Re: [Boost-users] [boost.geometry] buffer distance strategies

Subject: Re: [Boost-users] [boost.geometry] buffer distance strategies
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-11-08 05:58:04

Gu Grzegorz,

gchlebus wrote On 8-11-2014 2:33:
>
> 2014-11-07 19:30 GMT+01:00 Barend Gehrels [via Boost] <[hidden email] 
> </user/SendEmail.jtp?type=node&node=4668825&i=0>>:
>
>     Hi Grzegorz,
>
>     gchlebus wrote On 6-11-2014 19:08:
>>
>>
>>     2014-11-04 17:23 GMT+01:00 Barend Gehrels [via Boost] <[hidden
>>     email] <http://user/SendEmail.jtp?type=node&node=4668774&i=0>>:
>>
>>         Hi Grzegorz,
>>
>>
>>>
>>>         2014-10-29 23:18 GMT+01:00 Barend Gehrels [via Boost]
>>>         <[hidden email]
>>>         <http://user/SendEmail.jtp?type=node&node=4668617&i=0>>:
>>>
>>>
>>>
>>>             gchlebus wrote On 24-10-2014 16:44:
>>>
>>>             > Hi,
>>>             >
>>>             > I am wondering whether it would be possible to achieve
>>>             anisotropic buffering
>>>             > (distances in neg x, pos x, neg y, pos y can have
>>>             different values) of a
>>>             > polygon using the buffer function with
>>>             custom-implemented distance strategy.
>>>             > What I want to achieve is presented on the figure 2-b
>>>             in the following
>>>             > paper:
>>>             >
>>>             http://itcnt05.itc.nl/agile_old/Conference/mallorca2002/proceedings/posters/p_molina.pdf
>>>             >
>>>             > I would be grateful to hear from you whether it is doable, and if
>>>             positive,
>>>             > how one could implement such a custom distance strategy.
>>>             The current distance strategy has (currently) no means
>>>             to get the angle,
>>>             or a vector of the new point to be buffered. We can
>>>             consider adding that.
>>>
>>>             However, by writing custom strategies for join, side,
>>>             point (for
>>>             point-buffers) and possibly end (for line-buffers) you
>>>             should be able to
>>>             create this, because these have this information.
>>>
>>>             Attached a program doing similar things with polygons
>>>             and points (I vary
>>>             the distance based on angle - you will have to do
>>>             something with your
>>>             anistropic model).
>>>
>>>             The output is also attached.
>>>
>>>             The program defines three custom strategies, all based
>>>             on the same
>>>             mechanism, to create interesting output.
>>>             I did not do the end-strategy but that would look
>>>             similar, you can look
>>>             at the provided end-strategy (round) and apply the same
>>>             function.
>>>
>>
>>
>>         gchlebus wrote On 31-10-2014 18:13:
>>>         I really appreciate your example code, it helped me a lot.
>>>         Attached you can find my source code.
>>>         In my implementation of the anisotropic buffering I didn't
>>>         know how to make use of the distance strategy, as it was
>>>         possible to make it work using only side and join strategies.
>>>         I encountered strange behavior when changing number of
>>>         points describing a full circle. Using 360 points produced a
>>>         good output, whereas 90 points caused only the second
>>>         polygon to be buffered (see attached figures). I would be
>>>         thankful if you could help me to resolve this issue as well
>>>         as for any remarks to my code.
>>>
>>
>>         I could reproduce this. Basically the join-strategy should
>>         always include points perp1 and perp2 (these are the two
>>         points perpendicular to the two sides which the join-strategy
>>         joints). Either they are re-calculated, or they can be just
>>         added to begin and end. So I did the last option, and that
>>         piece of code now looks like:
>>
>>               double const angle_increment = 2.0 * M_PI /
>>         double(point_count);
>>               double alpha = angle1 - angle_increment;
>>         *      range_out.push_back(perp1);**// added
>>         *      for (int i = 0; alpha >= angle2 && i < point_count;
>>         i++, alpha -= angle_increment)
>>               {
>>                 pdd v = getPointOnEllipse(alpha);
>>                 Point p;
>>                 bg::set<0>(p, bg::get<0>(vertex) + v.first);
>>                 bg::set<1>(p, bg::get<1>(vertex) + v.second);
>>                 range_out.push_back(p);
>>               }
>>         *      range_out.push_back(perp2);**// added*
>>
>>         My sample code of course also suffered from that, so I added
>>         it there too if I use it in the future.
>>
>>         I tested your algorithm with various points and distances and
>>         it now seems always OK.
>>
>>         You ask for remarks on your code: it looks good ;-) one
>>         thing, many terms are recalculated such as
>>         pow(xPos*tan(alpha), 2)); or just tan(alpha), I usually store
>>         these into variables, to avoid expensive recalculations of
>>         the same terms, though maybe they are optimized by the compiler.
>>
>>         Regards, Barend
>>
>>
>>         P.S. this list discourages top-postings
>>
>>
>>     Hallo Barend,
>>
>>     I corrected the join strategy, but still the buffering doesn't
>>     work in all cases as expected. When using xPos = 1, and other
>>     values equal 0, the buffered polygon contains a hole (see
>>     xPos1.svg), whereas setting xPos to 2 produces a correct result
>>     (xPos2.svg). Do you know how to fix it? I attached also main.cpp,
>>     as I changed the code a bit and it contains the polygon for which
>>     causes the strange behavior.
>>
>
>
>     That is most probably caused by an error in some of your calculations:
>
>     The line y = sqrt(yPos2 * (1 - pow(x, 2) / xNeg2));
>     causes a NAN for this input:
>
>     alpha about PI
>     then xNeg2 = 0.010000000000000002
>     and x = -0.10000000000000002
>     and yPos2 = 0.010000000000000002
>
>     This adds a weird line containing NAN to the join, causing the
>     buffer process fail.
>     I got this using these parameters:
>     double xPos = 1.0, xNeg = 0.1, yPos = 0.1, yNeg = 0.1;
>
>     and not the parameters you have (that was fine for me).
>
>     I think you should make the calculations full-proof first...
>
>     For example add a line in the join-strategy:
>             std::cout << i << " "<< angle1 << " " << angle2 << " " <<
>     v.first << " " << v.second << std::endl;
>
>
>     Regards, Barend
>
>
> Thanks, I'll try to improve my calculations.
> By the way, I was playing with different strategies combinations and I 
> found out that when using only boost buffer strategies:
>         double points_per_circle = 36;
>         double distance = 130;
> bg::strategy::buffer::distance_symmetric<double> 
> distance_strategy(distance);
> bg::strategy::buffer::end_flat end_strategy;
> bg::strategy::buffer::point_circle point_strat(points_per_circle);
> bg::strategy::buffer::side_straight sideStrat;
> bg::strategy::buffer::join_round joinStrat(points_per_circle);
>
> the buffer function can still fail (produce no output) when the 
> distance is higher than 128 (e.g, 128, 130, 150, 300, 400). But this 
> happens up to a certain value, where the buffer function starts 
> producing a correct output (e.g., distances 900, 1000).
>

Hmm, I see (starting at different values, but I can reproduce).

I created a ticket, will be looked at. Thanks for reporting.
https://svn.boost.org/trac/boost/ticket/10770

Barend