Dear MoI3d community,

I have discovered that blending two curves gives different curvature from blending two surfaces with the same parameters (curvature: G2, Bulge: 1). I created two planar surfaces and blended them; then I copied profile curves of the those two original surfaces, blended them and received a slightly different curvature. I wanted to test that blending two curves and then extruding a surface out of them would have the same effect as blending two planar surfaces. However, it appears it's not the case.

The magenta curve on the screenshot below was created by blending two profile curves, while the surface was created by blending two surfaces created from those profile curves.



I suspected, that maybe the Bulge parameter was behaving differently for surfaces and curves. Can you please advise on where can I read/find an explanation to this behavior to better understand the Blend tool?

Thank you in advance,

Updated: I get a similar blend curvature for the mentioned surfaces and curves, if I use Bulge: ~1.26 when blending surfaces and bulge: '1' when blending curves.

Hi iv_one, it's because the curve blend and the surface blend use slightly different methods.

The curve blend applies an additional scale to try and make all segments of the control polygon equal lengths like this:






The surface blend doesn't do that, it only applies the same scale to each end and not any extra to make the middle control polygon span equal in size as well:






I don't really remember why they don't both use the exact same method but there are a lot of tricky things that can be going on with the surface blend when it's going around corners and possibly getting bunched up on top of itself. It's potentially dangerous for me to just switch it to behave like the curve one without doing a lot of testing.

It would probably be good if there was an option to do that or not, at some point I would like to rework the Blend command and give it an overhaul.

- Michael

Hi Michael,

Thank you for the explanation and the detailed examples.

I assume if I apply a G2 blend to the profile curves and then extrude, MoI will essentially produce a surface with G2 continuity (in this particular example). Thus, from the continuity perspective, both approaches (blend surfaces & blend curves + extrude) are kind of equal, and will give a nice smooth transition (regardless of bulge). I am curious is blend curve + network approach is also eligible for making continuous patches? (sorry if it's an obvious question - I am looking for alternatives to Blend for more complex connections).

Thank you

Hi iv_one,

I did a small experiment, and it seems possible to generate continuous surfaces with G2 Curve Blending and Networking in MoI.

Hi tuananhnp,

Thank you for your answer.
I just want to mention that I really appreciate all the information and how helpful and responsive MoI community is.

Hi iv_one,

re:
> I assume if I apply a G2 blend to the profile curves and then extrude, MoI will essentially
> produce a surface with G2 continuity (in this particular example).

Yes for this particular example that's true, because both of the surfaces being blended are also each extrusions and curves that are all in the same plane and extruded in the same direction will result in surfaces that have the same continuity between them as the planar generator curves.


> I am curious is blend curve + network approach is also eligible for making continuous patches?

In the general case, no - Network in MoI does not ensure that the generated surface will have continuity to separate neighboring patches.

It's usually better to build surfaces that you want to be continuous out of larger surface pieces and not try to build them in a "patch by patch" fill in type approach.

If you need to build things in that type of manner you might try doing it as a SubD model instead. One of the main properties of SubD smoothing is that it generates a continuous smooth surface from the control mesh and the control mesh does not have to be a completely regular row and column grid of points as is needed for NURBS surfaces.

- Michael

Hi Michael,

Thank you for the explanation and the recommendations.

Hi tuananhnp,

What software/plugin did you use to check the surface continuity on your last screenshot?

Thanks,
iv_one

Hi, I used Rhino’s zebra fuction.