Why do movie sets always spray water on the ground when filming outside shots?
電影片場拍外景時為什么總要在地上灑水？

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獲得1.4m好評的回答@Pat Roberts：

WARNING : AFTER READING THIS A LITTLE BIT OF MOVIE MAGIC WILL BE GONE FOR YOU. YOU’LL NOTICE THIS FAKE EFFECT ALL THE TIME - EVEN ON BRIGHT SUNNY DAYS.
警告：讀完這篇文章后，對你來說電影的某些魅力會消失，你總會注意到這個偽造出的效果——甚至在陽光明媚的天氣也會注意到。

It’s called a wet-down:
這被稱作 wet-down：

Excluding where it’s used to make it look like it it has rained:
那些為營造下雨效果而進行的灑水除外：

Daytime wet-down: Increased color saturation and contrast. Basically, it looks better.
白天灑水：提高色彩飽和度和對比度，主要就是視覺效果更好。

I can’t find a good before-and-after comparison, but you can do this yourself in your yard. Spray the plants and ground. The green will be more saturated and the dirt and rocks and bark will go darker and more saturated, leading to higher contrast.
我找不到好的灑水前后對比圖，但你可以在自家院子試試，在植物和地面都灑上水，綠色會更綠，泥土、巖石和樹皮顏色變得更暗，顏色更純，對比度也更高。

Nighttime wet-down: It looks prettier, plus it makes asphalt show up better. If you have a matte black street it just looks like a black or grey background. If it’s wet it’ll reflect light and make the street surface more visible.
夜間灑水：夜景更美，而且瀝青看起來效果更好。啞光的黑色街道看起來像黑色或灰色的背景，如果街道是濕的就會反光，使街道表面更顯眼。

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（翻譯：菲菲）