In this paper we present a novel measure of camera focus based on the Bayes spectral entropy of an image spectrum. In order to estimate the degree of focus, the image is divided into non-overlapping subimages of 8 by 8 pixels. Next, sharpness values are calculated separately for each sub-image and their mean is taken as a measure of the overall focus. The sub-image spectra are obtained by an 8 × 8 discrete cosine transform (DCT). Comparisons were made against four well-known measures that were chosen as reference, on images captured with a standard visible-light camera and a thermal camera. The proposed measure outperformed the reference measures by exhibiting a wider working range and a smaller failure rate. To assess its robustness to noise, additional tests were conducted with noisy images.
Afterwards, Kristan et al. REF proposed a focus measure based on the Bayes spectral entropy of the image spectrum.
A Bayes-Spectral-Entropy-Based Measure of Camera Focus Using a Discrete Cosine Transform
Pattern Recognition Letters
Computer Science Mathematics