Hi Brian, on your first example, you've got a 5-sided polygon and the outside offset is 5 sided with rounded corners, and the inner one has 5 lines.

5 components in each thing, I'm not sure why you don't consider these to be related?

It looks like the differences in this case from what you are expecting are due to some of the options that you've changed - when you set Corners : Round you'll get rounded areas on corner extensions. If you set this to the default "sharp", then the larger shape would also be a straight-edged 5-sided polygon closer in resemblance to the original.

Also, you've unchecked the "Trim" option - that's why the inside one is made up of 5 overlapping lines. If you turn Trim on then the inside one would also be another 5-sided polygon as well.


On your second example, you've got something with tight bends in it, and your offset distance is a lot bigger than the curvature of the bend - this would create an offset that loops back around on itself with curly-cue type things, but with Trim turned on it will try and remove the loops. Sometimes MoI will get confused with this type of offset and you might not get the portion that you need - that's why there is a Trim option that you can turn off so that you can get the entire piece with curly-cues if you want which can then be cleaned up.


Try it with the default settings on your polygon (corners: sharp, and trim on), and with a much smaller distance with your one with the tight bends, and the results should be easier to understand I think.

- Michael

Hi Brian, here is another example to show you what happens in the case when you've got a tight bend in a curve and you ask for an offset distance greater than the curvature of the bend:



This tends to be easier to understand when you see it happening in small steps. Here are a sequence of 6 different offsets, each one slightly larger in size.

You can see that the first 3 everything is pretty simple, the distance involved is less than the curvature of the bendy part.

But after that you can see that portions of the offsets from either side of the bend start to overlap one another - this makes that kind of "curly cue" situation that I was describing previously (the results shown here were with the Trim option turned off). If there are a large number of these, it is not unusual for the offseter to get confused about which parts are the "main" parts and which parts are the loop parts to throw out, that's what happens when you get a small result curve from a complex offset. But you can turn off Trim to get back some raw results that you can then edit by manual trimming to get the final result.

You might experiment more with offset initially using the "Through pt" mode instead of a specific distance, the through point mode makes it easier to see how the offset shape changes as you move gradually away from the curve in small steps.

- Michael

Michael,

If you ever decide to give up on coding, I suggest you get a job teaching!

Thanks for all the great examples and explanations. They clarified a lot for me.

that was a great explanation thank michael , simple enough that even I could understand it!

I have spent more time trying to get to grips with "Offset".

I have come to the idea that both for solids and lines this is an unnecessary and maybe confusing "extra"

"Cut and paste" and "scale" seem to be more "reliable" alternatives?

(Or am I missing something?)

Hi Brian, just plain scaling does not do the same job except for special cases like a circle.

Here's an example (.3dm also attached):



There isn't any way just through scaling to create a parallel curve a constant distance away from that one, give it a try.

Here's another example:



Here you can see how the lines of the offset are of different proportions with one another. Scaling will not produce this result.

- Michael

Thanks Michael. It just seems that there is less "logicallity" in offset.

Well--maybe different illogicallity in both options??

How are made the offset?
Seems easy in 2D but in 3D?
Extremities control the result for have a sort of perpandicular form between them?

Hi Pilou, the offset is created by a kind of "tracing" procedure that marches along the curve and displaces a point sampled from the curve by the given distance.

You can do it on a 3D curve, but the displacement direction will happen relative to a plane. It tends to be more predictable when you do it just on planar curves.

- Michael

Hi Brian, I don't really have any ideas on how to make it more logical...

I mean right now the basic procedure goes like this:

Step 1 - select a curve:



Step 2 - click on Construct / Offset to launch the offset command.

Step 3 - click a little distance away from the curve to create a parallel one going through the point where your mouse is at:



That's all that you do to create a basic parallel curve... It can get more complex if you want to tweak various options or work with more complex curves, but I can't really think how to get more logical steps than that for the simple basic case...

Is there anything that you can pinpoint here that doesn't seem logical?

- Michael