topic intersectWithCurve not returning the right count (it's missing intersections) in Fusion API and Scripts Forum

intersectWithCurve not returning the right count (it's missing intersections)

corbin2 — Tue, 13 Jan 2026 20:19:48 GMT

Hi!

I'm having trouble using Line3D.intersectWithCurve(). It seems to miss intersections at certain points.

1. Consider this basic sketch; it has one curve (a SketchControlPointSpline -- created in Illustrator) and point Point3D, near the top left of the curve (shown blue, selected).

2. Some basic code creates a Line3D starting at the point and going to the right; it should intersect this curve two times, but the result is only one. Code (full example attached):

design: Design = app.activeProduct sketch = design.rootComponent.sketches[0] point = sketch.sketchPoints[3].geometry # hardcoded to the right one print(point.x, point.y) endPoint = point.copy() endPoint.x += 100000 line = Line3D.create(point, endPoint) curve = sketch.sketchCurves[0].geometry intersects = line.intersectWithCurve(curve) hitCount = intersects.count # ui.messageBox(f'hitCount should be 1 but is: {hitCount}') print(hitCount)

I attached the .f3d file and the basic script to reproduce it. It is hardcoded and extracted from my more complex plugin.

Moving the point around seems to give the right result when it isn't at the apex/top of the curve. If yo move it in the middle of the apex (convex side), it will show 0 hits.

I'm guessing this is some rounding error or a bug...I'm open to suggestions on how to solve line intersections with arbitrary curves!

Corbin

Re: intersectWithCurve not returning the right count (it's missing intersections)

BrianEkins — Wed, 14 Jan 2026 04:25:18 GMT

I can reproduce this, and it appears to be a bug in the intersectWithCurve method. A bug has been logged. Out of curiosity, where did the curve come from? I'm guessing it was imported somehow.

Re: intersectWithCurve not returning the right count (it's missing intersections)

corbin2 — Wed, 14 Jan 2026 13:55:18 GMT

Thanks Brian! I mentioned it briefly, but the curve was created in Adobe Illustrator.

I'm working on a work around by implementing my own intersectWithCurve, but I'm hitting the same issue. I haven't debugged it much yet.

Corbin

Re: intersectWithCurve not returning the right count (it's missing intersections)

MichaelT_123 — Sat, 17 Jan 2026 11:00:24 GMT

Hi Mr Corbin2,

Consider checking:

(returnValue, intersectingCurves, intersectionPoints) = sketchControlPointSpline_var.intersections(sketchCurves)

returnValue = plane_var.intersectWithCurve(curve)

and also:

line = Line2D.create(point, endPoint)

Regards

MichaelT

Re: intersectWithCurve not returning the right count (it's missing intersections)

corbin2 — Sat, 17 Jan 2026 14:52:21 GMT

@MichaelT_123 wrote:

Hi Mr Corbin2,

Consider checking:

(returnValue, intersectingCurves, intersectionPoints) = sketchControlPointSpline_var.intersections(sketchCurves)

returnValue = plane_var.intersectWithCurve(curve)

and also:..

Hi MichaelT,

I should have mentioned that I also did try SketchCurve.intersections(..) and it fails too. I'm guessing that the algorithm subdivides the curve and compares distances, but not with enough resolution to realize the small corner change would cause a different hit. I also ran into this with my own algorithm, and increased the resolution to get the correct answer. However, mine has other flaws that I haven't worked out yet and need to debug. I have to shelve this for a while to work on other stuff.

Line2D doesn't have an intersectXXX method.

Corbin

Re: intersectWithCurve not returning the right count (it's missing intersections)

MichaelT_123 — Sun, 18 Jan 2026 02:59:03 GMT

As you seem to be capable... of writing your own numerical algorithm, try:

Find the closest point to the curve and use it as the starting approximation in fsolve()
With a failed "solve", run the process on a couple of approximation points lying on the curve
change fsolve() algorithm and/or change iteration parameters.

Regards

MichaelT

Re: intersectWithCurve not returning the right count (it's missing intersections)

BrianEkins — Sat, 17 Jan 2026 23:51:52 GMT

Another option is to call the evaluator property of the curve to obtain a CurveEvaluator3D, then use the getStrokes method to obtain a polyline approximation of the curve. Then the intersection becomes a line-line intersection.

Re: intersectWithCurve not returning the right count (it's missing intersections)

corbin2 — Sun, 18 Jan 2026 00:16:34 GMT

Thanks Brian - that sounds like a better idea than what I was doing! I'll experiment with this idea when I get back into it.

We're happy to report this issue has been resolved. Updat...

Closed-Loop-Bot — Fri, 04 Sep 2026 01:15:12 GMT

We're happy to report this issue has been resolved. Update Fusion to the most recent version to apply the fix. If this update resolved your issue, feel free to click the "Accept Solution" button to help other community members find the answer more easily. If the issue persists after updating, don't hesitate to reach out to us by replying to this thread. We're here to help! Let us know if you need any further assistance!

Re: intersectWithCurve not returning the right count (it's missing intersections)

corbin2 — Tue, 13 Oct 2026 20:22:20 GMT

Awesome, this looks to be fixed, thank you!