kernelUnitLength
The first number is the dx value. The second number is the
dy value. If the dy value is not specified, it
defaults to the same value as dx. Indicates the intended
distance in current filter units (i.e., units as determined by the
value of parent filter container's primitiveUnits) for
dx and dy, respectively, in the surface normal
calculation formulas. By specifying value(s) for
kernelUnitLength, the kernel becomes defined in a scalable,
abstract coordinate system. If kernelUnitLength is not
specified, the dx and dy values should represent very
small deltas relative to a given (x,y) position, which might be
implemented in some cases as one pixel in the intermediate image
offscreen bitmap, which is a pixel-based coordinate system, and thus
potentially not scalable.