Hi Burr, well a Coons patch is fundamentally a 4-sided thing.

When you do it with 3 curves there is an implicit fourth curve involved which is taken as a curve with all of its points collapsed down on to the same spot.

The location of that singularity point will have an effect on the shaping of the generated surface. Just turn on control points of the generated surface and you'll see how the points collapse down to a single point on one side, that's the difference.

It seems like the case you have here, your path curve is actually an exact 90 degree arc, with radius = 10.15, is that correct?

In that case it may better for you to use Revolve, just selecting one of those pieces as the revolve profile. That's usually the most efficient and accurate way to construct something that pivots on a center point and sweeps out a path of an arc.

- Michael

I was just poking for the knowledge of how it worked. I can control the collapse point by "order of picking". 2 are not good, and one produces nearly the same result as sweeping does. I was just inquiring for my better understanding of what was happening. But it is not nessasarily a method I would attempt to construct something.

No need to go any further with it. i thought it was a mechanism I couldnt find in the reference. The reference just said, using network with 3 or 4 curves is a coons patch.

Hi Burr, the singularity goes to be opposite of the first curve that was picked.

So for instance in your arrangement you probably would want the singularity/pole point to be at the very top, that would be most similar to a surface of revolution and make the 2 similar curves behave more symmetrical to one another:



So to do that, pick this curve first:



Now if you turn on surface control points, you will see that they kind of start at that first pick curve and collapse down to a single point opposite of it like this:



If you instead pick one like this:



Then if you turn on control points you can see that it will collapse down towards the opposite of it, but that gives a kind of asymmetry:




But really revolve or rail revolve is much better for something that pivots like this.

- Michael

That was it. Thanks. (first curve picked is swept through.)

Does this hold true for a 4 curve selection? So first curve slected is swept through 2 rails towards the opposite 4th curve in the selction? (So the only real determining factore of picking the curves would be the first one)

Hi Burr, no it doesn't really work like that for 4 curves - for a 4-curve Coons patch the curves are touching each other end to end forming a ring.

The ones opposite one another are not determined in that case by selection order, only by their relative placement in the ring which is a geometric arrangement, not a selection based one.

So like if you numbered the curves going around the ring like 1, 2, 3, 4 like this:



It pairs #1 and #3 opposite one another, and #2 and #4 as the other opposite pair.

In that case it is all determined by how the curves touch each other.

- Michael

Also in a Coons patch there is not exactly a "sweep" happening.

It's more like a Loft between the opposite pairs of curves, then those 2 lofts get combined together into a result surface.

There isn't really a sweep happening, it's more like a membrane attaching itself to all the curves.

There isn't one set that is the "profiles" and one set that is the "rails" for this particular kind of surface generation, they are all more being combined equally in a sense.

- Michael

Hi Anthony,

> It would be great for me and for other users
> to see precisely the workflow you've used to
> build the "boiteaproblem" :D
>I mean if you've got a bit of free time
> (I know it can be time consuming huhu)
> if you could take some screenshoot along the
> way it would be REALLY nice :D

I haven't forgotten, here is the run down for the body of the watch, I actually did this with a slightly different strategy than the previous model I did.

1. Line up the reference curves, I used the magenta line in the center to do this and place the center of the watch face on the origin,
it will be easier to work with.
Draw a circle around the outside (green) and use the second magenta line as reference to create the red profile curve, it's a simple line and arc.
Then transfer this red curve to the (top view) reference curves and orient the right way.


2. Revolve the red curve about the center to create a solid. (Construct > Revolve. with Cap ends)


3. Boolean difference so we just work with a quarter of the watch as it is symmetrical around the X and Y axis.
If you like there is a alternate Revolve script which allows you to revolve any angle segment, in this case 90°.


4. Draw the profile curves as shown with sharp corners, using the reference curves as a guide, join (Edit>Join) these curves together.
*Tip: if you are on V2 you can lock the reference curves so they don't get in the way or disturbed and still be used to snap to.


5. Draw an arc, (Draw Curve>Arc>3pts) as shown using the reference curves to snap to end points.
Make sure the curve is at the level shown in the second picture.


6. Draw a line (green) tangent to the inner curve of the reference as shown.
Again, make sure it is on the bottom level of the solid.


7. Extrude (Construct>Extrude) this line in Z+ (up) using the reference curves for the height.


8. Extrude the arc, but this time using the 'Set dir' button in the Extrude command and drag a construction line from
the middle of the bottom arc to the middle of the top arc of the watch to set the direction.
This surface should end up on a slight angle.


9. Select these two edges.


10. Create a blend (Construct>Blend) between the two surfaces and then join them (Edit>Join)


11. Now use the top curves, drawn previously, and the newly created surfaces to do a Boolean difference as show.
Hide or delete the pieces not needed.


12. Create a fillet on this edge first, I used 0.5


13. Select all these edges and create a 0.25 fillet.


14. Draw a line aligned with the top edge of the radius as shown.


15. Boolean difference the solid with this line and hide or delete the top part and line.


16. Mirror the quadrant through X and Y axis, then Boolean union the pieces together.
Done!


Cheers

Thanks so much Danny, for taking the time to explain your workflow - I'll definitely be saving this in my MoI tips and tricks document since once again I've gleaned insight on NURBS modeling techniques.
(Shame I've been ruined by poly modeling for so long...)

-Will

Superb and tricky tutorial!
Cool use of colors
Bravo!

PS You can find this tutorial in the French Section ;)