(New here, and didn't see this brought up before on a quick scan, but if it has been, sorry for the spam!)
I've been using the Lasso Tool in Photoshop to draw black and white shapes for a while now (smbhax.com or smbhax.deviantart.com for examples); it's sort of what Alchemy does, only you have to draw the selection first, then Cut (or Fill) to get the shape filled in, so it's somewhat slower.
I like how Alchemy auto-fills, and how responsive it is, but one thing is giving me some trouble as I try to get used to using it: because it keeps a solid shape connected between my start and end point as I draw, if I'm drawing around existing shapes, or doubling back on my line, the stuff I'm trying to draw around is obscured by the shape I'm currently forming.
What I'd love to see would be an option to have Alchemy render just the outline I've drawn so far--in a "marching ants" or "Difference" line or something so it's easy to distinguish against the canvas--without any fill or connection from start to end until I lift the pen off the stylus, at which point it would fill in the shape as usual.
Anyway, thanks for providing this tool. I've posted about it on my own little insignificant forum which won't do much for you, but I look forward to seeing where you take it from here.

