영상에서 Contrast 측정.

3.2. Contrast Measure

A good contrast measurement should have enough discrimination ability over image sequence. As

mentioned previously, various contrast measurements have been introduced and investigated in passive

AF systems. The Contrast Measure based on Squared Laplacian (CMSL) yields good performance in

terms of accuracy and unimodality [4,6,11]. It can be given as follows:

where G(x, y) is computed by:

where I(x, y) is the intensity value of an image pixel at location (x, y), the parameters J and K are the

height and width of focusing region in the image over which the contrast is evaluated.

However, under low light conditions, the higher frequency content in a focused image is just a little

more than that in the corresponding defocused or blurred image. A focused image obtained from low

light conditions possesses a small contrast value and it may be easily influenced by noise. Although we

formerly used the modified bilateral filter for noise reduction, it is still necessary to cope with the

influence of noise. As illustrated in Figure 3, due to the low contrast of image and noise influence,    

contrast measurement may generate a fluctuant curve with many local false peaks. In this case, it is

difficult to distinguish a well-defined peak, and this hinders the following peak search procedure from

performing its function in locating the best focused contrast peak.

sensors-11-08281.pdf