Help with half dome

Eyal · January 21, 2019, 2:24pm

Hi everyone,

I have a problem creating a half dome.
here is what I did: PIC
Instead of getting clean half dome , I get a little slit on the top of it and it seems to be less than 180 degree.

thanks in advance,
Eyal

tweenulzeven · January 21, 2019, 2:32pm

you have to make sure your quarter circle is perpendicular to the start of the path.
Because of the segmentation of the circle it isn’t right now.

eneroth3 · January 21, 2019, 2:37pm

Follow me, as well as offset, can act quite weirdly on segment ends. SketchUp doesn’t understand the end of the arc is supposed to have a tangent not aligned along the end segment. I’ve tried to explain this problem to the developers but don’t know if they have any plans to look into it.

Anyway, I would recommend to use a full circle as path, and then delete half the dome.

slbaumgartner · January 21, 2019, 3:07pm

The issue you encountered results from two things:

SketchUp represents a circular arc using a series of straight edges that are chords of the true circle. When a vertex of the arc is on a particular axis, the adjacent edges of the arc won’t be perpendicular to that axis, i.e. aren’t tangents to the true circle.

SketchUp’s follow-me operation works by extruding a cross-section along a series of edges. The cross-section is created from the selected profile by projecting it perpendicular to the first edge. When the profile isn’t drawn perpendicular to the first edge, the cross-section is distorted during projection and is moved to the first place it can be perpendicular. That causes the shortages at the start and end that you are seeing (it also stops at the last perpendicular to the final edge).

As @tweenulzeven wrote, one fixup given the existing implementation is to rotate the path or profile so that they are perpendicular at the first edge of the path. Or, as @eneroth3 wrote, to do a full circle and delete half the dome. An alternative that also works for more complicated paths is to add an extra segment beyond the end of the path, draw the profile perpendicular to it, and delete the extra when the follow-me is complete.

As @eneroth3 observed, SketchUp could in principle detect when the path starts with a circular arc and modify the way it creates the cross-section to take into account the non-tangent nature of the first segment. Unfortunately, when as here there is a known workaround the developers tend to give low priority to the change request.