Vector Drawing

Vector Data

The Pixel Data chapter discusses working directly with pixels, the fundamental, low-level unit of raster graphics. This chapter looks at vector graphics, which represent images as high-level shapes. Scalable Vector Graphics, or svg, is a common XML-based vector graphics format. SVG files look like this:

<?xml version="1.0" encoding="UTF-8" ?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360 144">
    <rect fill="#092" y="89.43" width="360" height="54.57"/>
    <rect fill="#0af" width="360" height="90"/>
    <rect fill="#fff" x="252" y="54" width="36" height="36"/>
    <polygon fill="#f00" points="288 54 270 36 252 54 288 54"/>
</svg>

SVG files are fairly easy to read and write. One way to get started with vector drawing and code is to write SVG directly by hand and then view them in software like Illustrator or Figma. While this works, SVG is more of an interchange format and isn’t really intended for hand coding. In particular, SVG is declarative and doesn’t have the imperative control structures—like if conditionals and for loops—you may be used to using in languages like Javascript.

In this chapter, the code examples will use Paper.js, a library for working with vector drawing in JavaScript. With Paper.js, you have the full power of JavaScript running in a browser and can export SVG files to work with them in other programs.

Talia Cotton (with Pentagram)

Central Park Typestester, 2021

The letterforms in this typetester are comprised of leaves that are individual SVGs.

Central Park TypetesterProject Pagetaliacotton.comArtist Site

YesYesNo

Más Que la Cara, 2017

An interactive installation in downtown Houston using paper.js and facial detection.

Más Que la CaraCase Studyyesyesno.comArtist Site

Matthias Dörfelt

Weird Faces, 2012-13

Matthias's procedurally generated illustrations capture a hand-drawn quality.

Weird FacesProject PageWeird Faces Vending MachineExtended Projectmokafolio.deArtist Site

Matthias Dörfelt

Cables, 2020

Matthias computationally generated these abstract forms and then printed them on risograph.

CablesProject Pagemokafolio.deArtist Site

Scott Turner

Here Dragons Abound, 2016—2021

This long-running process blog documents years of work generating SVG maps, detailing each feature and technique.

Here Dragons AboundProcess Blog

Meaning vs. Form

Vector graphics must be converted to raster graphics—a process called rasterization—in order to be viewed on a pixel-based display. Vector graphics contain all the information necessary to do so, but you can not directly convert from raster images to vector images: In order to do so you would have to make inferences about the meaning implied by the raster image. Form is ambiguous and inferences are just guesses; they may be wrong.

Consider the image below. What would the image look like if the black circle were removed?

When we look at the image, we quickly infer that it represents two overlapping circles on a blue rectangle, but the raster image doesn’t contain information about shapes at all. It contains only blue, red, and black pixels. Humans are very, very good at inferring meaning from visual forms. We fill in meaning without conscious thought. When we look at the image above we see the circles automatically.

Because we are so good at inferring meaning, meaning is always available to us. Computers are not good at inferring meaning from form. A human can guess that a red circle lies behind the black one, but a computer can’t—so the human can imagine the drawing without the circle, and the computer can’t.

This is the key advantage of vector graphics over raster graphics: vector graphics contain high-level meaning about the image that they represent. They are built from semantic units like rectangles and ellipses instead of nonsemantic pixels. This meaning allows computers to easily make high-level changes: we can scale the vector image up and perfectly fill in the needed additional detail; we can change the fonts used to render text; we can remove a shape and reveal what is behind it.

A Hybrid Workflow

Using a vector-based graphics library allows us to computationally generate images, export them in a way that preserves their high-level shape information, and then further manipulate them in vector editing software.

Building a workflow like this, that combines custom coding with manual manipulation and prebuilt vector effects, can be a powerful approach to exploring form.

Paper.js

This section introduces Paper.js, a JavaScript vector library developed by Jürg Lehni and Jonathan Puckey.

