The Perspective Projection tutorial is one of the Kickstarter 2015 tutorial rewards. It’s about something that humanity has known scientifically for a very long time, and decent formal training will teach you about this. But I think there are very very few tutorials about it in regard to how to achieve it in digital painting programs, let alone open source.
The tutorial is a bit image heavy, and technical, but I hope the skill it teaches will be really useful to anyone trying to get a grasp on a complicated pose. Enjoy, and don’t forget to thank Raghukamath for choosing this topic!