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how to find world coordinates of a object in a image using single camera

asked 2015-03-20 01:25:07 -0500

Jasdeep gravatar image

hi ,

want to find world coordinate of a ball in a image , already done camera calibration what i'll do next? (using single camera)

thanks

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answered 2015-03-20 02:43:17 -0500

David_86 gravatar image

updated 2015-03-20 02:48:34 -0500

With a single shot and no other information the answer is easy: you can't, because info about depth is being lost during the process of image formation.

You can recover 3D world coordinate using stereoscopic vision: take 2 shots moving the camera with a pure translation (in general you can even rotate the camera but this will give no advantages) and calculate depth of a common point in the 2 images using disparity (d), the camera focal lenght (F, which you know from calibration) and the baseline (B, which is the translation of the camera between the 2 positions):

Z = fB/d

Retrieving depth from a single shot is still a subject of research and can be done, but you need to have some additional info in your image like a known size object placed at the same distance of the object you're trying to identify.

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yes, the size of the object is known.

link below shows steps to get world coordinates but don't know what kind of camera is used single or stereo vision.

http://stackoverflow.com/questions/16...

Jasdeep gravatar imageJasdeep ( 2015-03-20 02:58:16 -0500 )edit

That's a different topic (as we've already discussed here a few weeks ago, if i remember well). Measuring distance between 2 objects on the same plane is different than recovering distance between a given object and the camera. The first one can be done with a single shot (taking into account some approximation), while the second one can not for physical reasons: it happens the same with human eye, as you are not able to clearly identify depth when you look with one eye only.

David_86 gravatar imageDavid_86 ( 2015-03-20 03:08:09 -0500 )edit

my current goal is also the same , " to measure distance b/w 2 objects on same plane using single camera." i have managed to calibrate camera , now want to measure distance by getting world coordinates for objects and applying simple Pythagoras distance measuring formula : d=sqrt( (x1-x2)^2 +(y1-y2)^2 ) , as shown in the link. thanks

Jasdeep gravatar imageJasdeep ( 2015-03-20 03:47:54 -0500 )edit
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A possible solution to this was given in the previous discussion (assuming you want to work with a single image): http://answers.opencv.org/question/55... . You need to know at a given distance how many pixels you have per mm (so you can get distance between 2 points by simply counting pixels). If this info is unknown you are not going to solve your issue in any way. Be aware that both balls have to lie on a plane parallel to the image plane (so at the same distance from the lens) and with a single shot you're not going to recover info about depth anyway.

David_86 gravatar imageDavid_86 ( 2015-03-20 03:56:15 -0500 )edit
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Asked: 2015-03-20 01:25:07 -0500

Seen: 2,170 times

Last updated: Mar 20 '15