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The stdDeviation
attribute defines the standard deviation for the blur operation. If two <number>s are provided, the first number represents a standard deviation value along the xaxis. The second value represents a standard deviation along the yaxis. If one number is provided, then that value is used for both X and Y.
A negative value is forbidden. A value of zero disables the effect of the given filter primitive (i.e., the result is the filter input image). If stdDeviation
is 0 in only one of X or Y, then the effect is that the blur is only applied in the direction that has a nonzero value.
If the attribute is not specified, then the effect is as if a value of 0 were specified.
Usage context
Categories  None 

Value  <numberoptionalnumber> 
Animatable  Yes 
Normative document  SVG 1.1 (2nd Edition) 
 <numberoptionalnumber>

A pair of <number>s, where the second <number> is optional.
numberoptionalnumber ::= number  number, number
In the SVG DOM, a <numberoptionalnumber> is represented using a pair of
SVGAnimatedInteger
orSVGAnimatedNumber
objects.
Example
Elements
The following elements can use the stdDeviation
attribute