how to obtain the world coordinates of an image

Question

After to calibrated a camera using Jean- Yves Bouget's Camera Calibration Toolbox and checkerboard-patterns printed on cardboard, I´ve obtained extrinsic and intrinsic parameters, I can use the informations to find camera coordinates: Pc = R * Pw + T

After that, how to obtain the world coordinates of an image using the Pc and calibration parametesr? thanks in advance.

EDIT

The goal is to use the calibrated camera parameters to measure planar objects with a calibrated Camera). To perform this task i dont know to use the camera parameters. in other words i have to convert the pixels coordinates of the image to world coordinates using the calibrated parameters. I already have the parameters and the new image. How can i do this convertion? thanks in advance.

score 1 · Answer 1 · edited May 23 '17 at 10:33

1

I was thinking about problem, and came to the result:

You can't find the object size. The problem is by a single shot, when you have no idea how far the Object is from your camera you can't say something about the size of the object. The calibration just say how far is the image plane from the camera (focal length) and the open angles of the lense. When the focal length changes the calbriation changes too.

But there are some possibiltys:

How to get the real life size of an object from an image, when not knowing the distance between object and the camera?

So how I understand you can approximate the size of the objects.

edited May 23 '17 at 10:33

Community

1
1

answered May 15 '14 at 11:29

PeterNL

630
6
21

1

Thanks for your response. Actually I know the distance between my object and the camera. The camera is fixed on a plane. The idea behind this experiment was to verify that the camera calibration is correct, taking the measurements of the object in the image, and comparing to hand-measured of the same object. This is the main reason why I need to get the world coordination from pixel coordinates of the image. I wonder if there any math way in matlab to do this? – AnnaGomes May 15 '14 at 13:51

score 0 · Answer 2 · answered May 15 '14 at 16:27

Your problem can be solved if (and only if) you can express the plane of your object in calibrated camera coordinates.

The calibration procedure outputs, along with the camera intrinsic parameters K, a coordinate transform matrix for every calibration image Qwc_i = [Rwc_i |Twc_i] matrix, that expresses the location and pose of a particular scene coordinate frame in the camera coordinates at that calibration image. IIRC, in Jean-Yves toolbox this is the frame attached to the top-left corner of the calibration checkerboard.

So, if your planar object is on the same plane as the checkerboard in one of the calibration images, all you have to do in order to find its location in space is intersect the checkerboard plane with camera rays cast from the camera center (0,0,0) to the pixels into which the object is imaged.

If your object is NOT in one of those planes, all you can do is infer the object's own plane from additional information, if available, e.g. from a feature of known size and shape.

how to obtain the world coordinates of an image

2 Answers2