About "Camera calibration"

Here are some notes I wrote last year regarding lens calibration. They should still be relevant I think, let me know if it works.

Summary : film checkerboard pattern, import 5 images to Agisoft, export calibration file, import calibration in Kinovea.

1. In Agisoft Lens: Tools > Show chessboard.
2. Film the screen with the camera from up close. About 10 cm for a GoPro. Film from 5 different angles: one central and the others from the corners. This assumes a flat screen. See example images below.
3. Open the video in Kinovea.
4. In Kinovea: Find 5 clear images (no motion blur) corresponding to the 5 various points of view and export them as images.
5. In Agisoft Lens: Tools > add photos. Add the 5 images.
6. In Agisoft Lens: Tools > Calibration. Check every checkbox except "skew" and "k4". Run calibration.
7. In Agisoft Lens: File > Save calibration…
8. In Kinovea: Image > Camera calibration. File > Import > Agisoft Lens. Import the file and Apply.

Done.

You can reuse the same calibration file for all videos filmed with this camera with the same lens settings. (If you change from wide angle to normal, another calibration file is needed.) Other cameras of the same model will have similar calibration files, but for the most accurate result you'll want to use a file specific to your camera.

The calibration computes the focal length, the misalignment of the lens center with regards to the sensor center, optical axis orientation with regards to the sensor plane, and distortion coefficients.

Verification

1. Go into the image tab in the calibration window and check the rectified image.

2. With a perspective plane.
- Reopen the checkerboard video in Kinovea.
- Add a perspective grid on the checkerboard and verifies that the lines are correctly distorted.
- Zoom to the max, adjust corners of the perspective grid and then calibrate the grid by the number of blocks covered horizontally and vertically.
- Display the coordinate system and check that it is correctly distorted. You can see the error accumulating at the periphery.
- Add a line covering a number of blocks, display its measurement and check that it matches.

My calibration images looked like this:

Yes it will work. It doesn't matter that you film from 10cm or 5cm. The program knows the expected geometry of the checkerboard and it compares it to what the image shows. Whether the squares occupy 25 pixels or 100 is not important for this. It is however important that all parts of the image are mapped to take into account asymmetries, so it should be from various angles.

It may appear that the distortion changes when you move the camera closer or farther away from the screen but in reality it does not. The periphery of the image still has the same amount of distortion. If you filmed a checkerboard ten times bigger from ten times farther away you would get the same image.

If you do the calibration several times you will see that the coefficients are not exactly the same. It should still work accurately and this margin of error is probably lower than the error introduced during digitization of points/objects positions.

The calibration of distortion works on the projected image so once calibrated it will work no matter how far the subject is.

Ok, i've done. Now I've the xml file. I upload it in "Camera Calibration" and it fix correctly the preview that i see on the tab "image" but i can't apply it on the vid. I push on "apply", but it don't modify the vid.