mask()
Produces an SVG "mask" element.
Syntax
mask(
   id STRING,
   x STRING,
   y STRING,
   width STRING,
   height STRING,
    node om.DomNode,
    name STRING )
- id is the SVG object identifier.
 - x is the X coordinate of the mask.
 - x is the Y coordinate of the mask.
 - width is the width of the mask.
 - height is the height of the mask.
 - maskUnits defines the coordinate system for x,y,width,height.
 - maskContentUnits defines the coordinate system for the content of the mask.
 
Usage
This function creates a "mask" SVG DOM element from the parameters.
To use a mask in an SVG DOM element, set the "mask" attribute with the string
generate by the url() function.