Measuring object size using webcam

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I need to calculate distance between two points using a webcam. Now the catch is I don't need it to be any way related to actual measurements in cm or whatever. What I want is to use different webcams of different resolutions and they should all give the same measurement. I'll explain.

Suppose I am viewing a square shape using a webcam of 640x480 and it measures as one unit. I then view the same object from the same positions using a webcam of 1024x768 and it should still measure as 1 unit. How do I do this?

Posted 8-Dec-14 18:35pm

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_Maxxx_ · Accepted Answer · 2014-12-08T19:07:00

Are you saying that you would have an object which you decide to describe as being one unit, then use it to calibrate each camera?
If so, easy! Measure the shape in pixels - and for that camera, that number of pixels = 1 unit.
From then on, you know that on webcam a, X pixels = 1 unit, and on webcam b, Y pixels = 1 unit.

If not, then I think it is impossible - you must calibrate in order that each camera knows what 1 unit is. (one may have a wide angle lens, one a zoom, for example - it's not just the resolution that makes a difference)