get position of an object from image

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You can take a look at visual servoing package. This out of the box thing will work with qr codes and it gives you the pose of the object you are trying to detect.

Or you can detect any other object that you want to detect and get it's pose, take a look here.

The short answer is you can't.

The long answer is that if you can make some additional assumptions then you can sometimes make a good estimate.

The x and y position within the image when combined with a calibrated lens model for the camera can tell you which 'direction' an object is in. If you know where your camera is then you know that the object lies somewhere along a line, but not where it is on that line.

However if the size of the object is fixed and known then you can use the width and height values to estimate it's range from the camera, although this will not be very precise.