Yeah, I think that these are the most mysterious.
For blend, you have to pick two surface edges, and a new smooth surface will be created between them.
One thing that can easily prevent blend from working is if you have selected an independent curve object instead of a surface edge. Sometimes this is easy to do if you have some generator curves around that overlap a surface, like some curves that you extruded or swept into a surface. Since those curves have priority for selection, it is easy for them to get selected and blend can't use them. You may sometimes need to select those curves and hide them in order to be able to select the surface edge instead. Let me know if you need more details on this part.
I was just thinking that blend is quite a bit like loft, except it takes the shape of the surface into account. Maybe I will try to combine these together at some point.
Ok, now for rail revolve.
Let's say that we're going to do a regular (non-rail) revolve, with a shape in the front view that looks like this:
From the top view, this would look like this (with a point here showing the revolve axis which we are looking straight down on):
Now imagine that we draw a new curve in that top view:
What rail revolve will do, is take the regular revolve, and scale it radially out from the axis point (which acts as kind of a center of scaling in this case) to match up to that guide rail curve. This is an attempt to illustrate that:
This is trying to show the profile of the standard revolve on the outside which is perfectly circular, the rail revolve one will be shrunken down towards the scaling axis to conform to the guide rail that you pick.
Because rail revolve works in this kind of shrink-towards-the-axis way, there are some restrictions on how the rail is formed, you won't get good results if you have a very wiggly rail that loops back on itself with regard to that scaling center point.
So that example will produce this result:
Rail revolve is rather handy for quickly constructing a few types of shapes, things that are slightly puffy and kind of jelly-bean like. Here is an example of a puffy valentine's heart constructed quickly using rail revolve:
The resulting rail revolve surface basically inherits a combination of the control point structure of the rail and the profile curve.
The rail can also be segmented, for things like this:
That one above is with an octogon polygon curve for the rail, and an arc curve for the profile.
So rail revolve can be useful for creating certain customized kinds of dome shapes, like imagine the top of a gazebo or something.
Let me know if you have any more questions about these, and if you have a particular situation where the blend or rail revolve is not working as you would expect after this, please post the model so I can see what is going on.
- Michael
