# Extracting range of an object from an image using object's dimesntions

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Alla am 18 Aug. 2017
Kommentiert: Image Analyst am 21 Aug. 2017
So I have a code that detects and isolates three objects and yields their dimensions in pixels (diamters, perimeter..), I have the ground truth data for the distance but I would like to be able to estiamte the distance from the inforamation I have efficiently, suggestions to how this could be done would be great.
The picture shows a frame from over a thousand frames I have that show the object slowly drifting closer, so obviously the closer it is the larger its measurements are.
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Image Analyst am 18 Aug. 2017
Measure the diameters and separation for images taken at multiple distances. Then divide the actual diameters and separation by the pixel ones to get a spatial calibration. Then plot spatial calibration vs. distance to get a curve. Fit to a polynomial with polyfit to get a formula where you can convert the real world diameters into a distance from the camera.
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Alla am 21 Aug. 2017
Thank you for your answer Image Analyst, and I must take the chnace to also thank you for your very helpful code for detecting blobs which I used a variation of for the early stage of this.
The image shows my plot from about a 100 samples because I only have ground truth at a 10 frame interval (the plot has distance on the x axis and diameter on the Y axis), when I use ployfit I get a third degree polynomial that is completely inaccurate, so I'm sure Im doing something wrong here, any advice?
Image Analyst am 21 Aug. 2017
Attach the actual data. Maybe a 1/x curve will be better.

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Matt J am 18 Aug. 2017
For a simple pinhole camera model (and ignoring off-axis effects), the relationship between distance and perimeter (in pixels) should be
distance=K/perimeter
Just fit the constant K with as many (perimeter,distance) measurements that you have.
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Alla am 21 Aug. 2017
Thanks Matt
My problem is both practical than conceptual,my goal is to have a good range estimator that relies on thresholding those objects, I'm not sure if this is the best way to do it or not but I am giving it a shot, now I'm struggling with getting it to work, it might be very simple for you but unfortunately I'm not the best at this stuff

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