Fillet surface vs Blend surface difference

blendsrf
filletsrf

#1

hi,

isn’t it a weird attitude that, fillet surface and blend surface commands are completely different approaches. sometimes i want to blend surfaces just like how i fillet surfaces by just clicking two surfaces not edges and thats it.

is it just me?

regads


(Tom) #2

isn’t it a weird attitude that, fillet surface and blend surface commands are completely different approaches. sometimes i want to blend surfaces just like how i fillet surfaces by just clicking two surfaces not edges and thats it.

Hello,

no it isn’t. Fillets do approximate an arc, whereas Blends do not.
However there should be a chordal fillet (with G2), which is not part of rhino or any plugin for Rhino.
If you want to blend chordal, you can offset curve on intersection with same distance, trim and blend from there.


#3

thank you @TomTom i know the work around for that and as i said before [quote=“Blastered, post:1, topic:41472”]
sometimes i want to blend surfaces just like how i fillet surfaces
[/quote]

without piping, trimming then blending. simple fast easy


#4

Not sure I understand here. _VariableBlendSrf works pretty much just like _FilletSrf… --Mitch


#5

stupid me


(Tom) #6

I think _VariableBlendSrf does not behave like a real Blend. It does not align controlpoints at all.
So a manual isotrim with an aligned blend, returns much more from what I expect to be a “blend”. I guess its a matter of how you define a blend for yourself…


#7

All VariableBlendSrf is is a G1 fillet that has been removed and replaced with a fixed G2 blend between the same edges. That’s why it works like FilletSrf and is simply a quick shortcut to have a “canned” G2 joint instead of a G1 at the edges. If you need more control than that, you need to use BlendSrf.

–Mitch


(Rob McPherson) #8

But watch out, in my experience BlendSurf gives a surface which has a ‘flat’ centre area in comparison to manually creating a ruled surface between edges and then using the match tool to get G2 which gives a more even spread of control points.

Regards,

Rob.


#9

Yep, especially if the angle is less than 90°. It’s the same as using the command Blend instead of BlendCrv. Seems that could have been improved long ago, but I guess nobody really uses Blend anymore.

–Mitch


(Tom) #10

but I guess nobody really uses Blend anymore.

No need to tell me that, I’m just answering this thread.
Equally spaced surfaces should have been part of Rhino Blend from day one.
I also believe that in most cases Rhino Blend produces a unnecessary amount of controlpoints.
A nice blend between two 6x6 patches usually only needs 6x6 cps for g2 on both sides.
If both are flat, even 2x6 is required, a thing Rhino Blend could have integrated long ago, That would
really safe time, in conjunction with real trim (not masking) and extrapolation functionality


(David Cockey) #11

Move the sliders to make the blend more curved in the center overall, or move the control points to change the shape locally. Add “Shapes” for more variation along the blend.

If G2 continuity is desired the possible shapes of the blend surface are limited depending on the position of the edges, distance between the edges, and curvature normal to the edges. That is due to geometry, not specific to Rhino.


#12

What do you mean by “true” trimming?


#13

Sorry, thought you were talking about _VariableBlendSrf here, not _BlendSrf. _BlendSrf can be adjusted any way you want.

–Mitch


(Rob McPherson) #14

Agreed, but the default setting of 1 doesn’t give you a surface with equally spaced control points. And moving the position of these is just by sliders (EDIT! Complete nonsense! It’s curve blending that is just by sliders, please ignore this comment!!! Sorry…) which makes doing a long run of blends quite tricky. Lots of going back and manually making sure that each surface matches up to it’s neighbour.

However, you get a different result by using the ruled surface then match method (equally spaced control points).

Not complaining (well, kind of…) but just giving a heads-up to anybody using the blend surf tool.

Regards,

Rob.


(Tom) #15

What do you mean by “true” trimming?


#16

Yes…

Philip


(Tom) #17

The full approach


#18

Well that might be nice but just isn’t possible with any sort of precision except for trivial cases. What if that trim curve has a kink in it, or is simply moderately curvy, what should it do? The “mask” trimming of NURBS is the #1 killer feature that explains why they rule the CAD world.


(David Cockey) #19

@TomTom Looks like you want surfaces to be rebuilt so the trimmed edge is an isocurve. That is only possible if the trimmed surface has four sides. Untrimmed NURBS surfaces have to have four sides although one or two sides can be “zero” length. Also rebuilding a four side trimmed surface as an untrimmed surface and retaining the shape of the untrimmed surface would usually require adding more, possible considerably more, control points were added. That is due to how the math of NURBS surfaces works, and not specific to Rhino.

Your illustrations appear to be lofted surfaces which are curved in only one direction, and the trim curve is a special case… Those are examples of surfaces which if the trim curve is suitable can be rebuilt without adding control points. That is why I said “usually” and not “always” above. Other surfaces or even your lofted surfaces with a different trim curve can not be rebuilt and retain their shapes without adding control points.


(Tom) #20

no this was made in Icem Surf, and it is 100% possible, and part of my daily work in automotive. <5 clicks to blend like this…