Skulls and Engraving
I have recently been inspired by the work of Diego from the Engravers Dungeon (brought to my attention courtesy of the Laser Cut Stuff blog). He's been doing some amazing engraving of custom artworks, he's really managed to bring out the best of the wood using different colours but what really caught my eye are the two significantly different colours of laser cut. The large areas are engraved to create the typical 'brown' engraving where the wood grain can show through. The details however have this amazing 'black' engraving effect, he's understandably protective of his methods but it got me thinking about how it's done.
The laser cutter has 2 main modes of operation it can trace out a shape and cut it out or it can engrave an area where it moves over every 'pixel' within an image and puts a single dot onto each spot. Fancy laser cutters are able to vary the power of this spot to produce greyscale images, the Leetro controller (what I have) is only capable of doing on/off engraving. The burn mark is either full power or off, this produces the 'brown' engraving seen here.
There is technically one other option with the laser that I love and have used to great effect many times before. I use it for Line art, it's a cut operation but with a very low power that only marks the surface of the material without cutting all the way through it. This would appear to be very similar to what is happening here, large areas of seemingly 'black' engraving. The stuff I have previously done is much more for outlining objects but I thought I'd give it a try anyway.
First things first, I needed a suitable image to work with. I showed Diegos work to some of my friends and one of them told me that Vesalius made some anatomical drawings that would work well in this style. A quick search led me to some works of his that were recently digitised, 5 minutes on there and I had some suitable skulls to work with. I vector traced the first one and cut it (image above) and it kind of works. The skull was painted white, the brown areas are engraved and there is lots of line artwork that is starting to look correct. It works particularly well when the lines are close together but it fails on the big gaps, I needed some method for filling the gaps.
Normally when I want to engrave in greyscale I have to convert the image with a halftone effect, this varies the dot densities to produce a grey engraving effect (like how newspapers used to work). More dots make a darker area and a pattern like this fills the whole area so would be better for this line art. I converted the original image using a halftone plugin for paint.net, most raster editors have a method for doing something similar and GIMP is my suggestion if you're not using windows.
If you zoom in to the image above you can see the lots of little dots that make the image appear grey. It was natural to assume that if I drew around each of these dots I would get a similar half tone effect but done with 'black' engraving rather than 'brown' engraving. I imported the image into Inkscape and traced the image which left me with vector artwork for a halftone image. I set this up to be a very low power cut and gave it a whirl. (svg here)
As you can see the result is very effective, as a proof of concept goes it is definitely one method to achieve these kind of results from any laser cutter. The engravers dungeon clearly spends a lot more time waxing, oiling and finishing their artwork which accounts for most of the difference. I love their final pieces and am seriously tempted to own an original.
This isn't the end of the story though, if you look closely at the piece I created it looks very badly burned. Where the lines are close together it acts like a normal engrave cutting the whole area away and it looks a mess. There are many different methods to create halftone images so there are probably other ways to get the base image. As an Inkscape user I'm keen to keep everything in Inkscape so I followed this tutorial in order to make a halftone image using tiled clones. I have to admit it wasn't the easiest thing to do, the tool seems to max out at 250,000 clones (500 x 500) and it takes a long time to do but I persevered and created an appropriate image.
This is the point where I got carried away and shared my intermediary success with the laser engraving and cutting forum, received a dozen requests to show my workings and ended up writing this blog post (wordy for me I know). There are more things I want to try with this method....
1. This halftone effect would probably work well, instead of lots of dots the noodle shapes would outline well.
2. It is apparently possible to make halftone images using filter methods in Inkscape these look like they would create more suitable images for engraving.
3. The Eggbot from Evil Mad Scientist using hatch filling to colour in regions on their pen plotter, this would be another way to fill a region and worth investigating.
Finally the main reason my 'black' engraving looks horribly burnt is because I have a 100W laser cutter than doesn't offer particularly fine control at the low end. If I had an RF tube like Diego I would have better control over the beam power. The Glowforge laser cutter is also showing great potential for this kind of line artwork, sadly it looks as though they're about to announce another long delay on international shipping otherwise I would have mine already and I'd be able to try it out (2 years delayed and waiting)