Chromatic abberation removal in picktorial

#Chromatic abberation removal in picktorial how to#

Detachment of the lens distortion from camera intrinsics becomes possible when the former is compensated separately, in advance.

#Chromatic abberation removal in picktorial how to#

When referring to camera intrinsics calculation, the question is how to avoid residual error compensation which is inherent for global calibration methods, the main principle of which is to estimate all camera parameters simultaneously - the bundle adjustment. Our aberration correction method, as opposed to existing software, demonstrates a final geometrical residual of less than 0.1 pixels, which is at the limit of perception by human vision. Our keypoint detection yields an accuracy up to 0.05 pixels, and our experiments show its high resistance to noise and blur. Combined with an accurate detection of pattern keypoints, the most precise chromatic aberration correction is achieved by using a polynomial model, which is able to capture physical nature of color channels misalignment. The latter may not be general enough to ensure stable correction for all types of cameras. In this context, the main idea is to search for a more general correction model to realign color channels than what is commonly used - different variations of radial polynomial. For the chromatic aberration problem, we follow a path of digital post-processing of the image in order to get rid from the color artefacts caused by dispersion phenomena of the camera lens system, leading to a noticeable color channels misalignment. Two main threads are followed and examined: lens chromatic aberration correction and estimation of camera intrinsic parameters. This dissertation focuses on internal camera calibration and, especially, on its high-precision aspects. Simulated results show the validity of the proposed method and experimental results demonstrate that the proposed full-field calibration method based on absolute phase maps will be useful for practical software-based CA calibration.

The systematic errors introduced by the LCD are analyzed and corrected. CA calibration is accomplished in Cartesian coordinates. These pixel deviations can be computed by comparing the unwrapped phase data of the red, blue, and green channels in polar coordinates. CA causes the unwrapped phase of the three channels to differ. The phase information of each color fringe is obtained using a four-step phase-shifting algorithm and optimum fringe number selection method. Red, green, and blue closed sinusoidal fringe patterns are generated, consecutively displayed on an LCD (liquid crystal display), and captured by a color camera from the front viewpoint. Based on an analysis of the distribution of CA, a full-field calibration method based on absolute phase maps is proposed in this paper. This characteristic of lenses causes images captured by a color camera to display chromatic aberration (CA), which seriously reduces image quality. The refractive index of a lens varies for different wavelengths of light, and thus the same incident light with different wavelengths has different outgoing light. The calibration method was able to accurately characterize the aberrations and produce transformations for efficient sub-pixel geometrical calibration over the entire spectral range, finally yielding better spatial resolution of hyperspectral imaging system.

The results indicated substantial wavelength dependent optical aberration that is especially pronounced in the spectral range closer to the infrared part of the spectrum. The calibration method was tested on an AOTF hyper-spectral imaging system in the near infrared spectral range. The acquired images were then automatically registered to the caliber model by both parametric and nonparametric transformation based on B-splines and by minimizing normalized correlation coefficient. Geometrical calibration of hyper-spectral imaging system was performed by stable caliber of known dimensions, which was imaged at different wavelengths over the entire spectral range. This problem is especially adverse in AOTF (Acousto- Optic Tunable Filter) hyper-spectral imaging systems, as the diffraction of light in AOTF filters is dependent on both wavelength and angle of incidence. Geometrical calibration must therefore be performed over the entire spectral range of the hyper-spectral imaging system, which is usually far greater than that of the visible light spectrum. In hyper-spectral imaging systems, the problem of optical aberrations is even more pronounced because optical aberrations are wavelength dependent. Geometrical calibration of an imaging system is therefore of the utmost importance for achieving accurate optical measurements. Optical aberrations present an important problem in optical measurements.