Fig. 1. Changes of image brightness before and after correction under different lighting conditions. (a) 2D-Gamma function; (b) enhanced 2D-Gamma function

Fig. 2. Image comparison before and after the enhanced 2D-Gamma function correction. (a) Before the enhanced 2D-Gamma function correction; (b) after the enhanced 2D-Gamma function correction

Fig. 3. Block diagram of homomorphic filtering algorithm

Fig. 4. Homomorphic filter transfer function

Fig. 5. Comparison of brightness histograms of images before and after correction. (a) Brightness histogram before correction; (b) brightness histogram after correction

Fig. 6. Influence of the weight coefficient on the uniformity of illumination

Fig. 7. Image comparison before and after linear weighted fusion. (a) Homomorphic filtered image; (b) linearly weighted fusion image

Fig. 8. Accuracy of edge extraction operator for disease recognition

Fig. 9. Influence of different weighting factors ε on the accuracy of city wall disease recognition

Fig. 10. Flow chart of the proposed method

Fig. 11. Four algorithm processing results. (a) Original images; (b) Gamma function;（c）homomorphic filtering；（d）Enlighten GAN；（e）proposed method