HISTOGRAM MATCHING AND HISTOGRAM FLATTENING CONTRAST CORRECTION METHODS - A COMPARISON

Objectives. To compare the results of two methods of histogram matchin g and two methods of histogram flattening for their ability to correct for contrast variations in digital dental images. Methods. A custom-b uilt, aluminium stepwedge with 0.1, 0.5 and 1.0 mm steps was placed ov er Ektaspeed films and exposed for 0.06, 0.12 and 0.25 s, respectively . Radiographs were digitized at 50 mu m spatial resolution and 12-bit contrast resolution. Contrast corrections were performed using Ruttima nn et al.'s algorithm (1986) for one method of matching (RM) and flatt ening (RF) and Castleman's algorithm (1979) for the other method of ma tching (CM) and flattening (CF). Mean pixel grey-scale values were det ermined for each step. The 0.12 s exposure was considered to be the ta rget image exposure. Absolute differences in pixel grey-scale values b etween the target images and the modified images were determined. Resu lts. The median values of the absolute differences in pixel grey-scale values between the target images and the contrast corrected images we re: CM = 4.3; RM = 4.1; CF = 70.2 and RF = 70.2. Conclusion. Castleman 's and Ruttimann's matching algorithms perform equally well in correct ing digital image contrast. Histogram flattening was less effective.