0059: Cairo III – Circles and Arcs

Let’s jump right in and look at…

Drawing a Circle

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

The essence of circle drawing is this:

bool onDraw(Scoped!Context context, Widget w)
{
	context.setLineWidth(3);
	context.arc(330, 160, 40, 0, 2 * 3.1415);
	context.stroke();
		
	return(true);
		
} // onDraw()

And even though you might expect a circle() function, we actually use arc()… and since an arc is just an incomplete circle, well… there you go.

The arguments to the arc() function are:

  • x position of the circle’s center,
  • y position of the circle’s center,
  • the circle radius (as if the circle were complete),
  • the point along the arc where we start drawing, and
  • the point along the arc where we stop drawing.

Those last two arguments are in radians. That’s why the point where we stop drawing is two times PI.

The Wikipedia/Cairo Anomaly

When I first started exploring the arc() function, I wasn’t getting the results I expected. My confusion was twofold:

  • I had assumed that 0 degrees (and therefore 0.0 radians) was true north like it is on a compass, and
  • I also assumed that, like a compass, degrees increase in a clockwise direction around the circle.

So, I Googled radians and read the Wikipedia page. However, their conversion chart showed 0 to the east, compass-wise, and both degrees and radians increasing counterclockwise.

Eventually, I realized that Cairo agreed with Wikipedia on one count, but not the other.

So, here’s the straight dope on radians according to Cairo:

  • 0 is to the east (compass east), and
  • radians increase in value in a clockwise direction.

Not being much of a mathematician, I have no idea why 0 faces east and I certainly don’t know why Wikipedia’s conversion chart has radians increasing in a counter-clockwise direction. But Cairo agrees with real-world clocks and compasses, so at least we have those as jumping off points for understanding what’s going on with Cairo arcs.

So I took the liberty of reproducing the conversion chart so it accurately reflects Cairo’s worldview:

Figure 1: Degrees and Radians, Clockwise
Figure 1: Degrees/radians conversion in the Cairo worldview.

Drawing an Arc

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

You may be ahead of me on this one, but drawing an arc is the same as drawing a circle except that the start and end points stop short of describing the entire circle:

bool onDraw(Scoped!Context context, Widget w)
{
	float x = 320, y = 180;
	float radius = 40, startAngle = 0.7, endAngle = 2.44;

 	// draw the arc
	context.setLineWidth(3);
	context.arc(x, y, radius, startAngle, endAngle);
	context.stroke();

	return(true);
		
} // onDraw()

If you consult the conversion chart, you’ll see that the arc starts at about 40 degrees (south-southeast in Cairo’s version of the universe) and goes to 140 degrees. And remember, drawing is done in a clockwise direction, so even though the start point is to the right of the end point (effectively, right to left) it’s still a clockwise stroke().

Conclusion

Next time, we’ll continue on and cover fills, then double back and have a bit of fun with arcs.

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© Copyright 2019 Ron Tarrant