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Camera Calibration : fx and fy

asked 2014-08-11 13:34:54 -0500

LadyUnknowing gravatar image

Hi, can somebody explain why I get focus lenghts fx AND fy? One focal lenght is "normal" for optical instruments using monochrom light. So what is the different of fx and fy? Thanks!

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A question, can i compare thees values with the original focal lenght of my camera? My camera has a focal lenght =60mm , and after the calibration in open cv i have fx=38246.8 fy 38074.0 the pixel size is 0,00571

Giancola gravatar imageGiancola ( 2015-06-10 05:01:45 -0500 )edit

Hi,

how can I get focal length in pixels from fx and fy? I need to get disparity using z = b*f / d,

where z is the depth (in meters), b is the horizontal baseline between the cameras (in meters), f is the (common) focal length of the cameras (in pixels), and d is the disparity (in pixels). At zero disparity, the rays from each camera are parallel, and the depth is infinite. Larger values for the disparity mean shorter distances.

I am using kinect with the focal length: fx_rgb = 5.1885790117450188e+02 * ratio; fy_rgb = 5.1946961112127485e+02 * ratio;

Kinect is not a stereo sensor, therefore I dont have a "baseline".

Please help Ayesha

ayeshas gravatar imageayeshas ( 2016-03-29 11:25:55 -0500 )edit

^^ @ayeshas , please do not make answers, if you have a question or a comment.

berak gravatar imageberak ( 2016-03-29 12:00:17 -0500 )edit

^^ @ayeshas , please do not make answers, if you have a question or a comment.

berak gravatar imageberak ( 2016-03-29 12:00:28 -0500 )edit

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answered 2014-08-18 11:18:21 -0500

You are right that a normal optical instrument has only one focal length. But here, you look at a combination of a lense and(!) a sensor that consist of a 2d-array of detectors (formerly known as pixels). Your focal length therefore has the unit meters/px. If your camera's pixel pattern is a perfect square, fx equals fy. If the pixels are a bit rectangular and not quadratic, fx differs from fy. This effect can be captured by using two different focal lengths.

There is even a version of the intrinsic calibration where there is a skew value that incorporates an angle between the rows and columns of your pixel array that is not 90deg.

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answered 2014-08-12 02:35:24 -0500

If you need more explanation about the theory, have a look at the first section of the calibration tutorial here. It will explain why fx and fy are needed.

But basically, as @Ziyou said, it's because image are not square (I should say not spherical) but a rectangle. Therefore, you need an estimation of the focal on Height and the focal Width.

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answered 2014-08-11 19:10:45 -0500

Ziyou gravatar image

An image has two dimensions, thus have two focal length values.

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Asked: 2014-08-11 13:34:54 -0500

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Last updated: Mar 29 '16