Hi Grzegorz,

2014-10-29 23:18 GMT+01:00 Barend Gehrels [via Boost] <[hidden email]>:

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(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

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