Re: Comments

Paul Topping wrote:
> CAD systems often want groups to contain objects from several layers. A
> "geometry" layer might contain objects that form the boundary of a
> mechanical part and a "dimensioning" layer might contain the arrows,
> dimensions, and annotations applied to the part. Groups would be used to
> group the geometry of sub-parts; one for "bracket A", another for
> "subassembly B", for example.
> 
> If SVG allows an object to be a member of more than one group, one can do
> this.

For that sort of structure, which cuts across hierarchical grouping, we
need the notion of set membership. thew class attribute is ideal for
this, since it is a space separated set of things. For example:

svg
  g class="a"
    g
  g 
    circle class="a"
    rect class="b c"
  g 
    path class="a"
    path class="c"

.a visibility: invisible
.b visibility: visible

If desired, the opacity property can be used to make semi-transparent
layers (eg, underlays).

--
Chris