From-scratch SSIM calculator

A dependency-light Python script that computes SSIM between two image or frame files from first principles. It prints the three comparison terms (luminance, contrast, structure) on whole-image statistics, then the proper windowed Mean SSIM using an 11x11 Gaussian window (sigma 1.5) per the 2004 paper — reporting both the mean and the worst window the mean hides. Includes the equivalent FFmpeg ssim/libvmaf commands and the ssim_db formula in comments, and notes why FFmpeg's 8x8 stride-4 result di