Extrapolate Arc? Or am I approaching this the wrong way?

[Hazz]

Hi all, 

 

I've the regular googling and forums searching, and its possible I'm using the wrong words or I'm going about this the wrong way cause I can't find much. 

 

I've drawn an ARC using the point on arc mode of the polyline tool that Ill compose and extrude, I want the arc to touch each side of the rectangle and then go through a specific point. 

The point on arc tool makes this easy. 

But I need to extend the arc along the arc path, I guess I want to extrapolate the continuation of the line to an unknown point in space. 

Is there a way to do this or am I approaching this wrong?

 

I thought I had seen this in a video on the university but can't seem to find that either. 

 

 

Thanks in advance,

Hazz

[line-weight]

You need to select it with the pointer tool so it has blue squares on the ends, then you should be able to grab one and use it to extend.

 

[Hazz]

Hmm yea, I tried this but I need the arc to stay attached to the corner of the cube. 

 

Thats why I think extrapolate is the best term what I want to do. I just want to continue the curve as is. 

 

Excuse the upside down working plane haha, as you can see, using the reshape tool means the arc does not pass through that point any more. 

[line-weight]

Sorry, I didn't read your first post carefully enough and thought you were using the "arc" tool but I see you are using the polyline tool.

 

I don't think you can extrapolate a polyline in the way you describe but I might be wrong.

 

I would perhaps draw your desired line with the "arc" tool, extrapolate that to where you want, then trace over it with the polyline tool in arc mode.

[Tom W.]

2 hours ago, Hazz said:

using the reshape tool means the arc does not pass through that point any more. 

 

I get this as well + using the Arc Tool. I didn't realise it was occurring until pointed out by @Hazz. You have to zoom in quite a lot to notice it as it's only off by 0.05mm...

[line-weight]

Oh yeah, I see that too.

 

This might just be the way VW renders the curve though (in segments), and mathematically it is still passing through the point. If I use the split tool to split the extended arc where it crosses the corner of the rectangle, the new "ends" appear to be properly on the corner point.

[line-weight]

There's a thread somewhere with some fairly detailed discussion of how this behaviour of VW causes problems, mainly with regard to polylines/NURBs.

(Hadn't necessarily realised it applies to arcs too)

 

Essentially, you can't really rely on VW to draw a curved line through any exact and consistent points unless there is a node there.

 

This causes problems with various things like splitting curved polylines and trying to keep them exactly aligned with an un-split version.

 

When absolute accuracy is required, basically you have to draw lines that are made up of straight segments.

 

I believe other software deals with this much better but I'm no expert.

Edited October 10, 2022 by line-weight

[Kevin McAllister]

7 hours ago, line-weight said:

This might just be the way VW renders the curve though (in segments), and mathematically it is still passing through the point. If I use the split tool to split the extended arc where it crosses the corner of the rectangle, the new "ends" appear to be properly on the corner point.

 

Its definitely related to that. You can see the curve facets in this screen shot. In my version, I started with an arc, converted it to a NURBS curve and then used the Extend NURBS command so I didn't physically touch the points.

 

 

The highlighting behaves weirdly too when significantly zoomed in. It flip flops around the point. I don't think all rendering has been switched over to the VGM yet, which may explain that issue. It happens with both the original arc and the NURBS curve.

 

Kevin

 

 

[line-weight]

yes and it behaves differently again if you go out of top/plan view and look at it in 3d.