Ah, I knew I had to be missing something! It's an interesting idea for sure. Have you tried doing it with the image itself? I don't know what exactly you're doing to produce toolpaths that are almost exclusively level, other than perhaps setting project Z depth to a very tiny size and pretending that's your cut depth for the Sharpie to traverse. If you took your image for the stripes and added a radial gradient to them and toolpathed off that using the contour-carving type operations (i.e. parallel, spiral, etc) then you could get the Sharpie to be lifted off more, or have less pressure, at either end of them. That likely won't be as good as changing the feed rate though.
I'll have to think about that because there's a lot of different ways to go about something like that. For instance, would you want to set a percentage of feed rate change per inch of depth/step-over? Or would you just want to say "at this point we feed at this rate" and then pick a distance point "at this distance and beyond we're at this feed rate". That would allow at least a feed rate gradient based on radial proximity to the original center point.There's really a huge array of possibilities, you could also have a set of points all over the project that are blendpoints for different feed rates, or have square/diamond/star shaped concentric zones of either blending between feeds or sharply stepping between them.
Then would you just want a linear transition from one feed to another or something more like a polynomial transition that gives a different dynamic? You could have plain old linear, but maybe something like an ease-in or ease-out cubic transition would be better. Maybe the user should just have control over the curve's start/end derivatives to change the feed rate blend dynamic.
There could even be a new marker/pen tool that just doodles on the project material surface. That might be neato too.
Well I'm not going to make any promises just yet, but I'll definitely keep a location-based variable feed rate idea cooking in the back of my mind, because there is definitely some value there as a feature for manipulating the final appearance of something. I think it could be one and the same with the variable feed system for maintaining a relatively constant spindle load - spawning visible points across the project for an operation which affect the feed rate as the tool approaches them. We will see! This is something that I probably won't be able to get around to until at least 6 months from now, or more, depending on how things go with the features currently lined up for me to knock out. In the meantime feel free to keep sharing your work here for other users to see. It's a pretty cool use case for PixelCNC!
Thanks,
Charlie