Calculating height of an object in the background based on the height of an object in the foreground

Question

I've been googling around all day and I can't really find an answer to this issue. I'm trying to create an image composed to multiple images. I have a person in the foreground who's height gives the viewer perspective for a painting hanging on the wall in the background.

bad rendering to give an example: poor rendering

I have a couple constants in this poor drawing. A is the height of the person, B is the distance between the person and the wall, and C is the actual height of the painting. Edit: Lets also say I have D which is the distance from the wall to the camera.

If anyone could point me in the right direction on how to solve this I'd be super greatful. Thanks.

Is there a specific formula I should be looking for or is there some reading that might help me with this?

Answer 1

You need one more thing: you need to know the distance from the camera (or painter) to the person (or the wall).

Why?

A six foot person standing in front of a six-foot mural -- say 6 feet away from it --- viewed from a point in the ground another 6 feet away will appear twice as tall as the mural.

That same person, viewed through top-notch binoculars from a mile away will appear almost exactly the same height as the mural. (The binoculars are just there to let you see anything at all!)

It's also helpful to know the "height of eye" -- how high off the ground the camera or viewing eye happens to be.

With those basic facts...you can read any book on computer graphics to show you how to make a nice rendering.

By the way, what's not clear from your "question" is just what you're asking. What is it that you actually want to know?