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Here's a high plateau on mars, rising up above the surrounding landscape.
这是火星上的一个高原，从周围的景观中升起。
Or is it a system of valleys and canyonlands cut into the surrounding plains?
或者它是一个切入周围平原的山谷和峡谷地体系？
And here's another – is this text cut into the wood, or is it bumping out?
再来看看这个——这段文字是刻进木头里的，还是突出来的？
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.
有些多稳态知觉的例子，经常出现在观看阴影地形地图或航空和卫星照片的地形的时候。
Like the famous duck/rabbit illusion, it can sometimes be hard to perceive both sides of these illusions, so let's try one more:
比如这个著名的鸭子/兔子错觉，有时很难同时感知这些错觉的两面，所以再来看一个：
a series of plateaus on earth? Or the grand canyon?
这是地球上的一系列高原还是大峡谷？
So why do we sometimes see these features as popping up or out of the image,
为什么我们有时看这些特征突然出现或从图像中消失，
and why do we sometimes see them as cut or dented in?
为什么我们有时会看到它们是突出或凹陷的？
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.
主要有两个原因：首先，在地球上，太阳总是在头顶上，第二，光与影的奇怪对称。
Ok, so if you're a human who grew up on earth, you're used to the fact that light mostly comes from above.
如果你是生长在地球上的人类，那么你已经习惯了光大多来自上方。
I mean, the sun is always overhead, illuminating things from above, or above and to the side,
我是说，太阳总在头顶，从上方或上方和侧面照亮物体，
but never from below the horizon, which means that shadows are pretty much always on the bottoms of things.
但从来不会从地平线下方照亮物体，这意味着影子总出现在物体底部。
This is so ingrained in us and our culture that religion talks about falling down into darkness and being raised up into the light,
这在我们以及我们的文化中根深蒂固，以至于宗教谈论坠入黑暗，然后被升起朝向光明，
western art exhibits a "top-left lighting" convention, as does computer interface design,
西方艺术呈现出一种“左上角照明”的惯例，电脑界面设计也是如此，
and the international space station has lights on one side to help give floating astronauts a consistent sense of "up" and "down".
国际空间站的一侧有灯光，帮助给漂浮的宇航员一种持续的‘上下’感。
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.
‘影子在下，光在上’，让我们的大脑通过阴影的位置感知3D。
If there's a bump sticking out from a wall, the shadow will be on the bottom of the bump.
如果墙上有一处突起，那么影子将会在凸起处的底部。
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.
如果墙上有一个凹陷，那么影子将在突出部分的底部，即凹陷处的顶部。
Roughly speaking, when we have no other context and see shadows on the bottom of a feature,
粗略地说，当我们没有其他背景，并在物体底部看到影子时，
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.
我们会觉得它是凸面，朝向我们的，而当我们看到的影子在物体顶部，我们就会觉得它是凹面，往下陷的。
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.
但自然界中有一种残酷的对称：从一侧照射的凹面可以投射出与从另一侧照射的凸面相似的阴影。
A bump lit from above has shadows on the bottom, and a dent lit from below also has shadows on the bottom,
从上面照亮的凸面，它的阴影在底部，从下面照亮的凹陷的阴影也在底部，
which makes it easy to misperceive a dent lit from below as a bump, or a bump lit from below as a dent.
这使得很容易将凹陷误认为是凸起，或是认为凸起是凹陷的。
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.
同样，从东边照亮的山脉，其阴影在西面斜坡上；而从西边照亮的山谷，它的阴影也在其西面斜坡上。
Not really a problem when you're standing next to the mountain or valley.
当你站在山或谷旁边时，这并不是一个问题。
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.
但从足够高的地方看，我们所有的大脑其实是根据它们的阴影来判断景观特征的凹凸。
So we'll tend to see these geographic features as mountains when the shadows are on the bottom,
所以当阴影在底部，我们会将这些地理特征看做是山脉，
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.
阴影在顶部，就是山谷。当然，只有当光线恰好来自图像的顶部时，这才是正确的。
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,
实际上，山脉或山谷错觉非常强烈，以至于北半球的地貌晕渲图上显示的光通常都是从北边照过来的，
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
这个方向上的大部分地方都没有太阳的照射。地图制作者更愿意牺牲太阳位置的准确性
to ensure that they accurately communicate the geographic features they're mapping.
来确保他们准确地传达所绘制的地理特征。
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,
他们这样做很好，其实我决定做这个视频，是因为我在桌子对面颠倒着看了一张地图，
and my brain thought that all the valleys were mountains and mountains were valleys!
我的大脑认为地图上的所有山谷都是山脉，而山脉是山谷！
I had to get up, go around to the other side of the table and close my eyes for a while before I could get them to switch to being the right way round.
我得站起来，绕到桌子的另一边，闭会眼睛，才能让它们转到正确的方向。
So if you're confused why a map or aerial or satellite photo looks weird, try rotating it 180 degrees to see if it makes more sense!
所以，如果你不明白为什么一张地图、航空或卫星照片看起来很奇怪，试着把它旋转180度看看是否更有意义！
