Can spherical ball get distorted to ellipse on image plane in a pin hole camera model

asked 2018-01-17 21:35:15 -0600

Sriram Kumar
31 ●1 ●3

I have a ball captured in an image. The ball is detected as circle when it is at the center of the image. when it moves to the corner of the image it is detected as an ellipse. We use a fish eye/wide angle lens and we are not correcting the image. We do the circle and ellipse detection on the original image. I want to know if this is a phenomenon of perspective distortion or due to the fish eye/lens distortion ? or anything else.

I did some reading around it and things are confusing me. https://books.google.com.sg/books?id=...

Any help or knowledge would be appreciated.

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answered 2018-01-18 02:20:38 -0600

LBerger
9317 ●2 ●20 ●88 http://www.traimaocv.fr

updated 2018-01-18 02:26:04 -0600

A pinhole model is a projection matrix : sphere becomes a disk in image plane (if skew parameter is 0). Pure pinhole model a sphere is a disk (skew parameter is 0 and fx=fy ) skew parameter is not 0 when pixel are not square or ccd plane is not perpendiculat to optical axis. fx!=fy strange optics.

Fisheye camera is not pinhole model, you have a distorsion coefficient theta=atan(r). Distorsion for a point near image center is less than a point far from center : sphere becomes ellipse.