# Please explain the following questions

1. What is the difference between “fillet edge” and “blend edge” and “fillet surface”?? In what situation do we have to use each of them?
2. What is the difference between “2 rails sweep” and “network surface”?? Do they create the same surface? In what situation do we have to use each of them?

Hi Alex -

`FilletEdge` and `BlendEdge` are very similar - they work on joined edges of polysurfaces and try to keep the integrity of the polysurface - that is, it should be joined up when its all done if all goes well. The difference is that fillets in Rhinospeak shoot for a true arc cross section and are tangent to the input surfaces, whereas `BlendEdge` makes a surface that is curvature continuous (G2) to the inputs and is not trying to be an arc in cross section.

FilletSrf is a surface operation in that it takes two surfaces as input (as opposed to a joined edge between surfaces) and it does not care if they are joined and makes no attempt to join the results.

`Sweep2` takes exactly two inputs in one direction (the rails) and any number of inputs in the cross section direction. It can only provide continuity to adjacent surfaces along the rails, if the rails are edges - it does nothing to adjust surface continuity on the ends ( the ‘short’ edges on a typical sweep)- edges used as cross sections are treated as curves with no surface info. `Sweep2` uses the cross section curve structure to make the surface - i.e. it’s advantageous - cleaner surface - if the cross sections all match each other nicely.

`NetworkSrf` can accept any number of curves in either direction. It always makes a degree three internally curvature continuous surface (no kinks) and does not care about the structure of the input curves - everything is refit to make the surface. Any edges used at the perimeter of the curve selection allow continuity settings to the surfaces there.

Both commands tend to make fairly dense surfaces - lots of knots and points - depending on settings - but they tend not to be ideal for point editing.

-Pascal

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Thanks.