I searched the forum and found that NURBS curves are actually better than Bezier curves.
http://moi3d.com/forum/index.php?webtag=MOI&msg=5256.11
I still wish MoI would allow to draw Bezier curves like in Illustrator because I can't get my head around using the Freeform Curve with Control Points effectively.

Here's a screenshot from Illustrator followed by screenshots of my attempt to recreate the curve in MoI


First I placed all the anchor points


Then I added all the handle points


Then I adjusted the handle points


It works but Michael says in the above linked Forum post that its better to start with the Freeform Curve rather than converting straight lines like I did. But I tried and the curve just doesn't behave :)

I understand that the first two and the last two points of the Freeform Curve are like Bezier handles but what is the correct strategy for placing the control points in between? Obviously the control points have to be set like above but doing that on the fly is not intuitive to me. In Illustrator I pretty much know where to set the anchor points. By clicking and dragging in the desired direction the handles emerge automatically.

An animation showing how to correctly trace the attached shape in MoI would be most helpful.

Just for a lough. Here's me trying to master the freeform curve. I feel like a two years old.

Short line segments can be placed at anchor points, or other points, and Blended between.
The Blend has adjustments.
After the Blend, the line segments can be rotated, which alters the Blend also.

2 point clothoids can also sometimes be used, but Blend is excellent.

- Brian

These are construction lines I would use here:

Hi Martin, basically you're making things more difficult by trying to use a very sparse number of points. Now while it's a generally good thing to have a light control point count using a somewhat larger number of evenly spaced points tends to make the specific job of tracing things quite a lot easier. You will still get a light enough point count that it won't be problematic and it is much easier to make the curve very close to your desired shape right from the start. Then go in and do some minimal point adjustment afterwards.

Also while drawing the curve have "Straight snap" on for only first first and last points, either turn it off in the bottom toolbar for the other points or hold down Alt to temporarily suppress it while drawing the in between points. Or leave it off entirely and true up the last 2 points on each side later on with "flat" snap by selecting them and squishing the corner of the edit frame down.

Anyway to draw the curve you follow along your shape placing down points at regular intervals with the points just slightly outside of the shape - the resulting curve will be kind of shrunken down and smoothed from the actual control points so you don't want the points themselves to be directly on the shape except at inflection areas, the points should go slightly outside the shape.

Here's how it looks - notice here how the curve that is created initially is very close to the desired result, rather than trying to start out with some totally different type of shape first:



It tends to be good to this kind of more regular spaced point placement for tracing over something - if you were more freely drawing a shape rather than trying to trace over one you can lighten the point density somewhat. But by having a somewhat higher density than what you were originally trying to do, it helps to limit the influence of an individual control point to a more local area and that tends to make it easier to guide the shape. With a very sparse control point density each point is having a much wider influence over the curve and the points will have to be more expertly placed in highly strategic areas rather than just being able to follow the shape.

The Bezier curve type process is somewhat problematic for CAD because there is a type of implicit segmentation to it - at every node where you have asymmetrical handles coming off the node, the actual shape has a break in curvature at that spot, basically the mathematical smoothness of the curve is kind of interrupted at those areas which is not the case with a NURBS control point curve.

- Michael

Nurbs power! :)

Pardon me for a possibly dumb question, but if the goal is to trace a shape, why not just use the "Through Points" curve and then adjust the control points as needed afterwards to make sure it is smooth?

Thankyou ttype, your curve looks good. Do I understand it correctly that you draw the blue construction lines first onto which you later place 3 control points using the freeform curve.

Thank you Michael, your explanation was what I hoped for. You're very skilled in placing the points. I tried it like you showed but my curves still have kinks in them and are far from smooth. I guess there's a lot of practise required.

Hi Dan,

that's a valid question. I don't use the through point method because the help file mentions this:

"Through points can easily introduce wiggles in the curve. Using control points is generally preferred since it will create a more relaxed curve."

<< I guess there's a lot of practise required.

Take away point when curve is smooth or flat
Approach point when curve is harping turn :)

Hi Martin, yes, I always draw lines first. As you see, without further refinement just two points are not on a construction line.

And when a nurbs curve is beautiful the polyline through the control points looks good, too!

Hi Dan,

> why not just use the "Through Points" curve and then adjust the control points
> as needed afterwards to make sure it is smooth?

That can work too, but often times the "Through Points" method can introduce wiggles in the shape more easily. There tends to be more constraint and force applied to the result with that method while using control points tends to make a more relaxed shape as the result.

But it is possible to use "Through points" as well - wiggles can be reduced if you use a fairly regular point spacing pretty much just like in my screencap above, just place the points directly on the outline rather than to the outside of it.

With "Through points" try to avoid irregular point spacing like having several points tightly clustered together and then a few more points a much longer distance apart from those, that really tends to make unwanted hooking and wiggles when that type of stuff is being done.

