Plotting raster images

Pen plotters can only draw lines, typically in a single color, so getting them to reproduce a photograph is difficult. This page collects some ideas for how to plot images that are represented as a grid of colors, or a raster.

Resources

• The Plotting page at Mitxela has a lot of ideas for accommodating raster images in plotters.

• Halftone with Points And Lines has a collection of algorithms for making halftone images with vectors.

• Daniel Piker tried dot clustering and line clustering.

• Drawing Bot V3 is a popular Java application for preparing raster images for plotting, in a variety of styles, with support for using multiple pens to recreate full color images.

• vpype-pixelart is a plugin for vpype that plots pixel artwork.

• Take an image, apply contouring, and then discretize hash marks into each successive region.

  • Tom Duff is working on a system that uses the Canny edge detector to find edges and tried it with a SpaceX image.

  • linedraw is a similar project.

• Geoffrey Bradwen gave a talk at Plotter People 2019 called From Monotone to Grayscale.

Stippling

Stippling puts more points in darker regions of an image.

• StippleGen is the standard generator from the creators of The AxiDraw pen plotter.

Line density

This system puts more discrete lines in darker regions, and includes approaches like cross-hatching.

• Ben Golder generated great cross-hatched plots of 3D fossil models and gave an explanation to Michael Fogleman.

• Mycelium draws little lines in denser areas of a bitmap, like the tendrils of a fungus.

• Generating TSP art from a stippled image is Evil Mad Scientist's wiki page on the subject and briefly covered by John D. Cook in Traveling salesman art.

• A Raytraced sphere uses scattered raytracing and then connects the points with a Voronoi from a Delaunay triangulation.

• The vpype flow imager uses streamlines of various densities for shading.

Line deviation

One way a plotter can generate a darker region is to deflect the pen's movement when entering a shaded area and add more lines in darker areas.

• SquiggleDraw is the original bitmap vectorizer for pen plotters.

• Use Pythagoras' Theorem to Draw Pictures relies on line variation of horizontal lines to provide shading.

• Spiral Raster script is an online tool for using line movement to represent shading.

Thickness-based

These approaches rely on changing the thickness to represent density and shading. A pen plotter usually cannot vary the thickness of the lines it draws, so these will need modification to work for plotting.

• Vectoraster is a macOS app that converts raster images to vectors, but which might require varying line thickness to work properly.

• Vertigo is a slick demo that plots density using thicker lines or circles organized radially.

Artistic

These techniques compromise the faithful reproduction of the image for artistic purposes.

• A domino representation of Joe Biden is an off-the-wall and artistic approach to plotting photographs.

• Geometric squares is an example of using a grid of squares with varying density.

Image vectorization

Vectorizing raster images is a bit more loose than techniques which can only target pen plotters, but there might be useful techniques to glean from these approaches.

• Differentiable Vector Graphics Rasterization for Editing and Learning is a SIGGRAPH Asia 2020 paper on image vectorization.

• Using AutoTrace's centerline option for pen plotting shows an effective technique for converting line art in raster format to vectors.

• VTracer uses machine learning to vectorize images.

• Inkscape's bitmap vectorization finds outlines and then applies contouring to shade thresholds.

• Watertight vector maps from raster images describes approaches to vectorizing maps, where edges are shared.

• oneliner is a Rust program using the ideas in A Sub-Pixel Edge Detector: an Implementation of the Canny/Devernay Algorithm.

• Vectorizor.ai uses AI to convert images to vectors and is free while in beta.