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A bitmap vectorizer that can trace along the centerline of a stroke. The builtin inkscape 'trace bitmap' can only trace edges, thus resulting in double lines for most basic use cases. It uses 'autotrace -centerline' and an optimal threshold to vectorize a pixel image.

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inkscape-centerline-trace

A bitmap vectorizer that can trace along the centerline of a stroke. The builtin inkscape 'trace bitmap' can only trace edges, thus resulting in double lines for most basic use cases.

Download Source code and Debian/Ubuntu package

Written with python-Pillow. It uses 'autotrace -centerline' and an optimal threshold to vectorize a pixel image. See centerline-tracing.svg for an illustration of the idea.

In inkscape it shows up under Extensions -> Images -> Centerline Trace ...

Installation hints

  • Install the extension (all operating systems)

    • the extension requires the installation of autotrace and python-pil (see below)
    • grab a binary package or
    • download the zip file of inkscape-centerline-trace and unpack it
    • copy the files centerline-trace.inx, centerline-trace.py to your Inkscape User extensions folder (see Edit > Preferences > System: System info: User extensions)
  • Install autotrace / python-pil

    • Windows
    • MacOS
      • Install autotrace MacOS.zip version 0.40.0 or later from e.g. https://github.com/autotrace/autotrace/releases
      • open a command line shell to install the python-PIL module:
        • sudo easy_install install pip
        • sudo pip install pillow
      • open a command line shell to install the inkscape extension:
        • cd ~/.config/inkscape/extensions
        • curl https://raw.githubusercontent.com/fablabnbg/inkscape-centerline-trace/master/centerline-trace.py -o centerline-trace.py
        • curl https://raw.githubusercontent.com/fablabnbg/inkscape-centerline-trace/master/centerline-trace.inx -o centerline-trace.inx
      • Please report success in the github issues. As of 2018-08-31, MacOS Support is back, but highly experimental.
    • GNU/Linux
      • Install the autotrace DEB package version 0.40.0 or later from e.g. https://github.com/autotrace/autotrace/releases
      • if you do not have pillow/pil installed, the extension will output an error message prompting you to install it. On Ubuntu and derivatives, run sudo apt-get install python-pil to install.
  • Finally, restart inkscape, and find CenterlineTrace in the Extensions -> Images menue.

  • Since Inkscape 1.0 the Autotrace library including the CenterlineTrace feature is integrated. That should obsolete this extension. Find an entry in the new Path -> Trace Bitmap dialog.

Algorithm

The input image is converted to a graymap and histogram normalized with PIL.ImageOps.autocontrast. Optional preprocessing: equal illumination, median denoise filter.

Autotrace needs a bi-level bitmap. In order to find the best threshold value, we can run autotrace at multiple thresholds and evaluate the result candidates.

We count the number of line segments produced and measure the total path length drawn. The svg that has the longest path but the least number of segments is returned.

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A bitmap vectorizer that can trace along the centerline of a stroke. The builtin inkscape 'trace bitmap' can only trace edges, thus resulting in double lines for most basic use cases. It uses 'autotrace -centerline' and an optimal threshold to vectorize a pixel image.

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