Ball and stick polyhedra

Ball and stick polyhedra

Hi

I am new to C3D, and have looked through the tutorial videos, so am familiar with the basics.

I am attempting to create “ball and stick” polyhedra, using cylinders and balls, as follows:

1. Create a polyhedron
2. Resize the polyhedron to specific edge lengths (not polyhedron radius)
3. Create a cylinder of a specific diameter and length (same length as the polyhedron edges)
4. Snap the centre of one end (cover) of the cylinder to a vertice point on the polyhedron
5. Snap the other end of the cylinder to an adjacent point of the polyhedron (so the edge of the polyhedron is now a cylinder)
6. Repeat this process for the whole of the polyhedron
7. Create a ball, paste one to each point

My questions:
• Step 2: How do I resize the polyhedron edge lengths, instead of resizing the polyhedron radius?
• Step 5: How do I drag the other end of the cylinder and snap it to a point on the polyhedron? (So the cylinder pivots and snaps to the polyhedron point)

Or can it be done this way:
1. Create and resize a polyhedron
2. Create a cylinder of a specific diameter (length not important at this step)
3. Snap one end of the cylinder to a point on the polyhedron
4. Pivot AND stretch the other end of the cylinder, until the end of the cylinder snaps to another point of the polyhedron (the cylinder resizes to the same length of the polyhedron edge)

Or is there another easy (and quick) way of doing this?

Here is an example of a 3D model https://sketchfab.com/models/35c08b030d624f2f98e6525519071965

Thank you for your help

Welcome to the forum AstroLogix.

I might not be understanding everything you want to do (nor am i anywhere near an expert). But from your sample image there is a script that might be able to do what you want called Polygon to Spline.

Once you convert the polyhedra into a spline object, you can use sweep creator (or even isosurface) to give the lines thicknes.

Here is a link to the script, one of many amazing and useful scripts by Hiroto:

http://www.cheetah3d.com/forum/showthread.php?t=10603&highlight=polygontospline

Brilliant thanks pegot!

I have looked at the examples on the forum and this looks like the sort of thing that will do what I need.

I've downloaded the script, and managed to create splines but am unable to thicken them, e.g. turn them into "cylinders". I'll keep experimenting with it, but in the meantime if you can give me some tips, or point me to a tutorial, that will be appreciated!

Thanks,
Rob

Use the Sweep creator and a circle spline for the sweep profile. Adjust thickens by scaling the circle up or down.

See screen shot.

As you can see in the object browser I've added the polyhedron, sweep creator and a circle spline for the sweep profile. My polyhedron is unchanged, with a green circle!

Is it possible to let me know how you ended up with the model in your screen shot? What are the settings for the sweep creator, circle spline and sweep profile?

What object do I drag on to the polygon to spline?

As you can probably tell I'm right at the bottom of the learning curve!

I have attached a sample file for you to review.

From your screen shot I see three things:

First - you have not used the script properly. In order to actually create the splines you need to drag and drop the object you want splines created from into the guide target field of the Polygone to spline properties.

Second, after you create the splines with the script, turn off visibility of the polyhedron so you can see and work on those splines more clearly. You probably also want to turn off the render property of that polyhedron as well so it doesn't appear in your final image. You can delete that polyhedron when you are sure you no longer need to make changes to it after collapsing (double clicking) on the Polygon to spline object.

Third, your circle for the sweep profile is way too large - you will need to scale it down considerably.

Also - one last thing. It is usually not necessary to upload such large resolution screen shots of the interface unless there is some small detail that must be shown. Or you are uploading a nice render to the gallery. The large images sometimes make it difficult to read the posts on smaller monitors or mobile devices.

Let us know if you're still stuck after reviewing my file.

You are a star! It all works now.

I will start creating some models

Sorry about the oversize image!

I'll post some examples when I've created them. Thanks again.

Alternative

* You can also use the script Particle Connector if you require curved ribs to replace the straight edges between vertices of the polyhedron.
* The method is identical, i.e. use the spline object generated by the script as a skeleton for a sweep with the required cross section.

That looks good Helmut, thanks for that!

I will also be "stellating" the models, so the edges will extend and meet at a point over the top of the face, forming a pyramid over the face. Is this possible with this script, or will this need a different script or tool?

Here is an example:

Hi AstroLogix

* You will have to subdivide (not the modifier, but geometry) the individual faces of the polyhedron to add the required centres and edges. Select the centres of the hexagons / pentagons (whatever, depending on your base polyhedron) and execute a normal move on those points by the required offset distance.
* The rest should be identical to the suggestions given below or above (depending on your sorting of responses).
* In my screenshot I only selected a few of the faces and executed a linear subdivision which creates the “wrong” topology. You need to replace that with the proper vertex-to-vertex diagonals (fix via point slides).

