其中一个构造函数以双精度浮点数作为输入，另一个以整数和换算因子作为输入，还有一个以小数的String表示作为输入。
One takes a double-precision floating point as input, another takes an integer and a scale factor, and another takes a String representation of a decimal number.
在内部，可以用任意精度任何范围的值和一个换算因子来表示BigDecimal，换算因子表示左移小数点多少位，从而得到所期望范围内的值。
Internally, BigDecimal is represented as an arbitrary-precision unscaled value and a scale factor, which represents how many places to move the decimal point left to obtain the scaled value.
此变换算法保证给出的控制栅格点的准确对应，可得较高的精度。
The algorithm guarantees precise correspondence between controlling points, higher precision can be obtained.
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