We now elaborate on how to minimize these errors and improve the accuracy.
Even the best cameras suffer from some errors due to imperfect lenses. A perfect lens would render straight lines as straight, no
matter where they occur. Most practical lenses aren’t that good, though, and instead bend lines outwards (barrel distortion) or inwards (pincushion
distortion). This is called lens distortion. Wide-angle lenses often suffer particularly badly from this.
Correcting this distortion in the image geometry is of critical importance in our digitizing application.
The two typical lens distortions that occur are called barrel and pin-cushion distortion. They are named by the effect that they have upon an image, as shown in Figure.
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Distortion free image | Barrel Distortion | Pincushion Distortion |
Barrel distortion is found in wide-angle views and it is the result of the squeeze that is applied in order to fit the image in a smaller space.
Pin-cushion is found in telephoto because of the stretching applied in the image.
The squeezing and the stretching of images vary radially due to the design of the lenses, making these distortions visually most prominent at the image corners and sides. Barrel and pincushion distortion typically appear at the shorter and longer ends of a zoom lens respectively, with continuous variation in between.
In order to avoid this distortion, we have to calibrate the camera. Camera calibration is the estimation of a set of parameters that describes the camera's imaging process. By calculating those parameters and using them while digitizing we will get the perfect image with minimum distortion error.
For calibrating the camera we need some photographs of a regular ChessBoard pattern.
Following are few guidelines that can help you get a better picture for digitization.
Reference Plane |
A lot of pixels are wasted in the left picture. In the RIGHT picture the maximum amount of pixels are utilized are devoted to the template, which is better. |
The photo should be clear so that edges of template are clearly visible. Focus the camera properly before snapping the picture. |