PS: For the balls on the apex of the stellations:
* Duplicate the original polyhedron and set diameter to the required distance.
* Use a particle mesh (mode: faces) with spherical particles of suitable geometry.

There is a trick with the inner extrude:
When pressing shift while dragging, the geometry extrudes outwards.
That would be the easiest way to achieve this stellarating(?) effect.

If the GIF changes too quickly, download it and examine the single frames in preview.app.

misoversaturated I'm stuck on step 3, I can extrude individual faces but if I "select all" nothing happens when I click and drag.....

Is there a way of setting the lengths of the edges of the stellations? These need to be in proportion to the length of edges of the polyhedron, e.g. for the dodecahedron:
Polyhedron edge length = 1
Stellation edge length = 1 x phi (1.618...)

I used Autocad for several years to built these models, but the tools and methods are totally different to Cheetah3D. Eg in autocad I would:

1. Make a dodecahedron
2. Place construction lines along the edges
3. These construction lines would intersect above the faces
4. Each point of intersection is the apex of each stellation
5. The construction lines were temporary so I drew the stellations with polylines, using a snapping too
6. Delete the construction lines

The C3D tools and methods are completely new to me, and I'm looking forward to learning to use it!

AstroLogix said:

misoversaturated I'm stuck on step 3, I can extrude individual faces but if I "select all" nothing happens when I click and drag.....

Click to expand...

The inner extrude tool has options: uncheck "Preserve group" to inner extrude individual polygons.

Cheers
Frank

LOL thanks Frank!

I can now create the stellations, is it possible to set these at a specific height?

Following on, my next step is to draw lines connecting the stellation points, resulting in a dual of the original polyhedron. E.g. start with the dodecahedron, create the stellations, bridge the stellations, which creates an icosahedron. The icosahedron will then be stellated, the stellations connected, creating a dodecahedron. And on and on....

The gif shows how these two polyhedra are duals of each other

AstroLogix said:

I can now create the stellations, is it possible to set these at a specific height?

Click to expand...

Unfortunately the "Status Bar" isn´t displaying anything useful.
After the inner extrude I´d use Polygon->Normal Move in orthographic views till I got some line-ups - or when you know the maths you can check the angle for instance:

Cheers
Frank

Good idea Frank.

OK, I've done this:

1. Created a dodecahedron
2. Created an icosahedron
3. Lined up the icosahedron points with the centres of the dodecahedron faces, by rotating the icosahedron 90 degrees in the Z plane
4. Using an online calculator, worked out the radius of the icosahedron that would fit around the points of the stellated dodecahedron
5. Dragged the stellations out until they lined up with the icosahedron points

When I go through this last step, is it possible to snap the stellation (extrusion) to the points of the icosahedron? This would satisfy me more than just lining by eye!

AstroLogix said:

When I go through this last step, is it possible to snap the stellation (extrusion) to the points of the icosahedron? This would satisfy me more than just lining by eye!

Click to expand...

Not in an automatic way although we have snapping-to-component.
What works in that case is to select the tip-point (after optimizing it´s just one) and drag it to the specific point of the icosahedron: bit work but precise.

Cheers
Frank

Sorry for forgetting to tell about the "preserve group" thing above.

I would say Frank's way to snap the tips of the stellated icosahedron to the vertices of the surrounding dodecahedron is the best.

For completeness two other ways:
Use the ruler tool on the original icosahedron and scale up all faces at once till the edges have a length of 1.000.
Now do the inner extrude till the triangles disappear, call selection menu: convert selection to points. You are in point mode now, change back to polygon mode and delete the downscaled middle polygons.
Call optimize to weld the tip points together, change to point mode and find that the middle points are now selected.

Now vou can use the normal move tool which has a parameter input.
You would need to calculate the height of a pyramid with edge lengths 1 and phi, type it in and apply.

Or you use the ruler tool again and scale up all points together (by dragging the center ball of the transform tool) till the number fits ( I could not get 1,618 exactly though).

When you have done this two times on the icosahedron and the dodecahedron you simply can duplicate those and scale those duplicates up to be the next iterations, and so forth.

Thanks misoversaturated

(I'm working with a dodecahedron) When you say "downscaled middle polygons" I presume you mean the triangles that are formed when perform the inner extrude?

I deleted these polygons and the whole model was deleted except for the points. Is this correct?