Paper.js is an open-source vector graphics scripting framework that runs on top of the HTML5 Canvas. It offers a clean Scene Graph / Document Object Model and a lot of powerful functionality to create and work with vector graphics and bezier curves, all neatly wrapped up in a well-designed, consistent, and clean programming interface.

About Paper.js

You can find an overview of Paper.js features, a set of tutorials, and a complete API Reference on the Paper.js site.

PaperScript vs. JavaScript

Paper.js is a JavaScript library, and can be used with JavaScript directly or with a special version of JavaScript called PaperScript. Paper.js is easier to use with PaperScript, at least for small projects. You can find info on setting up your workspace here: Working with Paper.js

You can also use PaperScript with this site’s code example editor. You can tell the editor you are using PaperScript with a special comment: // language paperscript.

Here is an example of a Paper.js program written in PaperScript:

Library Exports

In PaperScript, the library contents are exported to the scope of the program and can be accessed directly. If you use Paper.js with JavaScript, you will access the library through the paper object.

PaperScript:

var path = new Path();

JavaScript:

let path = new paper.Path();

Operator Overloads

PaperScript lets you use mathematical operators to work with Point and Size objects. You can see this on line 12 in the example above. circle1Location + new Point(100, 0) adds two points together by adding their components. {x: 10, y:10} + {x:5, y:5} = {x: 15, y:15}. Allowing custom addition procedures for specific types of operators is called operator overloading. JavaScript doesn’t have operator overloading so you’ll have to add each component separately or use Paper.Point’s add method.

PaperScript:

var point1 = new Point(5, 10);
var point2 = new Point(10, 20);
var result = point1 + point2;

JavaScript:

const point1 = new paper.Point(5, 10);
const point2 = new paper.Point(10, 20);
const result = point1.add(point2);

Let and Const Do Not Work in PaperScript

Modern Javascript allows you to declare variables with let and const. PaperScript is based on an older version of JavaScript and supports only var.

PaperScript:

var point1 = new Point(5, 10);

JavaScript:

let point1 = new paper.Point(5, 10);

Retained Mode vs. Immediate Mode

The p5.js library is an immediate-mode graphics library. When you call rect() or ellipse(), p5.js immediately draws the shape to the canvas.

Paper.js is a retained mode graphics library. When you call Path.Rectangle() or Path.Ellipse(), Paper.js creates object instances to represent those shapes and adds them to a scene graph. The scene graph is editable: you can add, remove, and reorder shapes in the graph and you change their properties. The shapes in the scene graph are all drawn to the canvas at once, when you are done making changes to it.

Creating an animation in p5.js involves constructing and drawing the entire canvas every frame. In contrast, Paper.js redraws the shapes in your scene graph every frame. To create an animation all you have to do is change the properties of your already existing shapes.

Notice that we don’t need to create the rectangle or the ellipse in every onFrame() call. They are created only once before the animation begins. We also don’t need to set the color of the circle every frame. We can set it just when it changes, and that information is also stored in the scene graph.

SVG Output

Since Paper.js works in retained mode and stores your drawing as a vector scene graph, you can export your scene as an svg drawing that you can open in a vector editor like Adobe Illustrator.

The Project.exportSVG() function will create an svg representation of your project as a string. You need to do a little more work to download this data as a file. You can use the downloadAsSVG() in the example below to export your sketches. To export the example click it to focus it, and then press the s key to download the export.

Study Examples

Basics

This pretty minimal example shows the basics of drawing lines with Paper.js. It shows how to create and configure a path object, and how to add points to the path with moveTo and lineTo.

Boolean Operations

A super-useful feature of Paper.js is the ability to combine shapes with boolean operations. This allows you to describe complex shapes by building them from primitives.

Project Case Study: Paper Garden

Paper Garden is a program that generates stylized drawings of plants arranged in a wall garden. It is written in about 300 lines of PaperScript using Paper.js. Read the case study to learn about how this program works and explore the code.

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