Scratchboard is an art material that is used for making high contrast black and white manual etchings. It's made of a board surfaced with white clay, which is then coated with a deep black ink.

Michaelangelo famously said "Sculpture is made by taking away, while painting is made by adding." - that is literally also the essence of Scratchboard art.. If you were to draw in the normal way on scratchboard you would in fact produce an image like a photographic negative, which is a common mistake among beginning scratchboarders. Instead, you scratch off the black surface to reveal the white below, which requires a thought process quite unlike that of normal drawing, making it a technically skilled art with relatively few practitioners.

Creating a scratchboard drawing is a time-consuming process, and before I started this project, one that I believe has never been done with computer assistance. People have generated scratchboard-like computer graphics but have not made them into real scratchboards.

In this instructable I will show you how to use a vinyl cutter or a CNC engraver to create real scratchboard art; I'll also briefly describe how to write custom software to convert a raster photograph into a set of curved lines in order to drive the plotting device. This is a project I've been working on for many months that has finally started to produce some tangible results; I will be publishing the source code once it capable of consistently producing good art, after after I've confirmed copyright clearance with the authors of some of the components that I've had to modify.

(This engraving of Steve Jobs, derived from the cover photo from his biography by Walter Issacson, is my very first success; I expect the quality of the rendering to improve as I perfect the software. The ICade in the photo has nothing to do with the Instructable, it was just the only thing I had handy that I could use as a stand that had a black background :-) )

Step 1: Converting a Photograph Into a Drawing

There's a lot of software out there that claims to convert a photograph into a drawing, but those of you who've tried the various IPad and Android photo filter apps will know that it doesn't do a very good job. I researched this problem at length and found that there's an academic subject called "non-photorealistic rendering" (NPR) that specialises in this sort of image manipulation, and the best paper I could find on it was this one by Kang, Lee, and Chui entitled "Coherent Line Drawing".

I produced a version of their code in C and used it to convert photographs into monochromatic bitmapped images.

The photograph above shows the original image, the converted image, and the converted image with the background removed (and very lightly photoshopped to make his rimless glasses more prominent)

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