Rhino - Moi : Circle on Sphere

 From:  Michael Gibson
4202.2 In reply to 4202.1 
Hi PaQ - well there are a lot of things that are more clunky in Rhino than in MoI, that's just one of many...

In several ways MoI is kind of a next generation after Rhino - I mean since I designed Rhino I was quite aware of limitations and irritations in its workflow, and starting MoI from scratch allowed me to make improvements to a lot of the fundamentals and basic operations such as how snapping and drawing works.

There isn't any way built in to Rhino's Circle command to make a circle aligned to a surface normal as you can in MoI. You can get it done but you'll have to use some combination of other tools.

A few ways that would work would be to set the construction plane first (type CPlane, then type S and push Enter to activate the Surface option, click the sphere surface to select it, then select the point you want to use as the center point of the surface, then right-click to accept default rotation), and then after the cplane is set you can draw a circle and type in 0 and push enter to pick the cplane origin as the circle center point and when you draw it, it will use the cplane orientation. Then you'll want to reset the cplane after that.

Another way which might be easier would be to draw a line along the surface normal (type N to activate the Normal option for line - it's a special option because Rhino doesn't have any automatic surface normal straight snap/ortho snap like MoI), then use the AroundCurve option in circle to draw a circle around the base of that line, then delete the line.

Another possibility would be to draw the circle elsewhere and then move it on to the surface with OrientOnSrf.

But yeah, drawing and snapping is just a lot better in MoI, I worked hard to make the really basic stuff be a lot more fluid and not require so many different kinds of special sub options and special modes and stuff inside of MoI.

> (I dont see this mode in the osnap bar)

That's because Rhino's osnap bar only has a kind of subset of snaps that are called "running object snaps" - they're ones that are persistent across many point picks. There's a different kind of snap called a "one shot object snap" that you activate just for one point pick, and the onsrf snap is one of those. Also as you may have noticed when you use Rhino's onsrf snap, it's a multi step process where you first pick the surface you want to use and then pick a point on it, you can't just do one click on any surface to activate it.

This kind of system with "one shot" and running object snaps is inherited from AutoCAD - a lot of stuff in Rhino is set up to be similar to AutoCAD so that using Rhino feels comfortable from someone who is coming from a background of using AutoCAD a lot. If you do not have that kind of a background much of that stuff will tend to feel more uncomfortable and archaic though.

You'll probably run into a lot of frustrations with various stuff like this - you'll probably want to do things more like draw stuff in MoI and copy/paste it over to Rhino when you want to use some Rhino function.

I mean when you use Rhino's drawing and snapping system you're basically going back in time to the system that I designed in 1993, and there were many aspects of that which were set up to be similar to AutoCAD's system which was quite a bit older than that... Of course they have made a lot of additions to Rhino since 1993 but all the new stuff is kind of layered on top of the same original design.

The reason why MoI works much more smoothly in some of these areas is that it's a more modern design that incorporates stuff like surface normal snapping more directly into its design rather than kind of stratified layers of design with new features layered on top of old...

I mean also keep in mind that Rhino was originally designed to run on a 90 Mhz Pentium system, with a wireframe display. That's why some of the stuff is structured the way it is - like for example with the onsrf snap, the reason why it requires 2 picks to do it (first one click to select the particular surface you want to snap on to and then pick the point on it), is because that works better in wireframe mode where you may want to place a point on a surface somewhere where there does not happen to be a wireframe edge or isoparm currently displayed. So that original focus on a wireframe display (in year 1993, remember) has a lot of side effects in the workflow that persist to the current version of Rhino. MoI is designed around a shaded display so stuff like having the whole surface as a displayed thing on the screen under your mouse is part of the workflow design - that difference is really why snapping points on to surfaces while drawing is so much nicer in MoI.

EDIT: by the way don't get me wrong - Rhino is definitely very useful and has a lot of functions that are not in MoI, especially stuff like deformation functions. But one of the reasons that they work well in combination with each other is that there is also stuff in MoI that works better, and that includes a lot of the really basic stuff like snapping, drawing, 2D editing with the object frame, exporting well to polygon meshes, ...

- Michael