How to create loop on a single point in fletcher diagram?

[Paul744]

I am trying to recreate the following Feynman diagram

but I cannot get the loop on the one vertex to work. I have

#set page(
  width: auto,
  height: auto,
  margin: 2mm,
)

#import "@preview/fletcher:0.5.8" as fletcher: diagram, node, edge, cetz
#import fletcher.shapes: circle, ellipse, cylinder, rect, chevron

#diagram(
  node((0, 0), name: <v1>),
  node((30deg, 2cm), name: <v2>),
  node((330deg, 2cm), name: <v3>),
  edge(<v1>, <v3>),
  edge(<v1>, <v2>),
  edge(<v2>, <v3>),
  edge(<v2>, <v3>, bend: 40deg),
  edge(<v2>, <v3>, bend: -40deg),
)

which does everything else just fine, but when I try to add a line like

edge(<v1>, <v1>, "--|>", loop-angle: 120deg, bend: 100deg),

all it does is place an arrow on top of the vertex. I know, in the manual, the loops only work when the vertex had an actual shape like a circle on it, so perhaps this is the wrong method, but if so, how do I get loops on points to work?

As a secondary question, are there any GUI editors yet for making cetz or fletcher diagrams? I have some much larger diagrams I’d like to make, but it’d be a lot easier ot make via drag-and-drop, and then getting the code for the diagram, something like:

CircuiTikZ Designer

Thanks for the help!

[Andrew]

@Paul744

I am trying to recreate the following Feynman diagram

FYI, there is inknertia – Typst Universe. If that misses something, the package can be improved to handle more use cases.

[Paul744]

Yes, I did see that one, but from what I saw, it also didn’t have a method for that. Also, I prefer to use a package that provides a more flexibility, and thus I like how fletcher provides an escape hatch to Cetz, while this package doesn’t seem to, at least from what I have seen.

I thought of a solution, but it’s hacky. I’ll mark it as a solution, but I’m not necessarily happy with it.

#set page(
  width: auto,
  height: auto,
  margin: 2mm,
)

#import "@preview/fletcher:0.5.8" as fletcher: diagram, node, edge, cetz
#import fletcher.shapes: circle, ellipse, cylinder, rect, chevron

#diagram(
  node((-0.5cm, 0cm), name: <v0>),
  node((0, 0), name: <v1>),
  node((60deg, 1cm), name: <v2>),
  node((300deg, 1cm), name: <v3>),
  edge(<v1>, <v3>),
  edge(<v1>, <v2>),
  edge(<v2>, <v3>),
  edge(<v2>, <v3>, bend: 25deg),
  edge(<v2>, <v3>, bend: -25deg),
  edge(<v0>, <v1>, bend: -90deg),
  edge(<v0>, <v1>, bend: 90deg),
)

Basically, I create another vertex to act as a second point to form a circle with (v0 and v1) form antipodal points for a circle. The output is:

[Jollywatt]

Yeah, it’s quite awkward to make loops…

This doesn’t work because the radius of a bending edge is determined by the points it snaps to a node, but in this case the nodes are zero-radius so you get an infinitely small bendy edge.

The situation will be improved in fletcher v0.6.0, where there will be better support for loops (i.e., an option to set its radius) along with the ability to just use any CeTZ path.

In the meantime, here’s another hack that you may prefer to your smart hack:

#import "@preview/fletcher:0.5.8" as fletcher: diagram, node, edge, cetz
#import fletcher.shapes: circle, ellipse, cylinder, rect, chevron

#let loop(v, angle, loop-radius: 5pt, ..args) = {
  let delta = 1e-2pt
  let b = 90deg + calc.acos(delta/loop-radius)
  edge((rel: (angle + 90deg, delta), to: v), (rel: (angle - 90deg, delta), to: v), bend: b, ..args)
}

#diagram(
  node((0, 0), name: <v1>),
  node((30deg, 2cm), name: <v2>),
  node((330deg, 2cm), name: <v3>),
  edge(<v1>, <v3>),
  edge(<v1>, <v2>),
  edge(<v2>, <v3>),
  edge(<v2>, <v3>, bend: 40deg),
  edge(<v2>, <v3>, bend: -40deg),
  loop(<v1>, 180deg, loop-radius: 3mm)
)

It works by nudging the end points of the loop edge in opposite directions and calculating a bend angle very near 180° that results in the desired loop radius.

[Paul744]

Thank you @Jollywatt. I’m eagerly waiting to see the updated API of v0.6.0.