Photocopies of the ballots challenged in the 2008 Minnesota elections, which constitute a public record, were scanned on a high-speed scanner and made available on a public radio website. The PDF files were downloaded, converted to TIF images, and posted on the PERFECT website. Based on a review of relevant image-processing aspects of paper-based election machinery and on additional statistics and observations on the posted sample data, robust tools were developed for determining the underlying grid of the targets on these ballots regardless of skew, clipping, and other degradations caused by high-speed copying and digitization. The accuracy and robustness of a method based on both index-marks and oval targets are demonstrated on 13,435 challenged ballot page images.
Nagy, George; Lopresti, Daniel; Barney Smith, Elisa H.; and Wu, Ziyan. (2011). "Characterizing Challenged Minnesota Ballots". Proceedings of SPIE, 7874. http://dx.doi.org/10.1117/12.876558