How to get the measures of an object from far away?

I am working on an App that intends to give an accurate measure of any object (building, desk, chair, people, etc.) Using the camera (either phone's or laptop) but I'm unsure as to how to do this without using a lot of resources, would someone mind giving me some options? I'm looking for a lightweight one that can be quickly processed by the computer/phone to give back the开发者_如何学运维 measures.

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Join the club! Computer vision is a notoriously difficult problem, and guessing the size of an object is a part of that. With a low-grade sensor like a phone camera, you are projecting a three-dimensional world onto a two-dimensional surface. Because two-dimensional images can't contain any depth information, without some kind of reference, it will be impossible to get a useful value.

Like Wile E. Coyote, your phone camera cannot tell the difference between a well-painted illusion and the real thing with only one point of reference. You'll either need at least two different views of the same object or a fixed reference point whose size is already known.

One possibility is to take a video instead of a picture. If you can store data like GPS readings along with the video, you will be able to infer the actual size of the object based on how much its projected size changes. At short distances (say, a few hundred meters or so) this is simple Pythagorean algebra.

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