Visualising Bézier Curves Part II

Experimenting with ClojureScript

Posted by Maarten Metz on July 12, 2017
ClojureClojure ScriptBézierFunctionalKlipseCanvas

In my previous post I described how to visualise simple Bézier curves. I chose Clojure as implementation language. Unfortunately I had to ask you to jump through a couple of hoops in order to code along: installing leiningen, installing and configuring the lein-try plugin and maybe even installing the JVM.

This post will be easier for you - at least in terms of setup. You won’t have to leave your browser and can experiment with Clojure(Script) right here on this page!

NO Setup

In this post we’re going to use the very useful klipse plugin. Klipse is a client-side code evaluator pluggable on any web page. It can evaluate Clojure, ClojureScript, ruby, javascript, python, scheme, es2017, jsx, brainfuck, c++, reagent and probably much more languages in the near future. It’s like jsfiddle on steroids, right here in the 040code blog.

Let’s start with a very simple Klipse demonstration. I’m going to map the inc(rement) function over the numbers 1, 2 and 3. The result will not surprise you:

(map inc [1 2 3])

What might surprise you though, is that you can change the code in the klipse evaluator and see the results immediately in your browser. So for instance change (map inc [1 2 3]) into (map inc [41 999 2]) and be amazed. Or change it into something completely different like (filter even? (range 10)).

Defining functions and calling them? No problem:

(defn factorial [n]
  (if (= 1 n)
    (* n (factorial (- n 1)))))

(factorial 4)

Now call the factorial function with some other natural number (whole number > 0) and see the results.

For those of you who don’t know Clojure(Script), this function basically says:

  • define a function named factorial
  • let n be its argument
  • if n equals 1, return n
  • otherwise return the multiplication of n with the factorial of (n - 1)

Lisp in the browser. Embedded in this blog. No setup required. Excellent!

But hang on, better things will follow soon.

Clojure is designed to be a hosted language

Why reinvent your own platform, your own runtime, your own garbage collector, your own ecosystem, when all you need is a decent language? Clojure is designed to be a hosted language. It runs on:

Clojurescript - a compiler for Clojure that emits javascript - will be the tool we use in this blog to visualise Bézier curves. We are going to manipulate a html canvas right from a klipse plugin.

Bézier in ClojureScript

In my previous post I tried to explain how (simple) Bézier curves ‘work’. You might want to go there or scan the Wikipedia page on Bézier curves if you have no clue what I’m talking about. Otherwise, let’s dive right in and try to play with Bézier curves directly on this page.

;; Get a grip on the html canvas element I inserted on this page
(def canvas (.getElementById js/document "canvas-2d"))
(def ctx    (.getContext canvas "2d"))

;; Draws a blue circle with radius `r` on the canvas on point [x y]
(defn draw-point [x y]
  (let [r 5] ;; radius
    (set! (.-fillStyle ctx) "blue")
    (.beginPath ctx)
    (.arc ctx x y r 0 (* 2 Math/PI))
    (.fill ctx)))

;; Draws a red Bézier curve and its blue control points on the canvas
(defn draw-bezier-curve [[x1 y1] [x2 y2] [x3 y3]]
  ;; draw curve
  (set! (.-strokeStyle ctx) "red")
  (set! (.-lineWidth   ctx) 2)
  (.beginPath ctx)
  (.moveTo ctx x1 y1)
  (.quadraticCurveTo ctx x2 y2 x3 y3)
  (.stroke ctx)
  (.closePath ctx)
  ;; draw control points
  (draw-point x1 y1)
  (draw-point x2 y2)
  (draw-point x3 y3))

