Newsgroup: it.comp.grafica
Titolo: Superfici di Classe A

Salve a tutti ,
ho una domanda da sottoporre al Newsgroup:
chi mi sa' dare riferimenti a documentazione , libri o riviste che trattano
in modo abbastanza chiaro cosa sono queste benedette superfici di Classe A
???
Qualunque indicazione / suggerimento / nozione e' bene accetto.
Grazie
Nicola

 ???
mai sentite... dove ne hai sentito parlare e qual e' il loro impiego?

 Mi scuso se ho detto una castroneria :)
La descrizione delle superfici di classe A fatta in modo tecnico é
abbastanza complessa, dicamo in sintesi e con enorme approssimazione che
sono superfici di alta qualità  che servono a descrivere geometrie tipiche
dell'industrial design specialmente in campo automobilistico) caratterizzate
da una bassa variazione di curvatura (un pò come l'antialiasing di
un'immagine) che é un obiettivo importante nel caso di reverse engeneering o
di prototipazioni rapide quando si ha a che fare con mesh poligonali dove
bisogna intervenire con un resampling per eliminare l'inevitabile
scalettatura che esse comportano.

 wrote in message
news:a7a9pr$h9e$1...@serv1.albacom.net...

la stessa domanda posta in un news di rhino
ciao gerry
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"A Class Surfacing"
At the Catia Operator exchange, Hakan wrote:
"Hear is a further understanding of Class-A surfacing based on experiences
with two automotive companies and whites goods manufacturers. They
independently have the same
definition for the classification.
The physical meaning:
Class A refers to those surfaces, which are CURVATURE continuous to each
other at their respective boundaries. Curvature continuity means that at
each "point" of each
surface along the common boundary has the same radius of curvature.
This is different to surfaces having;
Tangent continuity - which is directional continuity without radius
continuity - like fillets.
Point continuity - only touching without directional (tangent) or curvature
equivalence.
In fact, tangent and point continuity is the entire basis most industries
(aerospace, shipbuilding, BIW etc etc). For these applications, there is
generally no need for curvature.
By definition:
Class A surface refers to those surfaces which are VISIBLE and abide to the
physical meaning, in a product. This classification is primarily used in the
automotive and
increasingly in consumer goods (toothbrushes, PalmPC's, mobile phones,
washing machines, toilet lids etc etc etc). It is a requirement where
aesthetics has a significant
contribution. For this reason the exterior of automobiles are deemed
Class-A. BIW is NOT Class-A. The exterior of you sexy toothbrush is Class-A,
the interior with ribs and
inserts etc is NOT Class-A.
The consequence:
The consequence of these surfaces apart from visually and physically
aesthetic shapes is the way they reflect the real world. What would one
expect to see across the boundary
of pairs of point continuity, tangent continuity and curvature continuity
surfaces when reflecting a straight and dry tree stump in the desert????
* Point Continuity (also known as G0 continuity) - will produce a reflection
on one surface, then at the boundary disappear and re-appear at a location
slightly different on the
other surface. The same reflective phenomenon will show when there is a gap
between the surfaces (the line markers on a road reflecting across the gap
between the doors of a
car).
* Tangent Continuity (also known as G1 continuity) - will produce a
reflection on one surface, then at the boundary have a kink and continue.
Unlike Point continuity the
reflection (repeat REFLECTION) is continuos but has a tangent discontinuity
in it. In analogy, it is "like" a greater than symbol.
* Curvature Continuity (also known as G2 continuity, Alias can do G3!) -
this will produce the unbroken and smooth reflection across the boundary.
Please do not believe me! This is the real physical world. Look at your cars
rounded hood reflecting lines on the road or trees. Look at ripples of water
that are not turbulent,
reflection is everywhere but all blend into each other, as there is also
curvature continuity everywhere.
What about CATIA??
Traditionally CATIA has been used to create the "engineering" side of most
designs, rather then the exterior "aesthetic" shell (ie Class-A). These
traditional yet awesome tools
(like SURF2) are geared for this kind of engineering work. The best example
being BIW in the automotive industry.
CATIA comes from the aerospace industry. The exterior of aeroplanes (whose
panels buckle between frames and expand with every land-takeoff cycle) has
very little "need"
for curvature continuity and has 100% engineering factors driving its design
(aerodynamics and structures).
That is, there is zero styling in the design of an aircraft body. The fact
that aeroplanes looks good and "smooth" is by virtue of its operation
(streamlined as possible), their
general cleanliness and most importantly the distance that one generally
views them. If one was to look carefully down the fuselage of an aircraft on
the ground, there is nothing
smooth about it!
Having the capability to cater for these industries in an engineering and
process capacity with existing function and not requiring the ability to
create Class-A, has made CATIA
the de-facto standard for the aerospace and automotive industries.
As for Class-A, automotive manufacturers have utilised either or
combinations of Alias and/or ICEM Surf (or others) to achieve these goals in
a productive manner (remember
the word productive). Alias has the ability cover the entire industrial
design process from Sketches TO Surfaces on sketches TO Surface manipulation
and build and further
onto rendering and animation.
In retrospect, CATIA V4 can create Class-A surfaces with (1) compromise (eg.
this deviation is OK, because it can be polished by the toolmaker) and (2)
an idiosyncratic
approach by the CATIA operator - ie, it can be done but not as easily as
with Alias or ICEM Surf.
Historically, its been "difficult" of Dassault to create software in V4 to
easily create Class-A surfaces due to the use of Bezier (polynomial) based
mathematics. There is nothing
against Bezier based surfaces though. They are excellent for creating the
engineering surfaces we have all come to love (BIW etc) utilising
intelligent use of multi-patch surface
methodology. In fact, I doubt NURBS surfaces could do a better job.
And without a doubt, V5, with its new architecture and use of Bezier and
NURBS surfaces will go along way in being able to confidently and more
importantly competently
producing these Class-A surfaces for an ever growing aesthetic minded
world."
Hakan Ozcelik
Adelaide, Australia.
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