"""""""""""If you have a solid sphere with two points on its surface, how would you create an arc between those two points?"""""""""

If you use the arc by 3 pnts command, select the 2 pnts first, then in the 3d perspective viewport there is an "OnSrf" snap to get the 3rd selection on the surface.

Hallo BurrMan, so you get a bundle of curves.

@ Paolo, additionally you must say, that the second point of the plane must be in the middle between the two points on the surface.

Rudl

@ Burrman:

<< If you use the arc by 3 pnts command, select the 2 pnts first, then in the 3d perspective viewport there is an "OnSrf" snap to get the 3rd selection on the surface.>>

Thanks for the reply, but you would have to know where to precisely place the third point (since you could place it anywhere on the surface). And I don't have that information.

Hi YHWH_777,

> but it puts the curve on the z axis plane and not on the
> surface of the sphere.

The way that arc drawing command works is that it orients itself to the z axis of the construction plane.

The construction plane by default is set to be aligned with the world z axis plane, but you can alter the construction plane to the orientation you need first before drawing the arc to control the arc's orientation.

You'll need 3 reference points to set the construction plane - one at the origin of the sphere, and then the 2 points on the sphere.

Now hide the sphere, because it can easily get in your way with the following steps trying to snap on to it in a couple of ways that you don't want.

Then run the View > CPlane command to alter the cplane - set the origin point of the cplane at the sphere's origin point. Then grab then grab the x axis line of the axis triad widget, hold down the mouse button and drag and release it over the first point - now the cplane has its x axis aligned to that point. Now drag the y axis line to the other point - this will rotate around the xaxis line so that you will be at the proper orientation that you want. Now right-click to tell it that you're done manipulating the axis widget and want to place the cplane. Then you can draw an arc as you wanted. Afterwards, right click on the View > CPlane command to reset back to the default cplane.

It looks like this:




You want to hide the sphere because otherwise things will try to snap on to the sphere's arc seam edge when you snap to the center point there, and the orientation picker will try to align itself to that plane of that arc edge.

- Michael

Also some more information on setting the cplane here:
http://moi3d.com/2.0/docs/moi_command_reference5.htm#cplane

The little axis widget placement tool is used in a few different commands like Orient as well, some info on how that picker works here:
http://moi3d.com/2.0/docs/moi_command_reference11.htm#orientationpicker

If you want to orient it to have z perpendicular to the plane of 3 points, you use that method above though - place the orientation picker at one of the points, drag the x axis of the picker to point at one of them, then drag the y axis to spin it around the x axis line and align it to the other point.

One last note - the various planar drawing commands including the arc drawing command also will try to use an object that was snapped on to with the center point as a temporary construction plane to orient the arc when you are using the 3D viewport to place the points. This is to allow some easier direct drawing on a surfaces without necessarily needing to set a construction plane. But in this case that will probably get in your way which is why you want to hide the sphere so that you won't get that happening.

- Michael

Otherwise if you don't hide the sphere, you can uncheck the "Align to objects" option in the orientation picker when setting the cplane origin to avoid the snapping there, and then when drawing the arc instead of clicking on the point with the mouse (which will then try to align the arc to the plane of the sphere's seam edge which has its center point at that same spot) instead type in 0 and push enter to specify the construction plane's origin as the center of the arc, using numeric entry for that pick will not do any "temporary cplane" type snapping. Otherwise the newly drawn arc will basically try to stick in the same plane as the arc edge.

When you set the cplane it also affects how numeric input is used - when you type in coordinates they will be in that cplane's coordinate system. This is so you can temporarily use some local area of an object as if it were the world origin as far as entering in coordinates.

- Michael

Another solution: just make a line between the two points and use the project curve command to project the line to the sphere. For the direction clic the center of the sphere and then center of the line you just created and thats it!

<< Another solution: just make a line between the two points and use the project curve command to project the line to the sphere. For the direction clic the center of the sphere and then center of the line you just created and thats it!>>

That is actually how I did it (before I even opened up this thread), but I thought that there must be an easier way to do it with the ARC command! But it doesn't seem that there is.

Thanks to everyone for all of their help.

Hi YHWH_777,

> but I thought that there must be an easier way to do it
> with the ARC command! But it doesn't seem that there is.

Well, the arc commands are like other planar object drawing commands in that they want to draw the arc in a particular plane.

So if you want to use those commands you need to set the drawing plane to the orientation you want and then you can draw your desired result with the regular arc command.

Maybe in the future I'll be able to set up some drawing method for drawing on a curved surface making a kind of automatically projected result that adheres to the surface. Unfortunately it's kind of difficult to make that happen on just any kind of a surface with every kind of drawing tool without some kind of distortion happening in the drawn shape, and it might be kind of weird to say use the circle tool but get some kind of distorted result instead of a real circle. That kind of odd situation is basically why I haven't attempted to do that kind of thing yet.

- Michael