title: Histogram-matching and histogram-flattening contrast correction methods: a comparison.
abstract: OBJECTIVE
  To compare the results or two methods of histogram matching and two methods of histogram flattening for their ability to correct for contrast variations in digital dental images.
  
  
  METHODS
  A custom-built, aluminium stepwedge with 0.1, 0.5 and 1.0 mm steps was placed over Ektaspeed films and exposed for 0.06, 0.12 and 0.25 s, respectively. Radiographs were digitized at 50 microns spatial resolution and 12-bit contrast resolution. Contrast corrections were performed using Rüttimann et al.'s algorithm (1986) for one method of matching (RM) and flattening (RF) and Castleman's algorithm (1979) for the other method of matching (CM) and flattening (CF). Mean pixel grey-scale values were determined for each step. The 0.12 s exposure was considered to be the target image exposure. Absolute differences in pixel grey-scale values between the target images and the modified images were determined.
  
  
  RESULTS
  The median values of the absolute differences in pixel grey-scale values between the target images and the contrast corrected images were: CM = 4.3; RM = 4.1; CF = 70.2 and RF = 70.2.
  
  
  CONCLUSIONS
  Castleman's and Rüttimann's matching algorithms perform equally well in correcting digital image contrast. Histogram flattening was less effective.
authors.affiliation: Washington University, St. Louis, Missouri, USA.
authors.forename: C F
authors.initials: CF
authors.lastname: Hildebolt
authors.forename: R K
authors.initials: RK
authors.lastname: Walkup
authors.forename: G L
authors.initials: GL
authors.lastname: Conover
authors.forename: N
authors.initials: N
authors.lastname: Yokoyama-Crothers
authors.forename: T Q
authors.initials: TQ
authors.lastname: Bartlett
authors.forename: M W
authors.initials: MW
authors.lastname: Vannier
authors.forename: M K
authors.initials: MK
authors.lastname: Shrout
authors.forename: J J
authors.initials: JJ
authors.lastname: Camp
mesh_terms: D000465:Algorithms; D011856:Radiographic Image Enhancement / Q000379:methods*; D011861:Radiography, Dental; D012815:Signal Processing, Computer-Assisted; D013382:Subtraction Technique