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One True Path

A general-purpose library for working with curves and paths.

The primary aim is SVG paths, but the types and functions in this package can also be used for animations or other graphics backends (webgl, canvas).

Additionally, this package is meant to serve as an interchange format between packages.


The nicest module to use is the Curve module. It contains helpers for making all sorts of curves with different interpolation modes (how to connect points). For instance linear

or stepped

The code for drawing the letter H looks like this:

import SubPath exposing SubPath
import Curve
import Svg exposing (Svg)
import Svg.Attributes exposing 
    (width, height, viewBox, fill, stroke)

hShape : SubPath 
hShape =
        [ ( 0.3, 0.2 )
        , ( 0.4, 0.2 )
        , ( 0.4, 0.45 )
        , ( 0.6, 0.45 )
        , ( 0.6, 0.2 )
        , ( 0.7, 0.2 )
        , ( 0.7, 0.8 )
        , ( 0.6, 0.8 )
        , ( 0.6, 0.55 )
        , ( 0.4, 0.55 )
        , ( 0.4, 0.8 )
        , ( 0.3, 0.8 )

logo : Svg msg 
logo = 
    Svg.svg [ width "50", height "50", viewBox "0 0 1 1" ] 
        [ SubPath.element hShape 
            [ fill "none", stroke "black" ] 


When you need more control and want to move/rotate/scale or connect curves, the subpath module lets you do that.

  • connect: draws a straight line connecting two subpaths (end to start)
  • continue: make the start and end point of two subpaths coincide
  • continueSmooth: make the start and end point of two subpaths coincide, and rotate to make the transition smooth.

A SubPath can be ArcLengthParameterized, to sample the curve, animate along it or simply get its curve length.

<iframe src="" width="100%" height="400px" scrolling="no" frameBorder="0"></iframe>

full source

Finally, a piece of svg path syntax can be parsed into a list of SubPaths

import Path 
import SubPath

pathAsString = 
  M 213.1,6.7
  c -32.4-14.4-73.7,0-88.1,30.6
  C 110.6,4.9,67.5-9.5,36.9,6.7
  C 2.8,22.9-13.4,62.4,13.5,110.9
  C 33.3,145.1,67.5,170.3,125,217
  c 59.3-46.7,93.5-71.9,111.5-106.1
  C 263.4,64.2,247.2,22.9,213.1,6.7

path = 
    Path.parse pathAsString
        |> Result.toMaybe
        |> Maybe.andThen List.head


The Segment module is ideal for more advanced mathematical operations or for conversion between formats. A Segment is a mathematical shape: a single line segment, quadratic or cubic curve, or elliptical arc. A subpath can be transformed into a list of Segments.

Segment is a unifying wrapper around the equivalent OpenSolid shapes. If you really need full control, you can access the OpenSolid values and use that package to modify your geometry.


The Path module is a convenience for when you have a list of SubPaths and want to render them into one path element.

The LowLevel.Command module is meant for package authors. It allows more control over the generated svg instructions, but is pretty cumbersome to work with. Try to stay away from it.

Writing svg to a file

You can use the elm-static-html-lib js/typescript package for that. If you only want the path string, use SubPath.toString in combination with Html.text.


That's not part of this package, but I'm looking into it. The julia Compose.jl library has some interesting ideas.


Creating a nice interface for SVG paths in Elm







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