How do I find the distance between two pixel co-ordinates

I would like to find the distance between two pixel co-ordinates detected by the camera , whose screenshot has been attached below. the unit of co-ordinates is in pixel whereas I want to know the euclidean distance. By lukign at it, we can say that the distance is approx 4, but how I detect it from matlab? pdist doesn't work as the co-ords are in pixels.

Answer 1

[Image Analyst]

See my attached spatial calibration demo.

Answer 2

[Emmanuel]

awesome!