svgpoly is a package to support SVG to polygon conversion

Overview

This package supports manipulating a polygon object representation of the content of SVGs. The package is for converting an SVG file input into to a collection of such polygons, and the reverse process.

The primary use case is digesting SVG files from KiCad and automating the outline generation from overlapping polygons. The package can import SVGs using svger and output in SVG format via svgof.

Exploring with examples

To be able to run some of the examples, confirm gnuplot is available on your system. Try:

$ gnuplot --version

If it is missing, install it (Fedora: sudo dnf install gnuplot, Debian: sudo apt install gnuplot). Then:

$ git clone https://github.com/tinkerator/svgpoly.git
$ cd svgpoly
$ go run examples/outline.go --svg examples/test.svg --hatch 0.3 | gnuplot -p

Which should render this processed (union) image:

• Note how this output is visually oriented the same as the input examples/test.svg but to conform to gnuplot conventions, we’ve negated the Y axis values relative to the down the page SVG Y axis conventions.

This example is more faithful to the raw input SVG image in terms of the overlapping polygons.

$ go run examples/outline.go --svg examples/test.svg --before --after=false | gnuplot -p

Which should render this image:

We can inflate the polygons by the value specified with the --inflate option.

$ go run examples/outline.go --svg examples/test.svg --before --inflate 0.2 | gnuplot -p

Which should render this image:

Finally, you can output *polygons.Shapes in the form of an SVG. The SVG follows the conventions that .Holes appear white and non-.Holes as blue. A red line is used to identify the outside of a hole shape (or, said another way, the inner edge of a solid shape-ring):

$ go run examples/outline.go --svg examples/test.svg --osvg output.svg --hatch 0.3

Which renders as follows:

The outline example also supports generating a negative of the polygons rendered as an SVG or as gnuplot input. To see this, for our examples/test.svg input, we can do the following:

$ go run examples/outline.go --svg examples/test.svg --osvg negative.svg --hatch .3 --inverse .5 --inflate .1

The outline program can also input and output polygon.Shapes in JSON format using the --poly <in.json> and --opoly <out.json> command line options. This can be useful when debugging polygon manipulation code.

The example/inflate.go is provided to visualize how the (*polygon.Shapes).Inflate() function works. The basic idea is that the outline of the shape is grown by some distance, and where the resulting outline is to be found is where lines at that distance intersect. This example program only outputs in SVG format. A simple invocation of this code is this:

$ go run examples/inflate.go --dest inflate.svg

Which generates three lines (of length 50, 40, 50) where each line’s exterior angle to the next is 30 degrees, it looks like this:

Here, the blue line should be thought of as the three segment section of the original shape’s outline and we are focusing on the inflation around the corner points where three lines connect. The green dashed lines are the outline of the inflated polygon on either side of that original line and the surviving outline corner points are represented as solid green circles. For an actual polygon shape, only one of these two dashed green lines would represent the actual inflation.

What you can see is that these dashed lines are at tangents to circles of radius --inflate (which defaults to 10) centered on the blue line with centers at the midpoint and the end points of each line segment. This first example is the simple case and does not need to merge any points.

A more complicated example is where the bend at the joins is so tight, it eliminates the simple points (rendered here as empty red circles) in favor of a single merged point (solid green):

$ go run examples/inflate.go --dest tight.svg --alpha 70 --beta 70 --mid 10

TODO

Nothing planned.

License info

The svgpoly package is distributed with the same BSD 3-clause license as that used by golang itself.

Reporting bugs

This is a hobby project, so I can’t guarantee a fix, but do use the github svgpoly bug tracker.