/* ============================================================================
   PARALLAX
   ----------------------------------------------------------------------------
   ONLY the big green signs, the gantries they hang from, and the clouds move.
   Road signs, buttons and the boat stay on his marks - everything floating at
   once read as noise.

   --par is the amplitude, as a share of the SECTION WIDTH so it scales with
   everything else. --par-bias shifts a whole travel up or down the screen
   without slowing it: the gantry carries -0.12, which lifts it clear of the
   copy beneath it now that there is sky above to lift into.

   Progress is per SECTION, not per element, and an amplitude that would carry
   a piece of art off the top or bottom of its own section is capped to what
   fits - see parallax.js for both. --par-lock names a group that must take the
   SAME cap: the sign, its copy and the gantry it hangs from are one object and
   have to be capped as one. The clouds are capped one by one, so a low cloud
   running out of room does not slow the high one down with it.

   THE SPREAD IS THE POINT. Clouds and signs on near-identical amplitudes move
   together and read as if nothing is moving at all, however large the number.
   The first clouds run at 20 against the signs' 12, so they visibly overtake:
   224px of relative travel at a 1400px screen.
   ========================================================================= */
.par        { will-change:translate }
.par-first  { --par:20 }                    /* the clouds you meet first, hardest */
.par-cloud  { --par:8 }
.par-sign   { --par:12; --par-bias:-0.12; --par-lock:gantry }                    /* THE GANTRY SHARES THIS. A sign
                                               hangs off a truss: give them
                                               different amplitudes and the sign
                                               floats off its own gantry. */