If two values are provided, the first value represents a standard deviation value along the x-axis of the coordinate system established by the primitiveUnits attribute on the filter element. The second value represents a standard deviation along the y-axis.
If one number is provided, then that value is used for both stdDeviationX and stdDeviationY. Negative values are not allowed.
A value of zero disables the effect of the given filter primitive (that is, the result is the filter input image). If the stdDeviation method is zero in only one of stdDeviationX or stdDeviationY, then the effect is that the blur is applied only in the direction that has a non-zero value.
If the stdDeviation method is not specified, then the effect is as if a value of zero were specified.
- Scalable Vector Graphics: Filter Effects, Section 15.25.19
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