Very simple ;)

I want click annywhere on the Sphere and draw a tangent circle* in one click (the best)! or 2 (challenger) or 3 (last chance) ;)



* circle is given !

Maybe something with helper line

for the moment
Orient then Project Curve is cool but you must erase the second projection

Hi Frenchy,

how about this, in the upper right of the browser it tells you how big the radius of a given circle is. Then simply draw a new circle on the sphere surface with the same radius as the original one, were you want.

I did the following: I draw my (center) circle on the sphere were i want it, with the given size, then intrude the circle and do on both objects a "Curve" intersect command. Now i have a circle which is exact the size of a given circle, fitting on a sphere.

Regards
Stefan

http://www.mathopenref.com/chord.html
The chord length of the diameter of the circle of radius R, on the sphere of radius r, is given by the formula
2*R = 2* math.sqrt(r*r - d*d)
R*R = r*r - d*d
d*d = r*r - R*R


So the center of the circle is located at a distance "d" from the center of the sphere, where

d = sqrt(r*r - R*R)

So a simple script may be possible, if the radius of the sphere and the radius of the circle, can be extracted by MoI.

d is the Apothem, also denoted by a.
The depth of the center of the circle, from the surface of the sphere, is the Sagitta. Which sounds a lot like "sag" :-)

@Stephan
<< I did the following: I draw my (center) circle on the sphere were i want it, with the given size, then intrude the circle and do on both objects a "Curve" intersect command. Now i have a circle which is exact the size of a given circle, fitting on a sphere.

how do you make the circle perpendicular to surface of the sphere before the intrude ? Because the circle can like this has any orientation

I can also draw a cylinder from the center of the sphere passing by the point I want then Curve / Intersect and killl the cylinder ! ;)

Hi Pilou,

> I want click annywhere on the Sphere and draw a tangent circle* in one click (the best)! or 2 (challenger) or 3 (last chance) ;)

With just one click you would be only specifying the tangent constraint, there are an infinite number of circles of different radius values that could satisfy this description so you would at least need to specify that.

You can use the Draw curve > Circles > Center command to draw a circle tangent to a sphere, if you place the center point of the circle on to the sphere, you will then be able to pick a radius value and the circle that is created will be on a plane tangent to the sphere at that point.

It's not totally clear to me whether you actually want a circle tangent to a sphere like you're writing (which means on a _plane_ tangent to the sphere, that's what circle from center point will give you) or whether you're looking for a circular segment that is on the surface of the sphere but with the center of the circle actually submerged a distance below the sphere...

Probably the easiest way at any rate is to draw a circle with circle from center point, and then optionally project it on to the surface of the sphere if you need that instead.

- Michael

Hi Pilou,

> how do you make the circle perpendicular to surface of the sphere before the
> intrude ? Because the circle can like this has any orientation

If you use Circle from center point, and snap that center point onto the surface of the sphere (object snap must be enabled for this), the circle will then by default be drawn on that tangent plane from the start.

- Michael

<< It's not totally clear to me whether you actually want a circle tangent to a sphere like you're writing
say for drawing eyes of toon personnages ;)

<< If you use Circle from center point, and snap that center point onto the surface of the sphere (object snap must be enabled for this), the circle will then by default be drawn on that tangent plane from the start.
ok : I had also Snap gride so that's making interference :)

<< Probably the easiest way at any rate is to draw a circle with circle from center point, and then optionally project it on to the surface of the sphere if you need that instead.
That i made in first post with the Orient useless now by the circle by center :)
"I can use Orient then Project Curve"

For the moment the cylinder solution from the center sphere is the more funny :)

You can also create a construction line along the sphere's surface normal to help in some kinds of constructions like this.

To do that start the construction line's first point on the sphere (object snap must be enabled), then when you drag out there will be a "normal" snap that you can use to place the other point along the sphere's surface normal. Straight snap must be enabled to get that kind of surface-normal snap.

- Michael

So I've learned that the "On Surface" snap just works in the 3D screen. (?)

I was thinking the center of the circle would be submerged, inside the sphere, and the circle would be on the surface.
(Which construct-curve-project already does.)
A script, if possible, would be able to find that center point. Do not know if it is worth the effort though.
I see that UI PropertiesPanel has a read-only radius...

@ Michael
<<To do that start the construction line's first point on the sphere...
I don't succeded your last tip, i must miss something !

but I have found another one! ;) Maybe the more easy for the moment :)

I draw my circle on the top of the sphere in view Top

So in view Front I can easily move the Edit Frame circle vertically to the intersection of the "curve's generator" of the sphere!

So I have my tangent circle!

So now with the Orient / Copy I can draw anywhere my circle tangent on the sphere in one click! :) (+ a right click ;)

@Bemfarmer
Courage!

Hi Pilou, see here for a demo of doing a construction line along surface normal:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4720.3

- Michael

Hi Brian,

> So I've learned that the "On Surface" snap just works in the 3D screen. (?)

Yup, that's correct - in a 2D view it's pretty hard to see whether the point you placed is on a surface or whether it's on the construction plane so some of these kinds of snaps only apply to picks in the 3D view where you can more easily spin around and see the objects that you are drawing on.


> I was thinking the center of the circle would be submerged, inside the sphere, and
> the circle would be on the surface.

If you wanted to do it that way possibly drawing a plane and then using Construct > Curve > Isect to intersect the plane with the sphere would work.

- Michael

<< a demo of doing a construction line along surface normal:
Yes this gives the helper line on somewhere must be the center's circle inside the sphere
but not the deepth... ;)
Not yet found for example the use of the Orient only for place the circle on this line...and more helper lines for find the good point ...I must explore more :)

Hi Pilou,

> Yes this gives the helper line on somewhere must be the center's circle inside the sphere
> but not the deepth... ;)

Well you could use it for example to help draw a cylinder that had a central axis perpendicular to the surface of the sphere but partially submerged inside of it rather than sitting exactly on the tangent plane. Then with the cylinder intersecting the sphere you can intersect them.


> Not yet found for example the use of the Orient only for place the circle on this line..

It will be difficult to find the exact location to position the circle doing it that way - I'd recommend instead using a projection method so that you don't need to worry about the positioning in a manual way like that.

- Michael

<< Well you could use it for example to help draw a cylinder that had a central axis perpendicular to the surface of the sphere but partially submerged inside of it rather than sitting exactly on the tangent plane. Then with the cylinder intersecting the sphere you can intersect them.

it's that a sort of this I made some post above ;)
<< I can also draw a cylinder from the center of the sphere passing by the point I want then Curve / Intersect and killl the cylinder ! ;)

I don't deseperate to find something easy with the helper lines but for the moment
my above method of the move the Edit frame in Front view + Orient copy everywhere is very satifying and seems the more easy to do (for me) :)

So to generalize, how would it be possible to copy a clothing button, and place it tangent to a surface, such as a sphere, or catenary of revolution, with the "center" at an on-surface point of the surface?

Using the ArrayGem script with a tiny line segment, instead of a point object does such a copy.

Is it possible to modify the ArrayGem factory, to use a PointObject, instead of a Curve?
Changing objectpicker.allowCurves() to objectpicker.allowPointObjects() in the script, does not work, so the change would have to be in the ArrayGem factory, (I think).

Hi Brian, it should probably not be difficult to modify the ArrayGem factory to work like that, I'll take a look at that tomorrow.

- Michael

Hi Michael, I've just been messing around, practicing MoI. Ideas may or may not be useful or practical.
Seems like the orient command does the trick well.