If I can do it in one surface, I will.
I think my big caveat for that is if the surface is made of open or closed loops. That will make a big difference, as building off of those it is easier if they are all open or all closed. So if you are modeling a tube (like a handle, or an aero wing, etc), then it is important to decide if each rib is a closed loop, or 2+ open edges.
The only time I can think of when I will purposefully add new patches is when the surface changes direction over 90° from the normal of the existing surface. Once that happens there are bound to be issues somewhere, and it is best to make it multiple surfaces. Also, the obvious abrupt changes in curvature or non-curvy shapes.
Here I have an example of an interior of a mould cavity I did some time ago now, and I did it like is shown on the left, with the design intent being lines and arcs. So there really was no way around it, strictly technically speaking.
However doing a simple fit spline, and lofting that, I get the result on the right. Which has significantly less topology.
Both results sure look similar enough to each other, but the one on the left is unquestionably more work.
This is kind of a linear "loft" in one axis so perhaps here, the big surfaces are fine?
Still my intuition tells me that the layout on the left is just... better, period.
Any insight appreciated!
patch_layouts_mouldcavity.x_t.zip 193.2 KB
Your "straight" line edges will not be perfectly straight with a fit spline. Within manufaturing tolerance of straight? maybe... But if that was a hand created spline, and not a fit spline, it would be tough to edit it and keep the straight edges perfectly straight.
I've always been a fan of "if the model tolerances are tighter than the manufacturing tolerances than it is good enough", but there are plenty of perfectionists out there who would disagree with me. Then again I work mostly with aluminum castings.
There is also the modeling strategy of making a "theoretical model" first, just main slabs without any transitions. And then model the transitions. That will indirectly create a comprehensive patch layout.
But when is this a good way to model, and when is it better to build with smoothness right into your primary sweeps, lofts and boundaries?
No doubt that building it in directly is faster, but can it run you into a wall later on?
When I learned surfacing back in the day, manual drafting boards and early wireframe and surface CAD systems, we always developed surfaces using a primary, secondary, tertiary, etc techniques.
I actually have a hard time envisioning surfacing with the newer techniques of the CAD systems. Something that would be easier for me if I had more practice beyond the basics with the SolidWorks surfacing tools.
Yeah, I always think it is appropriate to think about this kind of stuff. My rule of thumb is to make as much surface area as a feature is comfortable with, but if you gotta break it up you gotta break it up. All other things being equal, it's better to have fewer edges in a model. Still, sometimes I'll make a single feature, then blast a hole in it and use Fill to patch it over because the single feature doesn't do a particular area very well. I agree its very model specific.
The only rule I have about this is that there are no rules. The unsatisfcatory answer is "it all depends". I wish I had some meta-rule that would help me figure out when I should make something with one surface or three, but I usually end up learning which way is best by trying all of the possibilities I can think of. And then someone else usually comes up with a better way of doing it. The closest I can come to a rule is that when the "character" of the surface changes, you should probably make it a separate surface. But sometimes you can do a single surface that has regions with multiple characters that looks better, or at least as good, as multiple surfaces.
Looking at my attached parasolid example file again, I do notice that the one with less edges still keeps 4 sided-ness.
Maybe that's at least one good factor to gauge when you can, and when you can't, make everything just one big surface.