How to create a half-circle

Version 1
Last Modified: Aug 12, 2012 10:42 AM

Use this tip to draw a half-circle that you can then use to slice into sub sections, add arrowheads, etc. Keynote doesn't allow you to "break" a segment, so in order to create a shape with two endpoints (instead of a continuous line) you have to use the Draw with Pen tool.


To prepare:

Go into Preferences and, under Rulers in the Alignment Guides section, select both Show guides at object center and Show guides at object edges.


Now, first draw a starter circle that you'll use as a guide

1) Create a circle by selecting Insert -> Shape -> Oval from the menu

2) Hold down Shift and enlarge it (say, 500x500) to give you room to work without zooming in

3) Use the alignment guides to move it to the center of the slide (it will snap into place with yellow lines extending all the way to the edges of the slide horizontally and vertically)


Now, draw the line that will become your half-circle

1) Start the line by selecting Insert -> Shape -> Draw with Pen

2) Create 5 points

  • One to the left of the first circle below the midpoint top-to-bottom
  • One close to touching the left side of the first circle near the top-to-bottom midpoint
  • One near the center of first circle
  • One close to touching the right side of the first circle near the top-to-bottom midpoint
  • and finally, one to the right of the first circle below the top-to-bottom midpoint.

3) Press Escape to stop drawing


When completed, the two shapes, one on top of the other, should look something like this:

Figure 1.png


Now, we'll use the alignment guides to help transform this angular shape into the smooth curve we're looking for.

Counting the points from left to right,

1) Select point number 2 (You'll have to click the shape to select it and show all the points, first) and drag it so that you snap to both the right edge of the full circle and the vertical center of the slide. In this case, the center of the slide and the center of the circle will be the same. Your alignment grids should show up like the below (ignore the red circle around the point,that was from the screen capture software I used).

Figure 2.png

Note that the vertical line stops at the bounds of the first circle, indicating the point is aligned with that edge. The horizontal line goes across the entire slide.


2) Select point number 3 (which should be near the middle of the first circle) and drag it so that it aligns to the top edge of the first circle and the horizontal center of the slide. It should look like this (again, ignoring the red circle).

Figure 3.png

One thing you may find interesting here is that my arrow isn't directly over the point! That's because the alignment guides are moving it to where it needs to be based on the general position of my cursor.


3) Finally, point number 4 (which was the one to the right, near the right edge of the first circle) should be dragged into position so that it snaps to the right edge of the first circle (basically, the alignment guides should look like the ones for the left point shown above).


Stepping back again to look at the big picture, your current shapes should look something like this.


Figure 4.png


Now, things get a little tricky as we've got to change the corner points into curves that match the more circular first shape.


Go back to point number 2 and double click it. You shoud get adjustment handles like these


Figure 5.png

The tricky part is that now you have to

1) Hold down the Shift key

2) Grab the top handle

3) and move it straight to the left


There's no tip or key that will end up with you moving it EXACTLY to the left, but holding the Shift key means that it will snap to vertical as soon as you move to the left far enough. Once it snaps over, release and it should be like this.


Figure 6.png

You may want to go ahead and do the right side now as well as it's basically the reverse of what you just did:

1) Double-click the point to make it a curve with adjustment handles

2) Hold down the Shift key

3) Grab the top handle

4) and move it straight to the right


Now you've got a curve on the left and a curve on the right, but that top point is still waaaay to pointy. As with the left and the right sides, step 1 to resolve this will be to:

1) Double-click the point to make it a curve with adjustment handles


Now that gets you really close to matching the circle below as you can see here:


Figure 7.png

All that's needed is to stretch the curve on the left AND on the right, by the same amount directly horizontally. This is how you accomplish that.

1) Hold down the Shift key and the Option key (Shift will constrain the move so that you can drag perfectly horizontally, Option will force the adjustment handles to mirror each other)

2) Click the adjustment handle on the right, then drag it to the right until you no longer see the circle below. For those of you who prefer the left handle, you are correct in assuming it will work the same way.


If you do this carefully and try to match the shape below as much as possible, you should end up with this:

Figure 8.png


The only steps remaining are to remove the first point and the fifth point and you should have a very good representation of a half circle that you can apply an arrow to.


Figure 9.png