Working with Triangulated Models

[ZooHead]

When working with imported OBJ or STL files in Cheetah,
you can't use tools like Loop Select, Ring Cut or Ring
Select because of the triangular polygon construction.

Here's a trick I figured out if you want to steal
some geometry from a model like that.

I built this in FreeCAD and exported it in OBJ format.

Here's the model in Cheetah 3D. Rendered with the Falcon renderer, Smooth Angle 30.

What if I want to utilize the outer contour as shown by the black line in the image below?
I could recreate it with a Spline. But what if it was much more complex?

This is how the "wires" look.

 

[misoversaturated]

Put a big cube under the object so that it intersects along the line you chose.
When you subtract the cube via boolean you are left with a big n-gon in the shape you want (plus some geometry you can delete).
From there you can work with extrude, cover or other operators creating quads.

I'm busy right now and cannot provide images but can do so later if my explanation is not clear.

 

[ZooHead]

Thanks for giving away the ending.

I appreciate the input though, in this case you could just delete everything
but the lower edge and your done, but there are other concerns.

More to come...

 

[ZooHead]

@misoversaturated Is right on track. A Boolean subtraction bisecting
at the point I want, and then deleting points below that plane.

The other consideration is when exporting to OBJ no edges
are placed where you want them for working in symmetry.

I can make edges where I want via the object I use for the Boolean subtraction.
In this case the model is obviously divisible by 3... tri-lobal??
Anyway, I just make a pie shaped object for my Boolean subtraction.

 

[ZooHead]

Now the Edges can be selected by Loop Select and extruded.

But you still have more edges than necessary, and they are irregular.

For the next trick you need the PolygonToSpline
script from Master Hiroto at Tres Graficos.
Maybe the most valuable script available imho.

 

[misoversaturated]

Thanks for giving away the ending.

Oops, I totally overlooked that this is the TUTORIAL section, sorry for that one.
Interesting that we came to the same conclusion though.

I have a hunch where this is going but will stay silent now, keep up the good work

 

[Helmut]

Stereolithography vs Quad meshes:
* FWIW, I have used (sparingly) the application Instant Meshes for generating a geometry more useful to the C3D selectors. This works quite nicely, producing a quad based topology where rings and loops can be selected for any required fine-tuning.
* There are a few parameters you need to experiment with to get an optimal topology.

PS: save in C3D as .obj for processing in InstantMeshes

 

[ZooHead]

Thanks Helmut! Already downloaded, and looking forward to working with it.

For the next step in this process, I added some extra
geometry to the object I'm subtracting, as you can see here.
This is so I'll be able to add another level of symmetry and then
I can work on 1/6th of the model to change the whole thing.

 

[ZooHead]

After the Boolean Subtraction, and deleting everything
below the cut line, it has the usual blue Crease lines.
You can ignore them or toggle them off if you want.

Before you delete the 5 other sections, select the outer edge.
That will make it easier than selecting the edge afterwards.

The original OBJ file was 12mb, but after using Cheetah's Simplify command
it became small enough to upload, in case anyone wants to play with it.

Next: Extracting a Spline and two ways to control a Splines vertex points.

 

[jdmac]

Thanks for giving away the ending.

Great minds thinking alike.

 

[ZooHead]

Thanks @jdmac I'm in good company here!

In the next step I have to make sure the edges are still
selected before I choose the PolygonToSpline script.

Then follow the instructions below without clicking anywhere else.

The PolygonToSpline script is quite intimidating at first because it has so many settings.

 

[ZooHead]

The new spline will be dependent on the object until it's made editable.

 

[ZooHead]

A Spline extracted this way is linear, even though the Type is set to Bezier Spline.

If you closely examine each Bezier point, you will see they are
all broken/bent slightly, directly dependent on the radius of the curve.

You could use the Spline/Soft Interpolation, but in this case the points
are so close together that when you do that, folds are created.

It's better to change the type to Linear, but I don't
think it's necessary for what I'm going to do.

You can use an Extrude Creator now, but the
points are still closer together than necessary.

 

[ZooHead]

Sorry for the long delay in my posting.
So much anxiety lately, I don't know if I'm going to be rich or homeless.

If the images in this gif change to fast, open with the preview app.
Then you can take your time.

 

[ZooHead]

My next step is to add a Symmetry Modifier and a Ring Modifier, to make the outer wall.