AND they do the same looking at various places on the door, so they see the "distance map" image from various perspectives. This makes it more analogous to the door as a mirror, just as you'd look at various points on a mirror and see a reflected scene from the point of view of that spot on the mirror.

If you imagine an economy based on hydrogen from splitting water, a lot of hydrogen would be lost as H2 is very small and fits through cracks that O2 or N2 or CO2 wouldn't. H2 in the atmosphere easily reaches escape velocity at ordinary temperatures and so is lost to space. While it could take years to get to the scale where this is relevant (much as dumping in the ocean took years to become relevant), this isn't simply a recycling issue, as it is truly lost from Earth. The incentives are there to conserve all H2 generated of course as it's wasted fuel. There is also incentive not to spill oil, but the Exxon Valdez shows us that's not always enough. In the meantime, cheap solar-generated H2 would be great, as long as we remember the Hindenburg disaster from 1937.