Thank you for the links! There could be some useful stuff in there. 
Ah, I see. Well, I look forward to seeing what comes of it.
Ah, fair enough; I’ll confess that some of the restrictions inherent in shaders can be frustrating sometimes (such as discovering that–on my machine, at least–variable array indexing isn’t allowed). 
Thank you for pointing out this particular issue, by the way: my pencil shader uses nested if-statements at one point, which I now may want to change.