The mid-point inference is easy to find.

SketchUp is a program that tries to make things easy. There are inferences at the ends and midpoints of every line. When you get close to one with the mouse cursor a colored dot appears along with a yellow tag. Click the left mouse button when the dot or tag is visible and the geometry you’re working with snaps to that point. But once in a while a point you think should be there isn’t. And if what you want to make doesn’t begin on an inference you need to do some head-scratching and take an extra step to get things where you want them.

A question from a student arrived in my inbox today. He wanted a big rectangle with a little square in each corner, and in the middle of each line. The corners were a piece of cake, but making a centered square between the corners was proving difficult. To draw the square centered between the corners would require the creation of guidelines to create a starting point for the little square. Not the end of the world, but not the most efficient way either. When I come across this kind of thing, my solution is this:

Draw it where it is easy to draw, then move it where it belongs.

There is an inference on the little square, but what happened to the one we used to draw that little square?

To make a rectangle you start with one of the corners. Click on the mid-point of the line, let go of the mouse button and drag diagonally. When it looks about right, type 3,3 and hit the Enter key. That creates the square the correct size, but not quite where it belongs. It needs to move (in this case to the left) so that the mid-point of the square is on the mid-point of the original line. Click inside the square, get the Move tool and click on the midpoint of the square.

Here is where things get dicey. When you start to move the little square there isn’t a mid-point inference where it ought to be, if you’re looking for the same point you used to start the small square. Where did it go? In SketchUp, lines drawn on top of other lines don’t pile up, they cut the lines below at the intersections. Drawing the square changed the line it was drawn on. Before we had one line between the two corner squares. After making the middle square we have three lines; two parts of the original line and the top line of the square. To get the little square centered, it needs to move 1.5″ to the left (half the length of one side of the square).

It may get frustrating, but SketchUp is behaving according to its rules. There is still a mid-point inference on the perimeter line of the larger rectangle, but there are now three lines where there once was one. This video shows you what happens, and how to work around it.

— Bob Lang

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