Why does cetz not allow to draw the arc with the circle element?

hexiongwu1995 · October 12, 2026, 4:48pm

#import "@preview/cetz:0.5.2"
#import calc:*

#cetz.canvas(length:1cm,{
  import cetz.draw: *
  set-style(line: (stroke: (paint: rgb("#55aaaa"), thickness: 1pt), 
mark: (end: (symbol: "stealth", fill: rgb("#55aaaa"), scale: 0.8))))
  line((-3, 0), (3, 0))
  line((0, -3), (0, 3))
  grid((-3, -3), (3, 3), step:0.5, stroke: (paint: rgb("#55aaaae8"), thickness: 0.1pt))
  // draw an arc
  arc((0, 0), start:45deg,stop:135deg, radius: sqrt(2) * 1.5)
  // how can I use the circle element to draw the same arc?
  // why does cetz not allow to draw the arc with the circle element?
})

Y.D.X · October 13, 2026, 5:40am

I’m not 100% sure, but I guess it’s because circles and arcs are implemented differently.
A circle in CeTZ turns into low-level directives for drawing circles, but an arc in CeTZ is approximated by directives for drawing Bézier curves, because the directive for drawing arcs does not exist.

Check Visualize - Typst Documentation and you’ll notice that Typst provides circle and curve, but not arc. Perhaps that’s limited by the SVG or PDF standard?

Further details on CeTZ.

Here’s the source of the approximation in CeTZ:

github.com/cetz-package/cetz

src/drawable.typ

2477845c6


      
          /// - y (float): The $y$ position of the start of the arc.
          /// - z (float): The $z$ position of the start of the arc.
          /// - start (angle): The angle along an ellipse to start drawing the arc from.
          /// - stop (angle): The angle along an ellipse to stop drawing the arc at.
          /// - rx (float): The radius of the arc in the $x$ axis.
          /// - ry (float): The radius of the arc in the $y$ axis.
          /// - mode (str): How to draw the arc: `"OPEN"` leaves the path open, `"CLOSED"` closes the arc by drawing a straight line between the end of the arc and its start, `"PIE"` also closes the arc by drawing a line from its end to its origin then to its start.
          /// - fill (color,none): The color to fill the arc with.
          /// - stroke (stroke): The stroke of the arc's path.
          /// -> drawable
          #let arc(x, y, z, start, stop, rx, ry, mode: "OPEN", fill: none, stroke: none) = {
            let delta = calc.max(-360deg, calc.min(stop - start, 360deg))
            let num-curves = calc.max(1, calc.min(calc.ceil(calc.abs(delta) / 90deg), 4))
          
            // Move x/y to the center
            x -= rx * calc.cos(start)
            y -= ry * calc.sin(start)
          
            // Calculation of control points is based on the method described here:
            // https://pomax.github.io/bezierinfo/#circles_cubic
            let segments = ()

Calculation of control points is based on the method described here:
A Primer on Bézier Curves

You can observe the approximation by drawing a red arc on top a green circle. The difference is noticeable when the radius r is 5000.

Code

#import "@preview/cetz:0.5.1"
#let r = 5000 // Change this number
#cetz.canvas(padding: (left: -2 * r, y: -r), {
  import cetz.draw: *
  circle((0, 0), radius: r, stroke: black + 0.5pt)
  circle((0, 0), radius: r, stroke: green + 0.5pt)
  arc((r, 0), radius: r, start: 0deg, stop: 90deg, stroke: red + 0.5pt)
})
$r = #r$

jwolf · October 25, 2026, 10:39am

You can use `arc(position, …, anchor: “origin”)` to use `position` as the circle center. Cetz uses up to 4 beziers for both circles and arcs, so the Typst primitive is not the reason both are different elements.

I guess adding a start/stop argument to circle would be fine, but I am not sure it is really needed.
Circles cannot have marks for instance, while arcs can, so `arc` is the more flexible element.