Structural similarity between two matrices

Three methods which can be used to find structural similarity each having different efficiency.

1. It is the classic method to find - by calculating root mean square deviation of two matrices
2. SSIM - Structural similarity Index Metric - This is a concept derived from image processing where it is used to find similarity of two images. It can be used for structural similarity if matrices.
3. MSSIM - Mean Structural similarity Index Metric - This is a improved version of SSIM where we use window sliding and implement SSIM for each window and calculate mean of those SSIMs. This gives better results than SSIM.