Figure 12.

Sample images at (top) low and (bottom) high initial distortion levels. At initial distortion level (MSE = 4), the best/worst SSIM and the best/worst MSE images are visually indistinguishable, resulting in 50% (chance) discriminability, as shown in Figure 11. At high initial distortion level (MSE = 128), the best SSIM image has clearly better quality than the worst SSIM image (with the same MSE), thus high percentage value was obtained in the 2AFC experiment (Figure 11). On the other hand, subjects have very different opinions about the relative quality of the best and worst MSE images (with the same SSIM), as reflected by the large error bars in Figure 11.