Rest pose and seven facial expressions as instructed to the volunteers.

<p>A: rest, B: voluntary smiling, C: pursed lips, D: raised upper lip, E: mouth open, F: depressed mouth corners, G: blowing, H: pouting.</p

The error values for <i>e</i>_<i>c</i> and <i>e</i>_<i>r</i> by volunteer with the mean optimal settings, i.e. volunteer-independent settings (see Eq (16)).

<p>The error values for <i>e</i>_<i>c</i> and <i>e</i>_<i>r</i> by volunteer with the mean optimal settings, i.e. volunteer-independent settings (see Eq (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175025#pone.0175025.e031" target="_blank">16</a>)).</p

The lowest error values for <i>e</i>_<i>c</i> and <i>e</i>_<i>r</i> by volunteer with the corresponding settings.

<p>The lowest error values for <i>e</i>_<i>c</i> and <i>e</i>_<i>r</i> by volunteer with the corresponding settings.</p