Ideally, the field of vision for a single human eye can be explained as a 180-degree hemisphere with the eyeball in its center. The hemisphere is constrained to the eyeball's orientation. Protractor handles the math to transform the hemisphere onto a flat drawing plane and applies the necessary distortion to the cursor guide.
The user sets the camera orientation and Field Of View and tilt, and Protractor then calculates the 2D positions of the vanishing points (which may or may not be off-canvas).
Protractor represents a month of research and observation and about 70 hours of coding and mathematical wrangling. I am considering releasing Protractor under a commercial license (whose price is yet to be decided, though there will be a limited demo version without save functions and a full-blown one with animation capability). After I make back my initial investment, all proceeds will then go towards Marfan's Syndrome to aid people that share my genetic condition.
The commercial release date for Protractor will be soon after TVPaint goes commercial. It's "bad form" to sell a plugin whose host application is still in beta.
Nicolas !!! )