MinkowskiSum() unexpected result

[3dodev]

When using C# and editing sample project MinkowskiSum() uses a path that is an open curve where points become colinear the resulting curve is not the expected swept result and instead segments are missing. Please advise. See code snippet showing MinkowskiSum expected result, pattern curve, and actual result with missing segments.

   public static void DoSweepAddition()
   {
     string filename = "../../../result.svg";
     SvgWriter svg = new();
     Paths64 p = new();

     //expected result
     Paths64 p0 = new() { Clipper.MakePath(new int[] { 2, 0, 2, 10, -28, 10, -28, 25, -50, 25, -50, 10, -68, 10, -68, 20, -100, 20, -100, 10, -100, 0 }) };
     p.AddRange(p0);
     p = Clipper.TranslatePaths(p, 0, 60);

     //pattern to be swept along a line
     Paths64 p1 = new() { Clipper.MakePath(new int[] { 0, 0, 0, 10, -30, 10, -30, 25, -50, 25, -50, 10, -70, 10, -70, 20, -100, 20, -100, 10, -100, 0 }) };
     p.AddRange(p1);
     p = Clipper.TranslatePaths(p, 0, -30);

     //pattern swept along line 
     Path64 pattern = Clipper.MakePath(new int[] { 0, 0, 0, 10, -30, 10, -30, 25, -50, 25, -50, 10, -70, 10, -70, 20, -100, 20, -100, 10, -100, 0 });
     Path64 path = Clipper.MakePath(new int[] { 0, 0, 2, 0 });
     Paths64 solution = Clipper.MinkowskiSum(pattern, path, true);
     p.AddRange(solution);

     //save SVG
     SvgUtils.AddSolution(svg, p, false);
     SvgUtils.SaveToFile(svg, filename, FillRule.EvenOdd, 640, 480, 10);
     ClipperFileIO.OpenFileWithDefaultApp(filename);
   }

[AngusJohnson]

Your path path (effectively the brush shape here) has no height, so any horizontal 'drawing' along pattern will have no output. So, given that the solution will only return the result polygons (ie not lines), I believe that the solution is correct 😁.

[3dodev]

Minkowski addition as I have read is always about the relation of sweeping one polygon along another polygon. I can understand how in the context of keeping the "inside polygon" this result is understandable, but I can't imagine much practical use. I'm after the points and something in Clipper is throwing away the lines and only keeping the polygons. My desire would be similar to the net result of Union, but because it should be swept Union will not always have the same result when the line is long or the polygon pattern is complex. How can I keep the outer boundary? For example where the black polygon is swept along the red line.

Additionally when I add height Clipper gives an even more "incorrect" result as there are now angles on the polygon that shouldn't exist. It seems like Clipper needs further work when the path or pattern is open curve rather than polygon. Is there any trivial solution here?

    public static void DoSweepAddition()
    {
      string filename = "../../../result.svg";
      SvgWriter svg = new();
      Paths64 p = new();

      //pattern to be swept along a line
      Paths64 p1 = new() { Clipper.MakePath(new int[] { 0, 0, 0, 10, -30, 10, -30, 25, -50, 25, -50, 10, -70, 10, -70, 20, -100, 20, -100, 10, -100, 0 }) };
      p.AddRange(p1);
      p = Clipper.TranslatePaths(p, 0, -50);

      //pattern swept along L-shape curve 
      Path64 pattern = Clipper.MakePath(new int[] { 0, 0, 0, 10, -30, 10, -30, 25, -50, 25, -50, 10, -70, 10, -70, 20, -100, 20, -100, 10, -100, 0 });
      Path64 path = Clipper.MakePath(new int[] { 0, 0, 2, 0 });
      Paths64 solution = Clipper.MinkowskiSum(pattern, path, true);
      p.AddRange(solution);

      //save SVG
      SvgUtils.AddSolution(svg, p, false);
      SvgUtils.SaveToFile(svg, filename, FillRule.EvenOdd, 640, 480, 10);
      ClipperFileIO.OpenFileWithDefaultApp(filename);
    }

[AngusJohnson]

  Paths64 path = {MakePath({0, 0, 0, 100, -300, 100, -300, 250, -500, 250, -500, 100, 
  -700, 100, -700, 200, -1000, 200, -1000, 100, -1000, 0}) };
  Paths64 pattern = {MakePath({ 0,0, 20,0, 20,1, 0,1 })};

  Paths64 solution = MinkowskiSum(pattern[0], path[0], true);

  SvgWriter svg;
  SvgAddOpenSubject(svg, path, FillRule::NonZero);
  SvgAddSolution(svg, solution, FillRule::NonZero, false);
  std::string filename = "mink_sum.svg";
  SvgSaveToFile(svg, filename, 400, 400, 10);
  System(filename);

[3dodev]

The SVG looks better only because of the fill rule. Can you advise on the best approach to make this one polygon? Is it reasonable to loop the Paths64 MinkowskiSum() result values to create a new Path64 and then apply TrimCollinear()? Or can I use Clipper to Union the results from MinkowskiSum? You have the result in step 2 of my image above and I need to get to the final result of a single polygon. Please advise.