“…Gradient-based operators take the first derivative of an image to compute the focus value, [13]- [16]. Laplacian-based operators measure the focus by calculating the second derivative of an image, [15]- [19]. Wavelet-based operators consider the wavelet transform to describe an image's spatial and frequency contents, [15], [20], [21], whereas, statisticalbased operators take into account different statistics of an image to measure the focus, [22], [23], [19], [16].…”