Before you can produce Augmented-Reality videos, you must calibrate your camera
system. Camera calibration accomplishes two things in one step. First, it calculates distortion parameters of
the camera’s optical system. This allows the processor in VideoF2B to “undistort” every video frame so that
straight lines in the real world remain straight in video. Undistorted frames are essential to many image
processing tasks. Second, it establishes a relationship between the size of objects in video versus the size
of the same objects in the real world. This is important for drawing Augmented-Reality geometry of the correct
size and shape in the video.
Calibration involves the recording of a special video and consists of three easy steps. To begin, start
VideoF2B and choose Tools ‣ Calibrate camera.. in the main menu. You will see the following
window:
You have two choices for the calibration pattern: display it on screen or print it to paper. The
recommended method is to print. However, if you do not have access to a printer, displaying it on screen is
also acceptable.
Important
To print the pattern you will need a PDF reader application, such as Adobe Acrobat, Foxit, or similar.
If you decide to print the pattern, make sure to mount it flat to a suitable piece of cardboard or
poster board for easy handling while maintaining accuracy.
Note
The absolute size of the pattern is not important. Whether you display it or print it, do not worry about
its true size. It is only important that the entire pattern is flat and visible.
Record the video using your camera system. The video should be fairly short; about 30-50 seconds is enough.
The pattern should be visible in its entirety throughout the video. Move and tilt the camera so that you
record as many perspectives of the pattern as possible. To see an example video, click the thumbnail under
Step 2 in the calibration window. An alternative method is to mount the camera on a tripod, then move and
tilt the printed pattern in front of the camera.
Attention
Configure your camera with the same video settings that you will use in the field to record the
flights. This means that your choice of lens, its focal length, and video resolution all
must be the same during calibration and during field recordings. If the focal length is adjustable
(also known as a “zoom lens”), then you must make sure to set the focal length to the same value during
field recordings as you did during calibration. When using the camera of a mobile device, always orient
the device in landscape mode (horizontally) and make sure you always choose the same zoom factor
and video resolution as you did during calibration. If you neglect to follow this rule, you will get
unexpected results in your Augmented-Reality videos. This rule does not
apply to the frame rate of the video.
If you chose the Display option for the pattern in Step 1, press the Esc key to return to the
calibration window after recording the video.
Transfer the video file to your computer. Under Step 3, browse to the file. Finally, press the
Start button at the bottom of the window. VideoF2B will begin processing the calibration video in
the main window:
Fig. 10 Main window of VideoF2B showing camera calibration in progress.
As stated in the message window, the calibration process takes a while. The video playback will appear in slow
motion, and it will seem to “skip” and “freeze” at times, but do not fret – all is well. The calibration
process is computationally intensive. If you do want to stop the calibration at any time for any reason, just
press the Esc key. Otherwise, grab a cup of coffee, relax, and wait patiently until the progress bar
reaches 100%. When finished, the video will disappear from the main window, and you will see some information
about the results in the message window:
Fig. 11 Main window at end of camera calibration. Take note of the messages in the message window.
If the calibration fails, most likely your video is too short and/or it does not show the complete pattern
from a sufficient number of points of view. In that case, record another video while paying attention to those
details.
If the calibration succeeds, VideoF2B will create a file named CamCalibration.npz and two image files in
the same folder as the calibration video. The CamCalibration.npz file is the calibration file for your
camera system. Do not lose it. You will need it for producing every Augmented-Reality video of the flights
you will record with your camera. You may also share it with others who have the same camera system as you.
For the technically inclined…
The two image files show a sample frame from the calibration video. The image calibresult_nocrop.png
is a full-size frame that is “undistorted”, i.e., straight lines of the pattern should appear straight in
the image. To achieve this, the calibration process transforms the original frame in such a way that empty
pixels appear around the edges of the undistorted image, giving the edges a “pincushion” look:
The strength of the pincushion effect depends mostly on the distortion inherent to the lens, and on the
focal length. Wide-angle action cameras typically show a stronger effect than longer lenses.
The other image file is calibresult.png. It is the same image as the “no-crop” image above, with one
important difference. It is cropped to the maximum usable area so that the empty pixels are no longer
visible:
Note that this always results in a smaller image than the full-size video frame that you see in the
camera. In the above examples, the “no-crop” image size is the original Full HD, or 1920x1080 pixels. The
cropped image size is 1910x1050 pixels. So a total of 10 pixels were lost from the sides, and a total of
30 pixels from the top and bottom of the original frame. It is important to keep this in mind when placing
the camera in the field. Give yourself some room, especially at the bottom of the frame, to account for
the lost pixels. VideoF2B will “upsize” calibrated video to the size of the original input video whenever
possible, but some pixels around the border of the original video will be lost due to calibration.
Congratulations, you are ready to record Control Line Stunt videos! The next
step is field setup.
