[3dprint] Plastic deposition flow control in IceSL

IceSL has a nice feature to control plastic flow that I never took the time to explain.

There are several ways to produce the toolpaths for plastic deposition, and some of them result in paths that can slightly overlap : if you were to follow the paths with a thick marker, some places would receive color twice. Well, the same happens with plastic resulting in overflow! The reason a slicer might want to do that is to avoid the opposite: leaving a gap. IceSL tends to squeeze more paths within a slice so as to reduce some of these gaps (it is not perfect though, as some thin walls are still problematic).

While often small, if such overflows occur always at the same place across layers, they will accumulate and result in bad looking 'bumps' (or worse, the nozzle will collide with the part).

The 'flow' module of IceSL takes care of this. It uses OpenGL rendering tricks and follows the path with the virtual equivalent of a marker. Only, in each pixel it counts how much color (plastic, really) is deposited. If this amounts exceeds the target, it tunes down the extrusion rate so has to avoid overflow. This works quite well in practice! I believe this could also be used for SLA / laser engraving where 'beam energy' can also accumulate more than desired (the beam footprint is not an exact dot, but rather a small circle).

The two images at the end of the post show a slice before flow optimization, and the same afterwards. The grayscale levels indicate a reduction of the plastic flow ; they are especially visible in the thin areas (you might have to zoom in to appreciate the changes.

Now is this the best way to do this? Possibly not ; geometry analysis of the path thicknesses could be faster than the brute force GPU approach -- which is also quite approximate.

I am currently exploring better ways to limit gaps and overflows in IceSL. Stay tuned!