I calculate similarity percent between two images. Another image is the same with changed brightness. I've already tried comparing by pixel (euclidean distance for grayscale and 0/1 values), comparing by HSV. I've tried also normalisation (pixel brightness-mean brightness). The results were quite good (80-90%), but the similarity rate with changed image was lower than similarity between images with compeletely different tables, for example. Is there any simple approach to solve this problem? Or should I use neural networks?