Work and World

The “Mountain Or Valley?” Illusion

Here’s a high plateau on mars, rising up
above the surrounding landscape. Or… is it a system of valleys and canyonlands
cut into the surrounding plains? And here’s another – is this text/image
cut into the wood, or is it bumping out? Like the famous duck/rabbit illusion, it can
sometimes be hard to perceive both sides of these illusions, so let’s try one more:
a series of plateaus on earth? Or the grand canyon? These are examples of a multistable perceptual
illusion that often comes up when looking at shaded relief maps or aerial and satellite
photos of terrain. So why do we sometimes see these features
as popping up or out of the image, and why do we sometimes see them as cut or dented
in? There are essentially two reasons: First,
the fact that on earth the sun is always overhead, and second, a weird symmetry of light and
shadows. Ok, so if you’re a human who grew up on
earth, you’re used to the fact that light mostly comes from above. I mean, the sun is always overhead, illuminating
things from above, or above and to the side, but never from below the horizon, which means
that shadows are pretty much always on the bottoms of things. This is so ingrained in us and our culture
that religion talks about falling down into darkness and being raised up into the light,
western art exhibits a “top-left lighting” convention, as does computer interface design,
and the international space station has lights on one side to help give floating astronauts
a consistent sense of “up” and “down”. And “Shadows on the bottom, light on the
top” gives our brains a way to perceive the 3D-ness of features just from the position
of their shadows. If there’s a bump sticking out from a wall,
the shadow will be on the bottom of the bump. If there’s a dent in the wall, the shadow
will be on the bottom of the overhang, which means the TOP of the dent. Roughly speaking, when we have no other context
and see shadows on the bottom of a feature, we perceive it as convex, coming towards us,
and when we see shadows on the top of a feature, we perceive it as concave, or dented away. But there’s a cruel symmetry in nature:
a concave feature, lit from one side, can cast very similar shadows to its convex counterpart,
lit from the other side . A bump lit from above has shadows on the bottom, and a dent
lit from below also has shadows on the bottom, which makes it easy to misperceive a dent
lit from below as a bump, or a bump lit from below as a dent. Similarly, a mountain range lit from the east
has shadows on its western slopes, while a valley lit from the west also has shadows
on its western slopes. Not really a problem when you’re standing
next to the mountain or valley. But viewed from high enough above, all our
brains really have to go on for the concavity or convexity of landscape features is their
shadows. So we’ll tend to see these geographic features
as mountains when the shadows are on the bottom, and valleys when shadows are on the top. Which, of course, is only correct if the light
happens to be coming from the top of the image. In fact, the mountain or valley illusion is
so strong that shaded relief maps of the northern hemisphere generally show light coming from
the north, a direction the sun never actually shines in most of those places. Cartographers are more than willing to sacrifice
the accuracy of the sun’s position to ensure that they accurately communicate the geographic
features they’re mapping. And it’s good they do – I actually decided
to make this video after I was looking at a map upside down from across a table, and
my brain thought that all the valleys were mountains and mountains were valleys! I had to get up, clear my head and go around
to the other side of the table before I could get them to switch to being the right way
round. So if you’re confused why a map or aerial
or satellite photo looks weird, try rotating it 180° to see if it makes more sense! Hey, Henry here, today I thought I’d try
something different. I’ve been wanting to learn how to use Adobe
character animator for ages, and it just so happens that this video is sponsored by the
online learning site skillshare and they have lessons on character animator. No joke, as of this morning I had no idea
how to use this software, and now you’re seeing me, in stick figure form, animated
in character animator. So yeah, skillshare clearly has lessons for
learning stuff, and not just animation: cooking, photography, programming, etc. It’s about 10 dollars a month, but you can
get two free months by going to, which also lets skillshare know you were sent
there by me in animated stick figure form. Bye!
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