Struggles using delta callback for variable offset

[lvjonok]

Hello! I am trying to run simple example with offset and I do not know how to change parameters or callback so expectations will be met.

I want to offset horizontal edges of square by 10.0 and vertical by 20.0. Code that I ended up with:

void test5() {
  // create a square and offset horizontal edges by 10 and vertical edges by 20
  Paths64 subject = {Rect64(0, 0, 100, 100).AsPath()};
  ClipperOffset co;
  co.AddPaths(subject, JoinType::Miter, EndType::Polygon);

  auto cb = DeltaCallback64([](const Path64 &path,
                               const PathD &path_norms, size_t curr_idx, size_t prev_idx) {
      // sin between path normal and y-axis
      auto sin_edge = sin(atan2(path_norms[curr_idx].y, path_norms[curr_idx].x));

      return 10.0 + std::abs(sin_edge * 10.0);
  });

  Paths64 solution;
  co.Execute(cb, solution);

  SvgWriter svg;
  SvgAddSubject(svg, subject, FillRule::NonZero);
  SvgAddSolution(svg, solution, FillRule::NonZero, false);
  SvgSaveToFile(svg, "tmp5.svg", 400, 400);
  System("tmp5.svg");
}

However in result I see this:

I went through the code and it seems I misuse callback rather than error in offset result.

My question is how callback or code itself can be changed to get as result something similar:

[AngusJohnson]

I want to offset horizontal edges of square by 10.0 and vertical by 20.0

It can't be done. You can only change the size of the offset, not its angle.
So given your rectangular path, the vertex at (0,0) could be offset by either 10 or 20, but not both 10 and 20, and the offset will be at -135 degrees (when mitered).

[lvjonok]

So there is no workaround except trying to "smooth" the path so it will not have sharp edges?
I also tried different JoinType and EndType, but neither could improve results.

[AngusJohnson]

So there is no workaround

No.

[reunanen]

I want to offset horizontal edges of square by 10.0 and vertical by 20.0.

Would something like this work in your use case?

1. Divide all x coordinates by 2.
2. Offset by 10.
3. Multiply all x coordinates by 2.

In steps 1 and 3, leave the y coordinates untouched.

In step 2, just a regular (uniform) offset should do – no need for callbacks or anything, I believe.

[lvjonok]

In this case it could work, but this is simplification of more complex polygons that I try to perform

[AngusJohnson]

  PathsD pp = { Ellipse(RectD(10,10,90,90), 49)};

  //PathsD pp = { MakePathD({50,10, 90,50, 50,90, 10,50}) };

  //PathsD p = { MakePathD({20, 80, 20,20, 80,20, 80,80}) };
  //PathsD pp = InflatePaths(p, 20, JoinType::Round, EndType::Round);

  //PathsD pp = { MakePathD({10,10, 90,10, 90,90, 10,90}) };
  
  int err;
  double scale = 100, delta = 10;
  Paths64 subject = ScalePaths<int64_t, double>(pp, scale, scale, err);
  Paths64 solution;

  ClipperOffset co;
  co.AddPaths(subject, JoinType::Miter, EndType::Polygon);
  co.Execute(
    [scale, delta](const Path64& path,
      const PathD& path_norms, size_t curr_idx, size_t prev_idx) {
      // NormalizeVector() see clipper.offset.cpp 
      PointD vec = NormalizeVector(
        path_norms[curr_idx].x + path_norms[prev_idx].x,
        path_norms[curr_idx].y + path_norms[prev_idx].y);
        return std::pow(delta * scale, std::sqrt(std::fabs(vec.x))) -1; }
    , solution);

  SvgWriter svg;
  SvgAddSubject(svg, subject, FillRule::NonZero);
  SvgAddSolution(svg, solution, FillRule::NonZero, false);
  SvgSaveToFile(svg, "tmp.svg", 400,400, 50);
  System("tmp.svg");