ee.Reducer.mean()

Returns a Reducer that computes the (weighted) arithmetic mean of its inputs.

返回一个计算其输入的（加权）算术平均值的Reducer。

No arguments.

Returns: Reducer

代码：

Map.setOptions("SATELLITE");  
var image = ee.Image("LANDSAT/LC08/C01/T1_TOA/LC08_123037_20180611")  
              .select("B[2-4]");  
Map.centerObject(image, 7);  
var visParam = {  
  min: 0,   
  max: 0.3,  
  bands: ["B4", "B3", "B2"]  
};  
Map.addLayer(image, visParam, "rawIamge");  
  
//统计影像的平均值（以平均值的形式合成影像）
var mean = ima