The only way I can think of doing this properly, is using the STALGO algorithm for the straight skeleton, but with a modification for the vertical angle of the edges in the motorcycle graph…this algo is pretty hard to implement, because you need to solve the collision problem at different angles. The straight skeleton is a collision based algo. other of the ways is by adding a parameter for the angle of the corner rays, instead of just using bisectors. But in this way you cannot specify the slope of the roof first. In the ideal roof generator algorithm, you have to be able to specify the slope of the different surfaces by hand, and the location of the principal axis. If your are gonna have to do this is probably just better to model the roof by hand.
Its funny because this problem is actually central to architecture, but I never seen anyone in the past 8 years having even a close solution to it
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