Drawing Arcs in Pyside

Question

I'm trying to do draw widgets on the circle, for this I need to paint the widgets as a arc. I know the number of widgets (let's say), then each widget is at 36 degrees from the origin to the circumference. The information I have is the center of circle, radius and I know the starting and end point on the circumference for each such widget.

This is computed by doing

    dx = int(round(400 + 300 * np.cos(angle)))
    dy = int(round(400 + 300 * np.sin(angle)))

where angle = 2 * np.pi / 15 I go over a for loop computing the new value for angle which is basically angle * i where i = (1, 10)

I don't understand the start angle and span angle for the arcs function in QPainter.QPainter Arc. I googled and not many terms came up. Maybe there is a different term for them.

So the problem is I have a starting point and ending point on the circumference and center and radius, how do I use them to draw Arcs such that I get something that looks like :

circos

What I have tried is, I can compute the center point (cx) of the two end points, if I draw a line from the center of the circle to this point cx, then I can compute how far this point circumference which essentially is my width, but how to get the orientation correct to represent them as circles.

Instead of circular I do have a layout with just lines like this, but would like to be like the circos one. My image

Answer 1

I don't understand the start angle and span angle for the arcs function in QPainter.QPainter Arc.

Why? The documentation is IMHO very clear:

The startAngle and spanAngle must be specified in 1/16th of a degree, i.e. a full circle equals 5760 (16 * 360).

This means that your units are 1/16º. E.g. 45º is 45*16 units.

Positive values for the angles mean counter-clockwise while negative values mean the clockwise direction. Zero degrees is at the 3 o’clock position."

This means that 90*16 points at 12 o'clock (goes 90º counter-clockwise from 3 o'clock), and -90*16 points at 6 o'clock.

Of course the "zero" degrees only has sense for the start angle. The span angle states how much further does the arc go, and in which direction.

For example, to draw an arc from 3 o'clock to 12 o'clock, you'd do

painter.drawArc(rect, 0, 90*16)
*or*
painter.drawArc(rect, 90*16, -90*16)

But to draw an arc from 3 o'clock to 6 o'clock, you'd do

painter.drawArc(rect, 0, -90*16)
*or*
painter.drawArc(rect, -90*16, 90*16)

The arcs are not specified using center and radius, but rather using a bounding rectangle. If the arc was a full ellipse, it would be inscribed in the rectangle - the arcs are implicitly elliptical arcs.

So, given x and y centerpoint, and r for circular radius, the bounding rectangle is

rect = QRect(x-r, y-r, 2*r, 2*r)