I’ve been struggling with this for ages, but yesterday at Hackspace – thanks to Barney (and I now realise, Tiff said this too and I got distracted and never followed it up) – I got it to work.
The issue was this: I’d been assuming that everything you lasercut had to be a vector DXF, so I was tracing bitmaps using inkscape in order to make a suitable SVG, converting to DXF, loading it into the lasercut software at hackspace, downloading it and – boom – “the polyline must be closed” for etching: no workie. No matter what I did it to the export in Inkscape or how I edited it, it just didn’t work.
The solution is simply to use a black and white png, with a non-transparent background. This loads directly into lasercut (which comes with JustAddSharks lasers) and…just…works.
As a bonus and for my own reference – I got good results with 300 speed / 30 power (below 30 didn’t seem to work) for etching (3mm acrylic).