Camera measurements after calibration

Hi all,

I have calibrated a camera successively using the calibrator app and a checkerboard pattern. I plan to use the camera to measure distances in an industrial process. In the Matlab documentation there is an example of measuring distances between two coins, which is similar to my plan.

http://se.mathworks.com/help/vision/examples/measuring-planar-objects-with-a-calibrated-camera.html

However, in my industrial process I cannot have the checkerboard to compare the objects I like to measure. My camera is calibrated keeping the distance from the camera to objects approximately the same as I aspect it be in the process where I plan to put the camera.

Can I measure distances without the checkerboard? What do I have to change in the example code to make it work if it is possible?

Answer 1

[Dima Lisin]

Hi Erik,

If you need to measure planar objects with a single camera, then you need to know the camera extrinsics, which are the 3D rotation and translation between your plane of interest (table, conveyor belt, etc.) and the camera's coordinate system. If your camera does not move relative to that plane, then you only need to compute the extrinsics once. You can place a checkerboard on the plane once, compute the rotation and translation using the extrinsics function, save the results, and re-use them for measurements.

Alternatively, you can use a calibrated stereo pair of cameras. In that case, all the measurements will be in the coordinate system attached to one of the cameras, and you do not need a checkerboard in the scene.

[Diluxshan Ariyanayagam]

Hi Dima Lisin and all,

I could extract the extrinsic parameters from the MATLAB command. Can you please show a simple example how to use the extrinsics in the coding to measure the planar distances without using the checkerboard square on the image?

[Ifigeneia]

Hello Dima, I am quite new in Matlab and trying to find out several things. I am an Oceanographer in the field of Coastal Observing Systems and I am going to install a single camera on a tripod at the beach in order to measure the swash elevation of the waves. My question is bit similar to Erik's. Once I calibrate the camera with the Matrix in the Lab, let's say, how can I re-use the obtained results and apply them in the images from the beach monitoring? Thank you in advance.

[Dana R]

Hi All, has anyone figured out this problem yet? I also am hoping to measure distances in an image without the calibration grid being present. Thank you!