有时候我们想进行遥感映射，而我们却使用的就是正常的加减法。最后得到的只有两幅影像相交的部分。我们先看看函数：
subtract(image2)
Subtracts the second value from the first for each matched pair of bands in image1 and image2. If either image1 or image2 has only 1 band, then it is used against all the bands in the other image. If the images have the same number of bands, but not the same names, they're used pairwise in the natural order. The output bands are named for the longer of the two inputs, or if they're equal in length, in image1's order. The type of the output pixels is the union of the input types.
用图像1和图像2中每一对匹配的波段的第二个值减去第一个值。如果图像1或图像2只有1个波段，那么它将被用来对照另一个图像中的所有波段。如果图像有相同数量的波段，但名字不一样，它们就按自然顺序成对使用。输出的带子以两个输入中较长的那个命名，如果它们的长度相等，则以图像1的顺序命名。输出像素的类型是输入类型的联合。
Arguments:
this:image1 (Image):
The image from which the left operand bands are taken.
image2 (Image):
Th