecos.ncsa.uiuc.edu [141.142.4.6])
	by mx1.ncsa.uiuc.edu (8.11.6/8.11.6) with ESMTP id g1LImlv00698
	for <neteler@itc.it>; Thu, 21 Feb 2002 12:48:47 -0600 (CST)
From: Michael Shapiro <mshapiro@ncsa.uiuc.edu>
Received: (from mshapiro@localhost)
	by pecos.ncsa.uiuc.edu (8.11.6/8.8.8) id g1LImkm24885
	for neteler@itc.it; Thu, 21 Feb 2002 12:48:46 -0600 (CST)
Message-Id: <200202211848.g1LImkm24885@pecos.ncsa.uiuc.edu>
Subject: Re: [cworth@east.isi.edu: Re: [GRASS5] [mshapiro@ncsa.uiuc.edu: bug in libes/gis/line_dist.c]]
To: neteler@itc.it (Markus Neteler)
Date: Thu, 21 Feb 2002 12:48:45 -0600 (CST)
In-Reply-To: <20020221180030.V18437@itc.it> from "Markus Neteler" at Feb 21, 2002 06:00:30 PM
X-Mailer: ELM [version 2.5 PL2]
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

I agree with Radim.  So now there are 3 votes for Radim, 0 for me!
Would you mind posting my agreement with Radim to the list for me?

> 
> Hi Michel,
> 
> one more comment attached. Just tell me if you
> don't want further comments forwarded.
> 
> Markus
> 
> 
> 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@east.isi.edu

--huq684BweRXVnRxX--