Results from processing the residue twice

Red dots are isolated pixels. There are so many of them, which affects the tone and structure a lot(The variant method)

BasicVariant

After distributing the residue once, then the residue left is distributed again based on the weight function without spreading the error on those pixels attaining black or white. It turns out the tone matching is getting better, the structure matching is getting lower. Basic method is 11 out of 12 better than Pang's method for PSNR, but 4 out of 12 for MSSIM. The variant is 6 out of 12 better than Pang's method for PSNR and 11 out of 12 for MSSIM.