STL format, again

From: Andre Dolenc (
Date: Mon Dec 04 1995 - 10:18:18 EET

Peter H. Gien writes:
> But coming back to the old STL format, how are we going to shoe-horn color
> information into it? As for textures, perhaps Mr. Brock Rooney might wish to
> enlighten us on how many triangles would be needed to represent realistic
> surfaces, such as grass and hair?

Peter, it boils down to the size of the model and the amount of accuracy you
want. No one is disputing whether you need more geometrical elements using
triangles instead of high-order surfaces. Besides, you don't really represent
*textures* explicitly in a CAD system, do you? A texture in many CAD systems
is a *property* of the surface used when a picture is rendered.

You will never NEVER represent textures like grass, glass, hair, explicitly in
a 3D solid CAD systems; most of the ones used nowadays---ACIS-based in
particular---use Breps, and doing so would blow away all your memory, even the
xGB in a workstation. For that you would need to use CSG-based reps.

When you have in your possession an RP machine, you want to build parts
starting from anything that resembles a 3D model. Do you have any idea how
many CAD systems there are out there? The last time I looked at a catalogue
from Silicon Graphics, there were more than 50 CAD systems for mechanical
design alone.

Data transfer is a critical issue. When done via STL, you are putting
the responsibility of obtaining a solid were it belongs: the designer.
In RP, there will never be one standard for data transfer. Users will
and must be able to choose the route which suits their requirements best.
You will always find a situation where an STL representation is better than, say,
an IGES files, and vice-versa.

I hear "contours"? Yes, some people will be very happy to send contours, but
that determines the surface finish *and* the costs of building a part. How
many users of RP have the necessary knowledge of a specific process to do *that*?

Regards, Andre'

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