ImageFilter filter

The image filter to apply to the existing painted content before painting the child.

For example, consider using ui.ImageFilter.blur to create a backdrop blur effect

Source 

ui.ImageFilter get filter => _filter;
void filter=(ImageFilter newFilter)

Source 

set filter (ui.ImageFilter newFilter) {
  assert(newFilter != null);
  if (_filter == newFilter)
    return;
  _filter = newFilter;
  markNeedsPaint();
}