Flash 8.
filter.knockout
Property; a Boolean value that specifies whether the filter is a knockout filter (true) or not (false). This property is defined for Filter objects with a value of "bevelFilter", "dropShadowFilter", "glowFilter", "gradientBevelFilter", or "gradientGlowFilter" for the filter.name property.
The following example sets the knockout property to true for the Glow filters on the selected object(s):
var myFilters = fl.getDocumentDOM().getFilters();
for(i=0; i < myFilters.length; i++){
if(myFilters[i].name == 'glowFilter'){
myFilters[i].knockout = true;
}
}
fl.getDocumentDOM().setFilters(myFilters);