I want to do a diff using this curve. I can get it to work if I scale it by 1.001, but that would give me trouble if I ever wanted to fillet in the future since it would create non overlapping edges directly on top of each other. Is it possible to get this to work without having to resort to scaling by a small number?

Hi MRAKGR, try with this attached version. I simplified the object by making a single plane in spots where you had 4 coplanar faces.

- Michael

One other thing to look out for is the cutting line has some control points stacked up on top of each other. If you select it and use Edit > Show pts you'll see some red squares, those are on spots that have duplicated points.

I don't think that is what is getting in the way in this particular but it can cause problems.

- Michael

I see. I played around a bit and can confirm that deleting the coplanar faces and making the 4 solids planar before unioning them fixes the problem. I am surprised this turned out to be the problem. I thought that the edges overlapping with the cutter is what was the problem, the fact that scaling by 1.001 worked for me only served to cement that hypothesis. I thought I was making a trivial question here, but the answer was wholly unexpected for me.

Considering the edges were so far away from the cutter, why were they a problem?

I see that if I separate the solid, the cut works, but it is a bit wonky on the vertical parts and I am not sure how to join it back in that situation.

Also thanks for the note that the points in the cutting object were overlapping, it slipped my mind to think about that. I had no idea that show points would indicate the duplicates.

Hi MRAKGR,

re:
> Considering the edges were so far away from the cutter, why were they a problem?

It has to do with the surface structure - for the areas with 4 coplanar faces each planar face has a kind of trapezoidal control point structure. That means that even though it is a planar surface it is a generic spline surface that happens to have all its control points in a plane.

That's a little different case than planar surfaces made by Construct > Planar which are able to use a specialized plane analytic surface that has some special case handling to get tighter accuracy on various operations like intersections.

It usually helps to have planar areas use the analytic plane surface and to have just one large plane instead of multiple coplanar fragments. That's a general cleanup step that is one of the first things I do with cases that are having difficulty.

I'm not entirely sure which step it is helping in this particular case, it would probably take a few days of work doing a deep analysis to determine that.

It's also possible to go about things in a little different way to avoid the overlapping. Like you can do a boolean with just these lines selected:



That will cut up the faces but won't be able to automatically separate it into different pieces but you can do that yourself using Edit > Separate, then Join together the pieces and use Construct > Planar to fill in the remaining pieces.

If the simplified planes didn't work then that would have been another way to do it.

- Michael

Cutting with just the horizontal lines selected works fine. I've cut the object, separated it, isolated the left side and what I need to do here if figure out how to fill the hole. You said that I just have to planar it. I've tried that on the separated object, tried joining all the surfaces together and doing that, tried joining the individual sides so they form a half finished cube, and also tried selecting just the edges I want to planar, but it is not working for me. I haven't gotten it to fill a side even once. Is it really possible to fill the hole using Planar or would maybe using Lofting be a better choice here?

As a way of simplifying the problem, maybe I could understand if you could explain how to Planar this cube which had two of its adjacent faces deleted. Right now I only understand how to close the sides using Loft. For that to work I'd need to put a line where the missing edge should be though. I thought that could help Planar, but I can't join a line and a joined surface.

Actually, I think I see how it could be done. I can just separate the edges that I need from the original structure by copy pasting them, draw the missing lines, join them and then Planar them. I'd have to go manually like that for each face. Is that the process I should be taking? It more complicated than using Loft, so I am not sure if I am missing something.

Hi MRAKGR,

re:
> Cutting with just the horizontal lines selected works fine. I've cut the object, separated it, isolated
> the left side and what I need to do here if figure out how to fill the hole.

The boolean will also leave behind some vertical planes inside the object and you can make use of those to
help speed things along without needing to draw in anything.

So doing a boolean difference with just these lines as the cutting objects:



Select one face and then do a right-to-left crossing window to select all these faces and use Edit > Separate to break them off:



Then hiding that broken off piece will reveal this as the hole to fill, there are already these pieces in place left by the boolean:



Ctrl+C copy those pieces and then join them together with the main piece, that will leave holes like this:



Select the whole object (not edges) and then one use of Construct > Planar will fill in those holes. Get the other half
showing and paste in the vertical planes and repeat on the other side.

- Michael


EDIT: corrected crossing window direction, it's right-to-left for making a crossing window

Hi MRAKGR, for your cube with 2 faces deleted,

re:
> As a way of simplifying the problem, maybe I could understand if you could explain how to Planar this
> cube which had two of its adjacent faces deleted.

For that case you would need to draw in a line and make one planar face by selecting the line and 3 edges using Construct > Planar, then join that, then for the final plane you can select the entire object and use Construct > Planar to fill that last one in.

But for the actual object when you do the boolean it leaves behind half of the planes that you need so you can use those and you don't need to draw anything extra since when they are joined the remaining planar holes are isolated from each other and so can be filled in with one use of Construct > Planar with the main object selected.

- Michael

Yeah, I understand now. Thank you for taking the time to answer my questions.

Oops I had an error in one part that I wrote above, for the window selection that should be a right-to-left drag to make a "crossing" window (with dotted outline) that takes anything that crosses it instead of only fully contained.

- Michael