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How to calculate the real size of an object from two photos in different types of perspective, knowing the dimensions of one of the objects in these pictures?

When I was in college, we did such calculations but it was a long time ago and I have no records. From what I remember,We need the coordinates of a known object from both photographs and the coordinates of the object searched from both photographs. And the realdimensions of the known object.

Is anyone able to give a formula how to calculate it?

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There are quite a few methods to determine this. The most common that you probably noticed would be in crime scene photos when an object of known size such as a pencil or even better a ruler is placed next to the object of interests. From this it is very easy to estimage the size of the object of interest.

As long as you know the focal length and the object distance, both of which some lenses return to the camera you can calculate the real size of an item in the image. The focal length will give you the lens field of view in terms of degrees.

Measuring size of objects in an image with OpenCV

Parallax

I'd try looking at these to start off

You also might find these of use:

visualsize

techcrunch on visualsize

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