Camera Calibration

ASRix has adapted an algorithm developed by George Karras and Dionyssia Mavrommati, which was presented to the CIPA International Symposium in Potsdam, Germany in 2001 with the title of: Simple Calibration Techniques for Non-Metric Cameras

By photographing and rectifying a grid ASRix will calculate parameters for lense distortion that can then be applied to any image photographed with the same camera using the same settings for zoom and focus.

You will have to use identical camera settings for both the calibration and the surfaces to be rectified which sounds easy but you will find that your camera will do everything in it's power to thwart you by changing the focus and aperture settings without warning.

The idea is that you are calibrating the geometry of the lens in relation to focal plane or CCD of the camera. The factors that matter are:

• Focal length (or zoom)
which you can usually control but which might change when you turn the camera off (or when it times out on it's own). If you are lucky the extreme settings, fully wide angle of fully telephoto, will provide a consistent geometry but you should calibrate several images taken during different sessions before you can count on this. Almost all photographs destined for rectification will be taken with the lens set to maximum wide angle. Don't worry that there is distortion, removing this is what the calibration routine is all about.
• Focus
which, on most digital cameras, is normally set to Auto which for calibration purposes is very bad. You will need to override this feature and set the focus manually to infinity. With the simple (point and shoot) cameras the best you can do is to set it to landscape mode which will fix the focus to infinity. If the focus on your surfaces is not as crisp as you might wish try to improve it by narrowing the aperture and decreasing the shutter speed.
• Aperature
has a limited effect on lens geometry but it is good to keep it constant if you can. Also a smaller aperture (higher number) increases the crispness of the focus and tends to use the central part of the lense which typically has less distortion than the extremities.
• Shutter Speed
should have no effect on lense geometry but the faster it is the less likelihood you will have problems with camera movement (tripods are a good idea in any event).
• Resolution
has no effect on lens distortion but it should be kept constant for calibration and rectified images.

Slightly better results can be expected from a larger target grid and for this pages 2-10 which are enlargements of the grid can be printed on a normal letter sized printer then glued or taped together to form a collage. When using multiple sheets care must be taken to accurately align the targets (pins through the target centres is a good way to do this).

You should then mount your grid on a flat surface and photograph it using the same camera settings you will be using (or used) to photograph the surfaces to be rectified.

When taking this image don't worry if it is out of focus. You will still be able to determine the centre of the targets. Remember you are trying to keep the lens the same as when the surface to be rectified was (or will be) photographed.

Also you should try to fill the image frame with the grid or, if you do not, you should try to use only the area covered by the calibration to collect the image of the surfaces you wish to rectify.

The next step is to download the image of the grid and open it with ASRix

Use the Database Connection feature to attach the database file ASR-grid.dbf which you will find in the directory where you installed ASRix.

Starting with point 1 in the upper left hand corner select each point in order, zooming in enough to ensure you have an accurate placement of each point.

When all 130 points have been accurately positioned you will have a display that looks something like the following:

When you are satisfied that all points have been accurately positioned go to the Tools Menu and select Camera Management

The camera selected as default when you close the Calibrate Camera dialog box will become the default for all future rectifications until you change it by calibrating another camera/lense combination or selecting a different camera via the Output Parameters dialog box at the time of rectification.

Whenever there is a current camera defined, which should be most of the time if you are using a digital camera, it will be possible to view the corrected, but un-rectified, image by selecting the

icon on the toolbar or the Corrected Source Image item in the View Menu

As well, under the Tool Menu there is an option Save Corrected Image which will save the source image corrected for lense distortion but before rectification. For an image named name.ext the corrected image will be saved as name.COR.ext.

There is no need to save this image, whenever rectification is performed the image will be first corrected for distortion (based on the current camera) and then rectified. Only the rectified image (name.ASR.ext) has any metric significance but sometimes it's interesting to see the steps such as below.

Source:

Corrected:
Rectified:

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Inquiries to: steve@icomos.org

Generated by: CART
(Thu Sep 20 17:56:23 2007 )