The commands \settoheight, \settodepth, and \settowidth invoke \setbox without producing output. To avoid fake pictures, we do the following.
<..latex ltlength..>
\let\:settodim=\@settodim
\def\@settodim#1#2#3{\PictureOff \:settodim#1{#2}{#3}\PictureOn}
-_-_-