  The Structural Similarity (SSIM) Index is a very widely used image/video
quality model that continues to play an important role in the perceptual
evaluation of compression algorithms, encoding recipes and numerous other
image/video processing algorithms. Several public implementations of the SSIM
and Multiscale-SSIM (MS-SSIM) algorithms have been developed, which differ in
efficiency and performance. This "bendable ruler" makes the process of quality
assessment of encoding algorithms unreliable. To address this situation, we
studied and compared the functions and performances of popular and widely used
implementations of SSIM, and we also considered a variety of design choices.
Based on our studies and experiments, we have arrived at a collection of
recommendations on how to use SSIM most effectively, including ways to reduce
its computational burden.
