| 摘要 |
A fingerprint image is divided into blocks of pixels. The blocks are determined to be within the foreground or background of the image. Poor quality images are examined to determine if the imaging process is corrupted, e.g. by a wet finger, by determining if a smudginess measure is above a smudginess threshold. For each foreground block, a mean intensity ( mu ) of pixels is computed for those pixels whose intensities are smaller than the mean intensity of all pixels within the respective foreground block. Further the standard deviation ( 3ROOT ) of intensities of all pixels within the respective foreground block is computed. For a foreground block with good contrast, mu is small and 3ROOT is large. But for a block with low contrast due to smudginess, mu is small and 3ROOT is small. Subsequently, to measure the contrast (a contrast measure) within a block, a function of the product (CS) of corresponding mu and corresponding 3ROOT is determined. A comparison is then made to determine whether a block is a smudged block. If the contrast measure is smaller than a smudginess threshold STH1, then the block is classified as a smudged block. If not, the block is classified as a non-smudged block. Embodiment, STH1 is set to 0.12. Lastly, the smudginess measure as the ratio of the number of smudged blocks to total number of foreground blocks is computed. If the resulting ratio is larger than a second threshold, STH2, a smudged impression is reported.
|
