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I want to know , what is function for measuring distance between 2 point in a image?? How can change distance (pixel)to mm or cm?

Asked by Zohreh Tavakkoli on 23 Jul 2013
Latest activity Commented on by Walter Roberson on 16 Oct 2013

Distance between 2 point

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Zohreh Tavakkoli

1 Answer

Answer by Image Analyst on 23 Jul 2013

Do you know the points? Use ginput() or inline() if not. Then just use the Pythagorean theorem to calculate the Euclidean distance. To convert to mm or cm, you need to image an object of known distance, such as a ruler. Then you know the spatial calibration factor in mm per pixel and you can multiply distances by that factor to convert the distance in pixels into a distance in mm or cm.

3 Comments

nikhil on 16 Oct 2013

for example i know the euclidean distance is 112.00 and image resolution is 90 dpi. Using these data any way to find the distance in mm or cm.?

Walter Roberson on 16 Oct 2013

To which I would add the note that when you see "72 dpi" or "90 dpi" or "100 dpi", you should ask for proof that the image really is that. Those three values are all commonly used as nominal resolutions: e.g., if you have a matrix of unknown size and save it as JPEG, chances are that the image will be arbitrarily marked as 72 dpi. 72 and 90 and 100 should be considered to be most likely display resolutions rather than actual resolutions that the images were taken at.

If you do not know for sure that the image was taken with something like a scientific microscope that always "knows" the current lens power and very precise positioning information, then you should expect that the dpi information is not much more than a number drawn out of a hat.

Image Analyst

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