# Straight Skeleton - or how to draw a center line in a polygon or shape?

**URL:** <https://discourse.processing.org/t/straight-skeleton-or-how-to-draw-a-center-line-in-a-polygon-or-shape/17208>\
**Category:** Processing\
**Created:** [January 20, 2020, 4:30pm UTC](https://discourse.processing.org/t/straight-skeleton-or-how-to-draw-a-center-line-in-a-polygon-or-shape/17208 "2020-01-20T16:30:24Z")\
**Posts on this page:** 1\
**Showing post:** 16

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**Author:** ![paulgoux](https://avatars.discourse-cdn.com/v4/letter/p/b9bd4f/32.png) [@paulgoux](https://discourse.processing.org/u/paulgoux)\
**Post date:** [February 2, 2021, 9:59pm UTC](https://discourse.processing.org/t/straight-skeleton-or-how-to-draw-a-center-line-in-a-polygon-or-shape/17208/16 "2021-02-02T21:59:36Z")

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I’ve made a sketch which does just this a while back. It was originally a canny attempt so essentially its calculating the skeleton of the edges, but is also able to calculate contours.

Theres currently a severe lack of comments on it and some of the other functions are nothing more than work in progress, but I’m happy to add comments and answer any questions you may have.

As for the algorithm used I just create a sobel of an image then for each black point I travel up down left right and diagonal in every direction until I find a white pixel store the steps required and then place a black pixel where the step count in opposite directions are equal to each other.

> [@OpenCV update, Heaviside Step Function, deriving parametric functions of shapes, Sobel filter](https://discourse.processing.org/t/opencv-update-heaviside-step-function-deriving-parametric-functions-of-shapes-sobel-filter/21931/10):
>
> They seem to use a wide range to techniques depending on the image type and the maths is hidden inside the Wolfram language. The HSF is used to limit the parts of individual curves to include in the calculation so is useful if you are trying to describe the shape using a single pair of parametric curves i.e. x=f(t) and y=g(t) which is what they have in the Einstein face drawing. In your code you are creating the twitter outline using a series of arcs and using the ca array to control the port…

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