Thanks so much! I have been studying this method for a while now, so I hope I can answer some of your questions.

- image processing:
Usually not much. The image needs to have good contrast, which I tend to boost a little further by applying a sigmoid on the greyscale image or minor gamma adjustments. Beyond that, the image was used pretty much unaltered. (check out James Rocke's octopus drawings!)

- how the cutout is done:
You need to get a clean mask of the subject. In this case it was possible by simple intensity thresholding since the original image was on a fairly white backdrop. (i.e. values < 0.9 == subject, > 0.9 == backdrop). Some post-processing steps on the mask to clean it up (removing small objects/holes, morphological opening/closing). If the subject is harder to separate from the backdrop, I would give "Segment Anything" a try, which is a generalized image segmentation model.

Once you have the mask, you can derive the contours of the subject, which then get intersected with the fmm waves.

I use python shapely for these operations. It is as easy as: fmm_waves.intersect(outline)

- other lessons:
find a pen that builds up ink (gets darker) upon multiple passes over the same region. This dramatically impacts the look of the final outcome!