;; Center the *drawing* canvas within the *html* canvas and draw the curves
;; Reason for centering: keep the coordinates simple AND
;; see the complete shapes (not cropped at the edge of the html canvas)
;; make the ratio 1 to see what I mean with 'cropped at the edge'
(let [wc (.-width canvas)  ;; width of *html* canvas
      hc (.-height canvas) ;; height of *html* canvas
      ratio 0.9            ;; ratio of *html* canvas to use as *drawing* canvas
      t  (/ (- 1 ratio) 2) ;; translation constant
      w  (* ratio wc)      ;; width of *drawing* canvas
      h  (* ratio hc)      ;; height of *drawing* canvas
      x  (* 1/2 w)         ;; x val in the middle of 0 and w
      y  (* 1/2 h)]        ;; y val in the middle of 0 and h

  ;; clear *html* canvas
  (.clearRect ctx 0 0 wc hc)

  ;; Center the *drawing* canvas in the *html* canvas
  (.save ctx)
  (.translate ctx (* t wc) (* t hc))

  ;; Draw a grey border around the *drawing* canvas
  (set! (.-lineWidth ctx) 1)
  (set! (.-strokeStyle ctx) "grey")
  (.strokeRect ctx 0 0 w h)

  ;; Draw main 'anchor' points
  ;; This is what I meant with 'simple coordinates':
  ;; (draw-point 0 0)
  ;; (draw-point x y)
  ;; (draw-point w h)

  ;; Increasing ascending curve
  (draw-bezier-curve [0 h] [w h] [w 0])

  ;; Swoosh
  ;; (draw-bezier-curve [0 y] [0 h] [w 0])

  ;; normal curve
  ;; (draw-bezier-curve [0 h] [x 0] [w h])

  ;; my pulse after a useless meeting
  ;; (draw-bezier-curve [0 y] [x y] [w y])

  (.restore ctx))

Before reading further you might want to experiment a bit by (un)commenting code and seeing the results. (Un)comment the different Bézier curves and different control points in the Klipse evaluator and see what happens.


Scanning the code quickly without going into detail, this is what happens:

  • Get a grip on the html canvas element
  • Define functions to:
    • Draw (control) points
    • Draw Bézier curves with 3 control points
  • Draw the actual curves and points

Since Clojure is a hosted language, it must be able to access its host platform and libraries. Clojure and ClojureScript have good interop documentation so I won’t go into detail here, but we’re basically using these forms in this blog:

  • js/document => the global document object
  • (.beginPath ctx) => ctx.beginPath()
  • Math/PI => 3.141592653589793
  • (.-lineWidth ctx) => ctx.lineWidth

The last form is used for instance in (set! (.-lineWidth ctx) 1) and translates to ctx.lineWidth = 1. The more general syntax is (set! var-symbol expr).

The second form (.beginPath ctx) can also have arguments. The general syntax is (.instanceMember instance args*) in that case.

Other than that it’s basic Clojure and HTML Canvas functionality


Thanks to the klipse plugin and a bit of preparation from my side, you can now play around with Bézier Curves in ClojureScript directly in this blog. I do realise this post is probably not a compelling case for using ClojureScript:

  • Javascript “in the small” is not really the place where ClojureScript shines, especially when the largest part of that small program is javascript interop
  • ClojureScript fits large browser applications better, where you need sane state management, immutable datastructures, lazy sequences, and a fast, stable and robust language

I’m not interested in ‘religious’ discussions about technology A versus technology B. Clojure and ClojureScript are THE sane way forward for me, in my context. I hope to share the fun I’m experiencing with it. Goethe said it best:

It is always better to say right out what you think without trying to prove anything much: for all our proofs are only variations of our opinions, and the contrary-minded listen neither to one nor the other.

But with all this talk about language, we’re almost forgetting what it’s all about: building useful stuff and having a great time doing it.

I hope you enjoyed experimenting with Bézier curves. The Klipse Blog has several great examples of using Klipse for ‘interactive programming’, for instance to write data driven documents.

Thanks Niek for sharing the 040code repo with me. Please share your comments, suggestions and thoughts about this blog post on Thanks for reading and Happy Coding!