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One-sided offset with EndType::ButtLeft #905

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One-sided offset with EndType::ButtLeft #905

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LarsSkiba
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Implement a one-sided offset by extending the EndType with ButtLeft.

@LarsSkiba
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LarsSkiba commented Oct 18, 2024 •
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Hi @AngusJohnson,

  • This implementation is not fully finalized yet but i created a PR so you can comment on the code early during my development and maybe give helpful feedback, if you recognize undesired changes.

  • Please leave it open unless you plan not to accept such a contribution at all, which would also be helpful feedback ;)

  • This only implements a left-sided offset

  • My testcases work fine so far, i'd also contribute them to your tests when i'm done.

  • Does the API extension makes sense to you? It's not really an EndType but so is polygon and this extension seems like the least "invasive" API extension.

  • I'd also add a right-sided offset, one really trivial approach would be reverse the EndType from ButtRight to ButtLeft, call OffsetOpenPath and reverse the last path in the solution. Do you have an opinion on this approach?

  • I can also try extending other languages once i have a finalized, accepted C++ PR.

The additional logic in OffsetPoint solves this problem:
image

To my knowledge this is not solvable by a FillRule so i went with calculation the intersection point.

LarsSkiba
LarsSkiba commented Oct 18, 2024
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CPP/Clipper2Lib/src/clipper.offset.cpp
const bool intersect =
end_type_ == EndType::ButtLeft // one-sided offset
&& cos_a < 0 // < 90 degre
&& !path_out.empty() // safety check, can this never happen?
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I think path_out can never by empty at this point, but i'm not 100% sure.

LarsSkiba
LarsSkiba commented Oct 18, 2024
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CPP/Clipper2Lib/src/clipper.offset.cpp
end_type_ == EndType::ButtLeft // one-sided offset
&& cos_a < 0 // < 90 degre
&& !path_out.empty() // safety check, can this never happen?
&& l < path.size(); // safety check, can this never happen?
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@LarsSkiba LarsSkiba Oct 18, 2024 •
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Basicely j must never be the last or first point, depending on direction given by the relation between k and j.
It seems given to me, but again i'm not 100% sure.

LarsSkiba
LarsSkiba commented Oct 18, 2024
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CPP/Clipper2Lib/src/clipper.offset.cpp Outdated
@@ -398,16 +429,23 @@ void ClipperOffset::OffsetOpenPath(Group& group, const Path64& path)
case EndType::Round:
DoRound(path, 0, 0, PI);
break;
case EndType::ButtLeft:
left_end = Point64(GetPerpendicD(path[0], norms[0], -abs_group_delta));
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Maybe this can be done more elegant later.

@LarsSkiba LarsSkiba closed this Oct 21, 2024
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I discovered further major challenges, closing for now.

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@LarsSkiba @LarsSkibaModuleWorks