title: Shading correction for endoscopic images using principal color components.
abstract: OBJECTIVE
  Inhomogeneous illumination often causes significant shading and vignetting effects in images captured by an endoscope. Most of the established shading correction methods are designed for gray-level images. Only few papers have been published about how to compensate for shading in color images. For endoscopic images with a distinct red coloring, these methods tend to produce color artifacts.
  
  
  METHODS
  A color shading correction algorithm for endoscopic images is proposed. Principal component analysis is used to calculate an appropriate estimate of the shading effect so that a one-channel shading correction can be applied without producing undesired artifacts.
  
  
  RESULTS
  The proposed method is compared to established YUV and HSV color-conversion-based approaches. It produces superior results both on simulated and on real endoscopic images. Example images of using the proposed shading correction for endoscopic image mosaicking are presented.
  
  
  CONCLUSIONS
  A new method for shading correction is presented which is tailored to images with distinct coloring. It is beneficial for the visual impression and further image analysis tasks.
authors.affiliation: Fraunhofer Institute for Integrated Circuits IIS, Erlangen, Germany. tobias.bergen@iis.fraunhofer.de.
authors.forename: Tobias
authors.initials: T
authors.lastname: Bergen
authors.affiliation: Fraunhofer Institute for Integrated Circuits IIS, Erlangen, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wittenberg
authors.affiliation: Fraunhofer Institute for Integrated Circuits IIS, Erlangen, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Münzenmayer
mesh_terms: D000465:Algorithms; D003116:Color; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D008029:Lighting; D010363:Pattern Recognition, Automated; D019047:Phantoms, Imaging; D011859:Radiography; D015203:Reproducibility of Results; D013382:Subtraction Technique; D018666:Ureteroscopy / Q000379:methods*; D001743:Urinary Bladder / Q000000981:diagnostic imaging*
keywords: Color shading correction; De-vignetting; Endoscopy; Image stitching; Principal component analysis