There are many ways to measure or estimate the distance between the objects or between the object and the camera in different situations. For example, if our objects of interest or our camera are moving in a known and fixed velocity, with the motion detection and object detection technology, we can estimate the distance between the objects in the view of the camera easily. Alternatively, if we get our hands on a stereo camera, we can follow https://www.ijert.org/research/distance-measurement-system-using-binocular-stereo-vision-approach-IJERTV2IS121134.pdf to measure the distance.
However, for our situation, we only have a common webcam in a fixed position, so how could we measure the distance from it? Well, it's possible to do with some prerequisites.
Let's talk about measuring the distance between the objects first. The prerequisites in this situation...