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Basic Parameters
Here’s a listing of the primary parameters: C02: a vanilla symmetric quadratic (2 nd order) term.
C22: additional quadratic term generating simultaneous barrel and pincushion distortion.
C04: a vanilla symmetric quartic (4th order) term.
C24: additional quartic term; barrel, pincushion, mustache distortions. C44: additional quartic term; barrel, pincushion, mustache distortions. C06: a vanilla symmetric sextic (6 th order) term.
C26: additional sextic term; barrel, pincushion, mustache distortions. C46: additional sextic term; barrel, pincushion, mustache distortions. C66: additional sextic term; barrel, pincushion, mustache distortions.
The quadratic and quartic terms are very similar in effect to what you find in the traditional “classic” SynthEyes lens distortion model, although they are scaled a bit differently. The sextic term tweaks only the furthest reaches. The three symmetric terms leave the interior of the image successively less changed, affecting only a ring successively further and further from the center.
The additional parameters create simultaneous barrel and pincushion or mustache distortions, again concentrating increasingly to the edges. There's no direct physical interpretation for additional terms. You can observe their effect by opening a lens grid in SynthEyes, selecting the Apply option on the Image Preprocessor’s Lens tab, then experimenting with the parameters. During a solve, they are combined in different proportions to match the observed distortion.
All nine parameters always create symmetric images, where the distortion is the same in all four quadrants of the image. That’s different than what happens in the Radial 4 th-order (Brown-Conrady-based) model, which was designed to correct for misaligned lenses. Even though the rotation parameter (to be discussed) creates/addresses asymmetric distortions, expanding the lens model to encompass more general asymmetry is a future possibility.
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