Fig. 1. Infrared images and three-dimensional (3D) intensity distributions. (a) Actual infrared image 1 containing small target; (b) 3D intensity distribution in small target area of image 1; (c) actual infrared image 2 containing small target; (d) 3D intensity distribution in small target area of image 2

Fig. 2. Infrared dim small targets in complex background. (a) Dim small target in sea-sky background; (b) bright target in sea-sky background; (c) dim small target in sky cloud background; (d) bright target in sky-ground background[12]

Fig. 3. Filter template structure of Robinson Guard with window size of 5×5

Fig. 4. Nine parts of sliding window

Fig. 5. Patch-image construction from original image by using image patches

Fig. 6. Steps of algorithm

Fig. 7. Representative images in four datasets. (a) Dataset 1; (b) dataset 2; (c) dataset 3; (d) dataset 4

Fig. 8. Four representative images in dataset 5. (a) Image 1; (b) image 2; (c) image 3; (d) image 4