>>14053>whats loomis? the book guy?
It's a board on imageboard I probably can't mention without being banned. Just know that someone asking almost your exact same question and using basically the same image asked another perspective question there making a half dozen threads just for this one question and got banned for it. If you're saying you're not that person fine, quite the coincidence though.
>ok i see…. so everything is based on ratio than actual real size? and a centimetres of a ruler is not exactly usable as a "real length"?
Not really no. For a very intuitive example, look at forced perspective. You should know that that the length between this thumb and finger is about 4 inches or so. If I were to draw a vertical line on the image, both the Eiffel Tower and the distance from his thumb to his index finger would be 4 inches. Obviously, the Eiffel Tower is not 4 inches tall. The length of anything is a mix between it's actual length and it's ratio relationship in regards to the horizon line, which, in a good perspective drawing, literally everything is. That's why things are in perspective, you've artificially constricted everything to a certain ratio relationship to 1 to 3 point perspective.
>what about angle?
Oh lordy that's a bitch just thinking about it. 45 is easy because if you make a cube anywhere in perspective, then draw a diagonal going from one corner to another corner on a single 'face" of the cube, by definition the angle formed between the "edges" of the cube and that diagonal must
be 45 degrees. I'll be honest though, they are very few times you're going to just want a random "angle" in your drawing that isn't just a smaller component of a different shape, so a better question that makes this less abstract to run into would be to break down everything into a set number of 3D shapes (cubs, spheres, cones, cylinders, pyramids) and then using perspective to draw those shapes. I'm pretty sure I could weedle out some way of deriving every angle you want out of a page, but honestly you're focusing a little too hard on perspective for most practical purposes. If you're not going into archetecture or being an engineering draftsman I don't see why you would ever need angles that
The entire field of perspective and it's rules is enough to write a book about, and people have, so go read one of the books suggest by >>14066