上一篇当中有介绍直接通过波段运算，但是更好的方式是通过内置的算法来实现，这样适配性问题会更好一些

normalizedDifference(bandNames)

Computes the normalized difference between two bands. If the bands to use are not specified, uses the first two bands. The normalized difference is computed as (first − second) / (first + second). Note that negative input values are forced to 0 so that the result is confined to the range (-1, 1).计算两个波段之间的归一化差异。如果没有指定要使用的频段，则使用前两个频段。归一化差值的计算方法是（第一-第二）/（第一+第二）。注意，负的输入值被强制为0，这样结果就被限制在（-1，1）的范围内。

Arguments:

this:input (Image):

The input image.

bandNames (List, default: null):

A list of names specifying the bands to use. If not specified, the first and second bands are used.

Returns: Image

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