[Gzz] vob_considerations

[Tuomas Lukka]

> In the second stage, invented by Tuomas in Spring 2002, modified
> the system by introducing *coordinate systems*. 

Doesn't parse. the system was modified / he modified the system.

> A coordinate system
> is defined by a coordinate transformation (translation and scale
> or possibly a full affine transformation) relative to the canvas.
> It is coordinate systems that take identities (keys) in this system,
> not vobs. 

---------------------

> A vob is placed in one or more coordinate systems;
> cell vobs are placed in a single coordsys, connection vobs are
> placed between the two coordinate systems they connect.

Ouch, misunderstood what you were saying: I thought at first
"rectangle used in two locations", not connections.

> This system allowed the practical integration of connections
> into the vob system proper.

> In this system, vobs do not necessarily represent
> an object with an identity; those showing cells do, those
> showing connections don't (these vobs represent the relation
> between two different identities). 

> Probably it can be said
> that some vobs in this pattern represent objects with
> identity; these vobs fill the corresponding coordsys with content
> (for example, the cell vobs fill the coordsys for that cell).

Not really; it's not identity; it's just the image of the cell.

> Other vobs represent annotations to these objects; these do
> not fill a coordsys, but draw over or near to it
> (for example the connections, but possibly also a vob
> that shows a little icon right next to a cell to indicate
> something about the cell).
> 
> It is still basically assumed in this system that there
> is a set of identities, and each coordsys represents one member
> of that set. In practice, however, even if most coordsys represent cells,
> it is common practice for some to hold other stuff (for example
> the name of the currently shown view etc.).

I'd like to tighten and reorganize the above a little (starting
at -------). How about

        Because coordinate systems have identities and are interpolated
        between keyframes, vobs are now less overloaded: they are simply
        graphical objects placed to be drawn in a coordinate system.

        The most important effect of this change is that it allows
        a single Vob to span two coordinate systems. The earlier system
        allowed animation of connections between vobs only in a kludgy
        way. The new system allows the connection vob to know both
        its start and end coordinate system and trivially draw itself
        from point A in coordsys 1 to point B in coordsys 2.

        It is still basically assumed in this system that there
        is a set of identities, and each coordsys represents one member
        of that set. In practice, however, even if most coordsys represent 
cells,
        it is common practice for some to hold other stuff (for example
        the name of the currently shown view etc.).

(the identity discussion seems confusing)



> (Note: A coordsys c1 inside coordsys p1 is interpolated
> to c2 inside p2 if the keys of c1 and c2 are equal, and if
> p1 is interpolated to p2. This is somewhat similar
> to the vob paths from above.)

Not true. Note that there are two DIFFERENT concepts of "inside":
there's the MATCHING-hierarchy, and then there's the TRANSFORMATION
hierarchy. This should be made clearer.

Otherwise, good job.

        Tuomas