All the friskets, masks and stencils can work either as masks, rulers, or guides.
If you take one of the curved stencils and right click on it, you get the option to turn it into a ruler or guide.
Then if you start drawing over the area of the stencil the line will be locked to the edge of the stencil.
Try it - drop one of the french curves on the canvas, right-click and choose 'guide mode' from the menu.
Now draw a line onto the french curve and you'll see it stops at the edge. You need to be a bit careful around the convex bits (just like if you were tracing around a real-world frisket with a pencil!).
Note that if you 'mouse down' on top of a guide or ruler stencil, the start of stroke will be at the point on the edge of the stencil that is closest to where you moused.
If you have a circle stencil set to 'guide mode' and just kinda click somewhere in the middle, and draw a quick rough circle in the opaque area of the stencil it is converted to a loverly smooth circle around the edges.
Things might get a bit odd if you use one of the softer masks as a guide - best to stick to the hard-edged stencils and masks if you want them to act nicely as guides and rulers.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"