Stipplegen

Introduction

StippleGen is a free, open source, and cross-platform utility from Evil Mad Scientist Laboratories that can create stipple drawings and “TSP art,” from image files.

Why do this? One of the perennial problems that we come across in a variety of contexts, including CNC artwork and producing artwork for the Egg-Bot, is the difficulty of creating good-quality toolpaths– i.e., vector artwork representing halftones –when starting from image files. One of the finest solutions that we’ve ever come across is Adrian Secord's algorithm, which uses an iterative relaxation process to optimize a weighted Voronoi diagram, mathematically producing a set of points (stipples) that can closely approach the appearance of a traditional stipple drawing.

Another important technique is TSP art, where the image is represented by a single continuous path. You can generate a path like this by connecting all of the dots in a stipple diagram. Designing a route that visits each dot exactly once (and minimizing the distance travelled) is an example of the famous Travelling Salesman Problem (or just “TSP”), and an optimal TSP path can give a surprisingly good grayscale representation of an image. From the standpoint of toolpaths (for the Egg-bot and most other CNC machines), a TSP path is even nicer than stipples, because little or no time is spent raising and lowering the tool.

StippleGen is easy-to-use software that can generate TSP and stipple drawings from input images. It saves its files as editable, Eggbot-ready Inkscape SVG files, which can in turn be opened by other vector graphics programs, or re-saved as PDF files for use in other contexts. It can also generate a TSP path from the stippled image, and either save that path as an SVG file or simply use that path as the order of plotting for the stipple diagram.

You can read an extended introduction to StippleGen at Evil Mad Scientist Laboratories.

Download

StippleGen is free and open source software, written in the Processing development environment. It comes ready to run on Mac, Windows, and Linux, and it is available for download here.

(StippleGen is hosted as part of the Eggbot project on Google Code.)

The download includes ready-to-run platform-specific versions of StippleGen for Mac, Windows, and Linux, as well as the source code. You will need to have a recent version of Java installed on your computer. If the platform-specific version of the software won't run on your computer, please install Java and try again.)

Usage

When you first open up StippleGen, you will be presented with this window, which shows the drawing in progress (in the top part) and a set of controls below that. Rather than present you with a blank screen, StippleGen automatically loads a demo image (source) upon launch, and begins calculating. This way, you can try out the program, even if you don't have a good image of your own to start with. And, you can stop it at any time by loading a new image.

All of Stipplegen's controls are accessed through the graphical user interface located in the lower part of the screen, in the gray bar. In what follows, we will go over the usage of the individual controls within the interface.

Loading an image file

To load a new image file, click the "Load Image File" button that is located above the "save" buttons. You can open any image file in .png, .jpg, .tga, or (non-animated) .gif formats. The file name must end in one of the following: .png, .jpg, .tga, .gif, .PNG, .JPG, .TGA, or .GIF.

As soon as a new image is loaded, StippleGen will "sprinkle" an initial weighted distribution of points over the image and then begin to calculate an initial Voronoi diagram from those points. During this process, you will see something like the image shown above, which is the set of Voronoi cells for the initial point distribution. Depending on the number of stipples being calculated, this may be a matter of moments or minutes.

After the initial Voronoi diagram is calculated, StippleGen will display the stipple locations that it has calculated. This "first guess" is usually quite crude— as you can see above —but will improve dramatically if you allow it to run for a while.

The calculation of those points through Secord's algorithm is an iterative process. For any given "generation," the process begins with an initial set of points— whether that's the initial set of "sprinkled' points, or the output from the previous generation. The Voronoi diagram of those points is then calculated, and each point is moved to the weighted centroid of its Voronoi cell. That distribution of points serves as the starting point for the subsequent generation.

Source Code

The StippleGen source code is available as part of the StippleGen .zip file. To run the code from source, you'll need to download Processing, and install the toxiclibs library and the ControlP5 library.