SSIM being a global metric is not sufficient for many cases. Images with soft gradient tend to be overcompressed.
MozJPEG is a good improvement. But its trellis cost model causes noticeable blurriness on fine details [1]. Compare with the original [2], Guetzli [3] or my Optimage [4].
So, 'Without (Visual) Quality Loss' is such a stretch.
The biggest difference that I see is that the leaf veins are clearer in the original image than in the other three, and that the MozJPEG one did lose some of the finest details.
Seconded. I can only see differences if I change my monitor settings and zoom the image. So, for the original size as shown without changing configurations from the monitor, there is no clear looser/winner.
MozJPEG is a good improvement. But its trellis cost model causes noticeable blurriness on fine details [1]. Compare with the original [2], Guetzli [3] or my Optimage [4].
So, 'Without (Visual) Quality Loss' is such a stretch.
[1] http://i.imgur.com/6naOWSf.jpg [2] http://i.imgur.com/jny5miJ.jpg [3] http://i.imgur.com/gkO2HsP.jpg [4] http://i.imgur.com/GEshKJD.jpg