| 摘要 |
In methods of digital image processing, it is a problem to automatically adjust the contrast of an image that contains a bimodal distribution of tone values, such as a back-lit scene, or some close-up flash pictures. The invention solves the problem in a digital image processing method employing a tone reproduction function generated by normalizing the histogram of a sample of tone values, by statistically testing the sample of tone values for bimodal distribution, and if the sample is determined to be bimodal, testing the statistics of the sample of tone values to determine whether the image contains useful information in the shadows, and if so, adjusting the contrast of the image by increasing the contrast of the shadow portions of the image, and decreasing the contrast of the highlight portions. A part of the sample of tone values representing the shadow portions of the image is tested for skewness, and the contrast is adjusted only if the skewness does not exceed a predetermined amount. The test for bimodality comprises testing for the satisfaction of the inequality 0.0385 beta 12+0.9217 beta 1+1.7662>/= beta 2>/=( beta 1+1.0), where beta 1 and beta 2 are the coefficients of skewness and peakedness of the sample of tone values respectively. |
