Introduction: Scratch!

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)

Step 2: Turn the Drawing Into an Outline

In order to engrave the image on scratchboard, you have to send vector commands to the engraving machine (in my case, a KNK Groov-E vinyl cutter). Specifically you have to send it "splines" which are curved paths. (I did experiment with using only straight line segments but it was less than impressive)

Fortunately there's a great open source utility for doing this conversion - GNU Autotrace. However it only creates outlines like the one above. For a good scratch drawing, we want solid white areas to be filled as well.

Step 3: Add Some Magic!

This step requires magic. "ImageMagick" to be precise - an open source image manipulation program that can do almost anything with a raster bitmap.

We fill the white areas as follows: start with the regular image, and then "shrink" it by one pixel. This isn't a simple linear shrink like zooming out in a camera, it's a special kind of shrink where every shape in the image shrinks independantly, a little like watching a petri disk full of germs grow into colonies of germs, but run in reverse. (I'll update this later with a movie if anyone feels it would make the process more clear)

Run autotrace on the shrunken image and you'll now have a set of vectors that will scratch the drawing one pixel in from the outline. Keep repeating this step until the image has shrunk away to nothingness, and you'll find you have scratched out the entire inside area of all the white parts of the original bitmapped drawing. The image above shows the vector paths from a part of the image - zoomed in a lot so you can see each line.

Step 4: Preview

You can use the ImageMagick software to render the SVG file as a bitmap and print it on a regular printer before you start to cut, because printing takes seconds and cutting takes much longer! At the current stage of development, the preview actually looks better than the real thing, but I hope to make the engraving look more like the preview as I improve the quality of the conversion code and driver.

Step 5: Make It Real!

Having generated the cutting paths (as an SVG file), I load it in to the "Make The Cut" program, which I'm using effectively just as a driver to send the paths to the device. I kick off the cutter, and 30 minutes later (not a long time by human scratchboard standards!) we have the physical object that you saw on the first page.

You should be able to make scratchboard art with a CNC machine and an engraving tool if you have one. I bought a used vinyl cutter specifically for this project. Not all vinyl cutters will work by the way, some of them don't have enough clearance to fit the scratchboard below their pinch-rollers.

I think it would be cool to use a 3D printer to lay down a single layer of plastic to make art with as well. I'ld love the chance to experiment with that some day! I have an idea that a plastic print could work as the equivalent of a linocut or woodcut for making prints...

You can also apply this exact technique for regular drawings, using pencils, ballpoint pens, and felt-tipped pens. Because this much drawing would wear down a regular pencil, you need to use an automatic self-propelling pencil or one of those great new pencils that uses liquid graphite. The second image above is actually of a white gel pen drawing on black paper. It gives a result remarkably similar to scratchboard for a fraction of the cost. (I use that technique as a second proof because the blank scratchboards are quite expensive)

You'll also get much better results by scratching or drawing high contrast line art rather than photographs, but that's actually pretty easy to do - it was the challenge of creating images of faces in scratchboard that appealed to me most.


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