The reason for wanting to see the curve before setting the end point is to be able to snap to the ‘tangent at vertex’ (purple arc) right away without resorting to using the tape measure when working with polygons.
It is because the inference system can not choose from which edge it should ‘tangent’ to. The vertices of a circle are ‘part of an endless’ curve, whereas the vertices of the polygon and the 4-sided polygon (rectangle) ‘belong’ to two edges.
AutoCAD has a “Tangent to” object snap, that allows the construction of arcs and circles tangent to one, two or three lines or edges. When this appeared (Version 12, if I remember correctly) it made life WAY easier.
Yes, it is possible to do the same thing by other means, but why, when there is obviously a solution.
AutoCAD object snaps may also be forced, rather than depend on an inference system, and THAT would be a super neat thing for SU/LO to implement.
@DaveBryce, you beat me to the punch. If all you need is a profile with a double tangent round or fillet, you COULD build two intersecting planes, round or fillet them using that extension, then cut across the result for the 2D representation. Lots of work, but hey, there are lots of operations in SketchUp that need workarounds. The only problem I’ve found with using Round Corner, is that the point where the curve begins isn’t always on a perpendicular to the tangent edge from the center of arc. A situation that shows up using Follow Me to perform rounds or fillets, and which is not unique to SketchUp. The extrusion tool in AutoCAD did that too. My guess is that it’s easier to code, and it’s pretty accurate for such things as bent tubing, but doesn’t work for cast pipe bends or similar fabricated corners (I can think of a dozen examples).
The comment by @MikeWayzovski, above, is pertinent, because arcs and circles in SketchUp, while calculated like circular curves, are in reality polygons with a fixed number of straight line segments (set in advance of creating the circle or arc). In programs such as AutoCAD, only the graphic representation of circles and arcs are straight line segments, and finding tangents between curves and straight lines, or two curves, is performed accurately. In SketchUp, it’s hit or miss. The straight line segment of a SketchUp curve may “jump” away from the true arc to perform the connection. You can minimize this by using a large number of segments BEFORE creating the arc, but you can’t eliminate this idiosyncrasy of SketchUp.
Until SketchUp implements forced object snaps on demand, an a true tangent join that is based on a calculated arc rather than a polygonal simulation, you may be forced to continue to use whatever work around you find least objectionable.
And this information is used (just like in programs such as AutoCAD) to calculate the tangent accurately. There are some extensions that overcome the jumpiness of the graphical representation (TrueArc), i like to change the number of segments AFTER I created the arc, to check if the graphical representation of the curve is acceptable.Then give it the accurate radius, and have that adjusted radius ‘in memory’ for quick filliting other corners by double click.
I have found that this method, altering calculated arcs into user-defined arcs, is least objectionable and give me more control than the forced object snaps in AutoCAD (I always forgot to switch them on or off)
It’s been years since I used AutoCAD (I much prefer SketchUp), but if I’m not mistaken, you could define hot keys that would temporarily override automatic object snaps, AND, if you wanted, you could define a hot key to toggle auto snaps on or off.
Speaking of hot keys, AutoCAD allowed not just single keys or ALT/CTRL/SHIFT+key combos, but things like “os” to toggle object snapping. One of my students had so many of these key combos that he needed a glossary to remember what everything did. But he was a really fast drafter!