The lack of this option is a huge downside of the VariableOffsetSrf tool. Not only the user is left without a way to create a variable offer surface with the simplest possible structure (same as the input surface), this also causes various problems such like: misleading preview, self-intersections, bad objects, and even crashes of Rhino. Here is an example with video posted in another topic.
By the way, both tools (OffsetSrf and VariableOffsetSrf) also lack an option to keep the tangency around the border of the surface. As a result, the end points look weird and destroy the expected tangency, eventually forcing the user to fix that manually. Notice the distorted control points on the upper left corner. The offset surface is the top one. The 2nd row of control points is supposed to be vertical, but instead Rhino made them heavily inclined at approximately 45 degrees.
In car design and product design in general, preserving the tangency of the offset surface at the cost of some deviation in the middle of the surface is much more important than keeping the thickness the same at the cost of compromised tangency.
Any chance for a “KeepTangency” option, as well? Presumably, the 2nd row of control points should preserve the same direction as those of the input surface.
I believe that I already explained it in the first post above. Preserving relatively the same tangency helps a lot in the product design, especially on 3d models for actual production that must have a certain (variable) thickness.
I also created a simple example with a comparison between:
Regular surface offset (red surfaces);
Surface offset with “KeepTangency” option (blue surfaces);
Variable surface offset with “KeepTangency” option (green surfaces). Note that the upper surface edge is 10 mm away from the input surface, whereas the bottom edge is offset by 5 mm), but the 1st and 2nd row of control points of the green offset surface perfectly match the adjacent surfaces, because the “KeepTangency” option will force them to follow the same direction as the corresponding control points of the input surface, while also trying to maintain the relation to the control polygon in the middle between the surfaces to gradually increase the thickness from 5 mm to 10 mm (the so-called “blending”, which some CAD programs have sliders for to control the bulging).
Preserving relatively the same tangency helps a lot in the product design, especially on 3d models for actual production that must have a certain (variable) thickness.