Measuring an object from a picture using a known object size

Question

So what I need to do is measuring a foot length from an image taken by an ordinary user. That image will contain a foot with a black sock wearing, a coin (or other known size o

Answer 1

You don't have to calibrate the camera if you have a known-size object in your image. Well... at least if your camera doesn't distort too much and if you're not expecting high quality measurements.

A simple approach would be to detect a white (perspective-distorted) rectangle, mapping the corners to an undistorted rectangle (using e.g. cv::warpPerspective()) and use the known size of that rectangle to determine the size of other objects in the picture. But this only works for objects in the same plane as the paper, preferably not too far away from it.