Nell: An SVG Drawing Language

Nell: An SVG Drawing Language

by Stefan Hollos and J. Richard Hollos

Copyright © 2020 by Exstrom Laboratories LLC

All rights reserved. No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher.

Abrazol Publishing

an imprint of Exstrom Laboratories LLC

662 Nelson Park Drive, Longmont, CO 80503-7674 U.S.A.

Publisher's Cataloging in Publication Data

Hollos, Stefan

Nell: An SVG Drawing Language / by Stefan Hollos and J. Richard Hollos

p. cm.

ISBN: 978-1-887187-40-4

Library of Congress Control Number: 2020932538

1. Computer graphics 2. SVG (Document markup language) 3. Programming languages (Electronic computers)

I. Title. II. Hollos, Stefan.

T385 .H65567 2020

N7433.8 .H65567 2020

006.66 HOL

About the Cover: Images created with NELL.

Table of Contents

Introduction

Drawing Definition File

Polygons

Circles

Ellipses

Arcs

Bezier Curves

Gallery

Useful Identities

Acknowledgments

About the Authors

Thank You

Introduction

Nell is a language for describing two dimensional vector drawings. We have been using versions of the language for several years to create illustrations for books, web pages and various art projects. It can even be used as a tool for physics and math simulations and experiments. You can for instance do optics ray tracing with it. There are a couple of examples later in the documentation that show how to do this.

We created the language because of our frustration with how tedious and time consuming it can be to create complex vector drawings. Why sit in front of a screen for hours clicking and dragging with a mouse when you can spend a few minutes thinking about your drawing, solving a few geometry and trigonometry problems and then writing a simple program that will create the drawing for you. It's less tedious and a lot more fun, plus you get to sharpen your math and programming skills.

A big advantage of using Nell is that you can parameterize the drawing. Lengths, angles, numbers of polygons, circles, ellipses, arcs, and curves can all be parameterized so you can easily experiment with different values and quickly see how the drawing changes. Another advantage is the simplicity of the language itself. It's so simple that it's easy to write programs that create Nell programs. We have used this technique to explore domains of possible drawings and have created thousands of interesting patterns in the process.

We usually use Nell to create just the basic outline of a complex drawing. Then if we want to decorate the drawing with different colors and line styles we use an interactive vector drawing program such as Inkscape. This is because there is currently no way to set stroke width and color for individual elements of a drawing or to set the fill color for individual polygons, circles and ellipses. We have thought about adding colors and styles to Nell but have decided against it to keep the language as simple as possible. Would you like to see colors and styles in the Nell language? Let us know what you think.

To understand how to use Nell it is helpful to be at least vaguely familiar with the rudiments of Cartesian or analytic geometry. This is geometry in which points, lines and curves are defined in a coordinate system. Coordinates are sets of numbers that are used to locate the position of a point in the coordinate system. Equations are often used to relate coordinates thus defining a set of points that make up a curve. Just the basics of high school algebra and geometry is all you need to use Nell. To be a power user you probably also need to know a little basic trigonometry.

Drawings in Nell are defined in a standard two dimensional Cartesian space where the positive x