- Michael

Hi Martin,

> Thank you Michael, your explanation was what I hoped for. You're very skilled in placing
> the points. I tried it like you showed but my curves still have kinks in them and are far
> from smooth. I guess there's a lot of practise required.

Yes, practice does definitely help. The thing that's nice about it compared to Bezier drawing though is that you're more able to directly shape the curve right during the initial creation of it, you don't have to do that type of multi-pass Bezier drawing approach with having some totally different straight lined shape to start with and then putting in bulges as a separate step.

The Bezier method with bulge adjusting is really not so bad for tracing in particular - but it becomes significantly more weird when you're just freely drawing something though, that's when the NURBS method of being able to directly draw the curve in one go through becomes way better.

If you are still not comfortable with control point curve drawing, maybe using "Through points" would work better for you - since you can then place the points directly on the shape it removes some amount of experience. Then the thing to focus on is to try and place the points at fairly regular spaced intervals, do not cluster a whole bunch of points tightly together and then place the next one after that a far distance away, only slowly increase the distance between points as you go to a less bent shape. That will help to reduce the amount of wiggling.

Again the key difference from what you were showing previously is that you were trying to use a too sparse set of points, put in somewhat more of them in at more regular intervals and that will help a lot.

- Michael

Here's an example using "Through points" instead, which can work pretty ok for tracing like this - it has a tendency to apply more force to the curve though and it's easy for there to be hooks and wiggles in the shape especially if your point set is sparse and if you make sudden changes in spacing. When you get to less detailed areas, don't just put a single point some far distance away from the tightly clustered ones, just gradually increase the distance to sort of ease-in/ease-out the distances rather than any sudden changes.

It tends to be easier to get a smooth result using control points, especially when you're freely drawing rather than tracing.

Anyway, here's what using "Through points" looks like:



- Michael

Hi Michael,

I think this is a great place and moment to let you know that an option for the AddPoint function would be great so that it does not leave you with a new curve (2) after adding a point, but tries to move the control points you just added one to in a way so that you get a curve(3) that looks like your curve before adding the new control point(1). This is my regular end user's ultimate dream!

1.


2.


3.

I just copy/paste from Illustrator to Moi. Works great and I can leverage my existing skills easily.

Hi Strube, re:

> I think this is a great place and moment to let you know that an option for the
> AddPoint function would be great so that it does not leave you with a new curve
> (2) after adding a point, but tries to move the control points you just added one
> to in a way so that you get a curve(3) that looks like your curve before adding
> the new control point(1).

Add point actually already has a mode for doing that, some information on that here:
http://moi3d.com/3.0/docs/moi_command_reference4.htm#addpoint



So when you use Edit > Add pt, there are 2 modes to it, one mode is if you click on a control polygon hull line, that's the mode that adds in a new control point and keeps the control points in their current locations which will result in the curve shifting shape somewhat just like you have seen.

The second mode is if you click the point down directly on the curve rather than on the control polygon. You can do this with the control points being shown but if the control point hull is getting in your way you can turn off the points to make it easier to click directly on the curve. In this second mode the shape of the curve will not be modified at all, the curve will remain in its exact previous shape and instead the control points will shift somewhat and get refined with a new point inserted into the curve in the general area where you clicked.

Here is an example of that in action - again note if the control polygon is getting in your way of targeting the curve itself turn the points off and then use Add pt and click directly on the curve:




So note there that unlike the other add point way the shape of the curve remains completely fixed and does not change at all with this style of added points, which is what you were asking for if I understood correctly.

- Michael

Thanks, Michael.

Whenever something doesn't work I use the search function, but I never thought that there is something to find for AddPt...

"...if I understood correctly."

Yeah, lost in translation... I guess drawing a simple curve using as few points as possible, adding points using the second mode - click point down directly on the curve rather than on the control polygon - and then, having enough material, using rebuild to even points out is going to be a new workflow for me for creating curves.

But what I was suggesting, and I still think that is very intuitive for users, is, adding a point on the hull and the curve just remains as fixed as possible. Especially because, even though it is a bit emberassing I haven't read the manual properly, using mode 2 right now you cannot decide where on the hull you get a point.

Hi Strube,

> But what I was suggesting, and I still think that is very intuitive for users, is, adding a
> point on the hull and the curve just remains as fixed as possible. Especially because,
> even though it is a bit emberassing I haven't read the manual properly, using mode 2
> right now you cannot decide where on the hull you get a point.

The problem is I don't know of any way to make something like what you're describing work... - it's kind of a fundamental side effect that you can either keep the control points in their current location and have the curve shape change, or have the curve stay the same shape and have the control points shift around with some refinement in the general area you want to add detail (the second mode)...

I don't know how it would be possible to make things work to put in a control point in a specific location but then change the other points around to keep the curve the same shape, there just isn't any established mechanism that I know of that can be done to a NURBS curve to get that particular behavior...

The second mode that I describe above is often called a called "knot insertion" operation for NURBS curves - it's a way to refine the curve but keep the curve in its exact same current shape.

- Michael