Fig. 1. Algorithm flow chart

Fig. 2. Non-subsampling filter model

Fig. 3. Diagram of nonsubsampled pyramid decomposition and reconstruction

Fig. 4. Structure diagram of dual-channel nonsubsampled fan filter banks

Fig. 5. Diagram of the Sigmoid function

Fig. 6. Sigmoid curves with different values of a

Fig. 7. Histogram after logarithmic function processing

Fig. 8. Histogram processed by Sigmoid function

Fig. 9. Effect of the bilateral filter near the edge of the image. (a) Image area containing noise; (b) effect of value-domain filtering; (c) effect of bilateral filtering; (d) spatial domain filter function; (e) combination function of spatial domain filtering and value domain filtering

Fig. 10. Comparison of two algorithms. (a) Original image; (b) traditional bilateral filter result; (c) improved algorithm result; (d) comparison of data

Fig. 11. Comparison of experiment results. (a) Orginal images; (b) multiscale Retinex method; (c) Kawasaki's method[10]; (d) Wang's method[12]; (e) method in this paper

Fig. 12. Objective evaluation data. (a) Standard deviation; (b) information entropy; (c) PSNR; (d) run time