Single-Sided Offset of Open Paths

[mariuszhermansdorfer]

Hey everyone,

is there a way to offset open paths only in one, user-defined direction? This is the default behavior:

And the desired result:

Ideally, we'd keep various EndType types to switch between Butt/Square/Round/None options.

[AngusJohnson]

is there a way to offset open paths only in one, user-defined direction?

It wouldn't be very hard to modify the existing code to do that.
But you'll have to do that yourself as it's not something that I'm considering at the moment.

[mariuszhermansdorfer]

Thanks for the heads-up @AngusJohnson! To wrap my head around the logic, I started with commenting out this loop:

Clipper2/CPP/Clipper2Lib/src/clipper.offset.cpp

Lines 450 to 451 in 81b01d2

	for (size_t j = highI, k = 0; j > 0; k = j, --j)
	OffsetPoint(group, path, j, k);

It does work for higher delta values, but breaks with lower ones. (Please ignore the automatic closing of the resulting path for now). Any ideas why it's happening?

2023-07-14.11-50-33.mp4

[EDIT] The union operation at the end seems to be the offender here. Commenting it out yields consistent results.

Clipper2/CPP/Clipper2Lib/src/clipper.offset.cpp

Lines 617 to 620 in 81b01d2

	if (groups_[0].is_reversed)
	c.Execute(ClipType::Union, FillRule::Negative, paths);
	else
	c.Execute(ClipType::Union, FillRule::Positive, paths);

[mariuszhermansdorfer]

It seems that this part is responsible for cleaning up after the offset operation:

Clipper2/CPP/Clipper2Lib/src/clipper.offset.cpp

Lines 617 to 620 in 81b01d2

	if (groups_[0].is_reversed)
	c.Execute(ClipType::Union, FillRule::Negative, paths);
	else
	c.Execute(ClipType::Union, FillRule::Positive, paths);

Skipping this step results in corrupt geometry for some edge cases:

While working for others;

Any ideas how to address this issue?

[mariuszhermansdorfer]

OK, I ended up doing a bit of a loop:

1. Offset the path to one side
2. Add the original path to the solution to have a closed path
3. Clipper.Union everything to clean up intersections
4. Remove the original path from the result

It's a bit cumbersome but works for now:

2023-07-14.13-55-29.mp4