| The Panoramic annular lens optical system(PAL)belongs to a special catadioptric panoramic optical system,which is based on the principle of plane-cylinder projection imaging.It projects the scene information around the optical axis 360° onto a cylindrical surface,thereby generating a panoramic annular image(PAI).Therefore,PAL have excellent characteristics such as large field of view,compact structure,and are widely used in exploration,aerospace,military,and other fields due to their advantagesThe PAI formed by the PAL requires good visual restoration,and the distortion correction of the PAI is the bottleneck of image restoration quality assurance.Due to the impact of the imaging principle of the system itself,the image of the PAL has serious tangential and radial distortion.Therefore,in order to ensure the restoration effect,this article divides the correction of PAI into two parts,one of which is tangential distortion correction,the second is radial distortion correction.The tangential distortion correction methods rely on the selection of the unwrapping center point(UCP)of the PAI.If the center of the detector’s photosensitive surface is directly regarded as the UCP of the PAI during the selection process,nonlinear errors will be introduced while correcting the tangential distortion.In response to the above problems,an improved least squares method(ILS)based on the 3σ-Guidelines(ILS-3σ)is proposed to improve the accuracy of determining the UCP and the unwrapping radius(UR)of PAI.This method can overcome the impact of traditional tangential distortion using Hough transform(HT)on the influence of non-uniform background illumination.In addition,it promotes the effective unwrapping of image restoration algorithms and ensures the accuracy of distortion correction in PAL.The radial distortion correction methods rely on calibration experiments for PAL.To address the above issues,this paper proposes a correction method that predicts radial distortion based on lens design results(PRDLLR).This method eliminates the dependence of radial distortion correction on calibration experimental conditions,improves the convenience and timeliness of correction,and can also achieve feedback on the installation effect of PAL to a certain extent by predicting the correction effect.In terms of tangential distortion correction,PAI that have not been corrected for tangential distortion are circular annular images,while PAI that have been corrected for tangential distortion are rectangular images that do not have straightness.In terms of radial distortion correction,based on the PRDLLR that the maximum conformality error after radial distortion correction is 2.7% and the average value is 0.73%,while the maximum conformality error of PAI without radial distortion correction is 124% and the average value is 37.54%.Compared with the two conformality errors,it is found that the conformality error after radial distortion correction has increased by 51.42 times.The simulation results show that: using the ILS-3σimproves the positioning accuracy of the UCP of the PAI,better improves the tangential distortion of the PAI,and uses a correction method that PRDLLR,significantly improving the conformality of the PAI,and eliminating the cumbersome practical calibration process.On this basis,this paper also built an experimental platform for the PAL,using an ILS-3σ to achieve a mean straightness of 1.43 × 10 for the unwrapping image.The correction method for PRDLLR has a mean conformality error of 1.45%,while the mean conformality error of traditional radial distortion is 3.36%.The experimental results coincide with the simulation results,verifying the effectiveness and correctness of the method proposed in this paper,and providing a new reference idea for panoramic optical image restoration. |
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