[Bf-committers] Temporary storage for Edit{Vert|Edge|Face}

[Martin Poirier]

Chris is talking about the Edit* structs, those aren't
saved to files, so no SDNA problem.

Pretty neat idea if you ask me, thumbs up (god knows I
abused that vn pointer more than my share of times).

Martin

--- Ed Halley <ed at halley.cc> wrote:

> 
> I think the big reason that stuff like Unions (and
> other things like 
> symbolic array size constants) is not used in many
> structures right now 
> is that the SDNA scanners can't read them.  That is,
> every member of 
> every structure which has a hope of being saved to
> disk in a .blend must 
> be given a stable name, must be a type that SDNA
> knows about, must be 
> aligned in all kinds of kookie ways, and the moon
> must be gibbous 
> waxing.  Nested unions are a nice idea, but the
> makesnda/ code has to be 
> made smarter before you can just pile multiple
> variables at the same 
> struct offset.
> 
> Chris Want wrote:
> > Hi folks,
> > 
> > You might recall that earlier this week I got
> > into trouble while trying to abuse short flags
> > f1, f2 of the EditEdge structure to hold a long
> > ... this was a bad thing to do because it will
> > fail on 64 bit platforms.
> > 
> > I was told by Ton that I should use the field
> > 'vn' ('vertex new') to hold my long -- this is
> > a pointer to an EditVert and so has sufficient
> > storage to hold the long.
> > 
> > Looking through the code, there are many instances
> > of this vn field of EditEdge and EditVert being
> > used to hold all kinds of things:
> > 
> > * pointers to EditVerts, not necessarily new ones
> > * longs
> > * pointers to EditFaces
> > * pointers to floats
> > * pointers to EVPtr (whatever that is)
> > 
> > And these uses involve ugly looking casts, and are
> > usually accompanied with a comment that states
> that
> > this use is an abuse of the pointer (which it is).
> > 
> > So I was grumbling about this on irc last night
> > when brecht had a great idea: why not use a union
> > to hold this temporary storage? I haven't used
> > unions much, but this seems like the kind of
> > application that a union was designed for.
> > 
> > Here's my proposal: replace vn with a union called
> > tmp:
> > 
> >         union {
> >                 /* some lean storage for temporary
> usage
> >                  * in editmesh routines
> >                  */
> >                 struct EditVert *v;
> >                 struct EditEdge *e;
> >                 struct EditFace *f;
> >                 float           *fp;
> >                 void            *p;
> >                 long             l;
> >         } tmp;
> > 
> > This union uses *no more storage* than what is
> currently
> > being used now. How does it work? Well, if you
> have
> > an EditVert eve and you need a long for temporary
> uage,
> > you would use eve->tmp.l ('temporary long') to
> store the
> > long, rather than doing a cast like (long)
> eve->vn.
> > If you need a pointer to an EditFace, you use
> eve->tmp.f
> > ('temporary face') You don't need to cast
> anything, you
> > don't need to add an apologetic comment, and in
> both of
> > these cases the same storage is being used for
> each task.
> > A real life example in the code would look like
> this:
> > 
> > BEFORE:
> > /* the vertices, abuse ->vn as counter */
> > ...
> > eve->vn= (EditVert *)(long)(a++);  /* counter */
> > 
> > AFTER:
> > /* the vertices, use ->tmp.l as counter */
> > ...
> > eve->tmp.l = a++;  /* counter */
> > 
> > I think that the AFTER code is much nicer, and the
> > name we store it in gives a better clue about
> what's
> > being held ('temporary long' instead of 'vertex
> new').
> > 
> > Anyways, this email is getting kind of long. I've
> put
> > a patch in the patch tracker that implements these
> > kinds of temporary storage unions for EditVerts,
> > EditEdges, and EditFaces. I'd like to commit this,
> > but I think it would be best to have some
> discussion
> > first. The patch is here:
> > 
> >
>
https://projects.blender.org/tracker/?func=detail&aid=3695&group_id=9&atid=127
> 
> > 
> > 
> > Regards,
> > Chris
> > _______________________________________________
> > Bf-committers mailing list
> > Bf-committers at projects.blender.org
> >
>
http://projects.blender.org/mailman/listinfo/bf-committers
> > 
> > 
> > 
> 
> -- 
> [ e d @ h a l l e y . c c ]
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>
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> 



		
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