DevLoft is a new command in Rhino WIP that makes lofted surfaces between two rails.
The rulings of the surface are adjusted to make the surface developable if possible.
Developable surfaces are ones that can be flattened to a plane by bending along ruling lines without causing stretching or compressing distortions.
They are useful because the resulting flattened surface can be cut from sheet material that can bend but not stretch, and the non-planar original surface shape can be made by bending the flat pattern.
Only certain curves can be connected by developable surfaces. You have to provide good input to get a good result. At least for now, DevLoft always makes a surface using the entire lengths of the input curves.
Depending on the specific application, surfaces that are nearly developable may be useful since all real materials are somewhat deformable. The specifics of what will be acceptable are beyond the scope of the DevLoft command since they involve material properties and manufacturing processes.
Just because DevLoft is able to make a surface between two rails is not a conclusive indication that the surface can be manufactured in any particular case. Using the Unroll command and looking at the area comparison that Unroll makes is one way to get more information about how close to fully developable a surface is. Gaussian curvature display is also sometimes helpful, but it is scale dependent and absolute numbers are pretty hard to evaluate meaningfully. Very few surfaces will have zero Gaussian curvature.
In the immediate mode, run by the DevLoft command, a surface is made without further interaction, with the rulings adjusted to produce the best approximation the command can find without more interactive input.
Often this immediate mode will make an acceptable surface if the rails you pick make sense.
In the interactive mode run by -DevLoft, there are some options available to help find other approximations of developable surfaces.
Density - Represents the number of rulings across the surface initially found before the process of adjusting the rulings for maximum developability starts.
Untwist - Runs the optimization process to find better rulings. In some rare cases, it may help to run Untwist more than once.
In the interactive mode you can place rulings at arbitrary positions by picking points on the rails. When you pick a point on one of the selected rails, a suggestion of an untwisted ruling will be drawn if one can be found. Sometimes several rulings from one point are equally, or nearly equally, untwisted and the suggested ruling will follow the cursor along the opposite ruling.
Once you place one or more rulings, you can run Untwist to find a new surface starting with your rulings in addition to others found by DevLoft according to the Density setting.
In this mode, pressing Enter will produce a surface result if possible.