Possible to add support for Three-component SSIM ?

[pastelsky]

According to wikipedia, this algorithm is a variation of SSIM that has following advantages

Three-component SSIM (3-SSIM) is a form of SSIM that takes into account the fact that the human eye can see differences more precisely on textured or edge regions than on smooth regions.[4] The resulting metric is calculated as a weighted average of SSIM for three categories of regions: edges, textures, and smooth regions. ...
This SSIM variant gives results which are more consistent with human subjective perception

Adding this would help in cases when preserving the image texture is important.