dxf2fds and Other Stuff

[Kevin McGrattan <kevin.mcgrattan@nist.gov>]

All

I forwarded some of your comments and questions about the
CAD to FDS converter
program called dxf2fds. I apologize for using the term
``AutoCAD'', which is a 
popular package for doing CAD (Computer Aided
Drawing/Design/Drafting, pick one). 
At the moment, dxf2fds picks up the objects labelled
``3DFACE'' and ignores all
other objects in the dxf file. For example, if the CAD
package dumps out a sphere,
it will often just dump out a center point, radius and
color, taking up far less
space than if it were to dump out a series of triangles or
other polygonal planes.
dxf2fds has no feature of parsing anything but the 3DFACE's.

Here are the replies from the author of dxf2fds, Dave
Sheppard, to some of your
comments:


Comment:

We used AutoCAD to make a DXF file that consists a simple 2
3d-faces 
(18 x 5 x 4 m), the program seems to do the following:

1- it read the objects from DXF with 10 times bigger scale,
the above
object will be then ( 180 x 50 x 40 m), and

2- it is only able to read 3D-faces but not the solids
within AutoCAD.

Response:

1- The DXF2FDS utility only converts the 3DFACE information
from DXF file.

2- FDS requires all coordinates in meters.  DXF2FDS utility
converts all
coordinates in the DXF file to meters from the units
specified in the
dropdown list labeled 'Length Units In DXF File" located in
Step 3.
Please let us know if more units should be added to the
dropdown list.





Comment:

I downloaded a DXF file from the internet of the Eiffel
Tower, and the
program worked for me.  My question for you is, how does it
decide on
cell size?  I never saw that as an input anywhere in the
program.  I
know you can change it once the input file has been written,
but I would
think it needs to know the cell size so it can 'cut' the
mesh, in other
words, if you have a diagonal in a DXF file, how can you
determine how
it is going to stairstep it, as it would need the cell size
before it
could stair step it.

Answer:

The program  currently uses the exact dimensions for faces
that are close
to orthogonal to an axis and throws away all other faces.
You are correct that you need to put in grid size for stair
stepping.  I
was going to add stair stepping in when I had a case that
required it
(probably within the next couple of months).  Do you have a
stair-stepping algorithm
that I could plug into the program?  The DXF file provides
12 numbers to
define each face representing four corners for each face
where each corner
is defined by an X, Y, and Z coordinate.




Comment:

How hard/possible is it to program in the ability to see
other cad files
besides DXF.  I don't like AutoCAD, but if say I had an iges
file or
something else more universal and not cad package specific,
how hard would
it be for smokeview to have an option to view it?


Answer:

DXF is a common data file format that is used by many
programs other than
AutoCAD.  The DXF2FDS conversion utility was developed for
use with the
'Chief Architect' Cad package.  It was only a happy
coincidence that
DXF2FDS works with the DXF files from many other programs.

Smokeview and FDS do not view the DXF file.  The DXF2FDS
utility attempts
to convert the 3DFACE information from the DXF file into the
FDS input file
format. The DXF2FDS utility was given to NIST by an FDS user
and was not
developed by NIST staff.




Comment:

I understand that you developed the DXF2FDS program, and
that it isn't 
supported by NIST.  I also know that it isn't reading in the
DXF file, 
but instead is reading in the FDS blockages from the input
file created 
from the DXF2FDS program.  But when you open the geometry in
Smokeview, 
you can see the nice blockages created from your DXF2FDS
program.  You 
can also turn on the CAD faces now in Smokeview.  I was
wondering how 
Smokeview is allowing the user to view not only the
blockages from the 
FDS input file, but also the CAD.  I think Kevin previously
told me the 
DXF2FDS program is creating, besides the FDS input file, a
separate file 
that smokeview can read the planes from the DXF file.  I was
wondering 
how that special file for Smokeview was created, or what the
file format 
was so that when I create an FDS file, if I have some CAD to
go with it, 
I could somehow create a similar file from the CAD that
smokeview can 
read in.  Anyway, I hope that makes sense,

Answer:

The CAD view in smokeview is derived from a separate file. 
This file is
defined in the FDS input file on the input line (example
below).  FDS
ignores this parameter except to pass it through to the SMV
file.

&MISC ...,RENDER_FILE='c:\Model\noosa.GE2' /

The render file consists of a series of two line face
information read
directly from the 3D face lines in the DXF file.  The first
line has 12
numbers representing 3 coordinates for each of four
corners.  The second
line has seven items

1) layername from DXF file : string
2) Red : Real (0 to 1)
3) Green : Real (0 to 1)
4) Blue : Real (0 to 1)
5) Not Used : Real
6) Not Used : Real
7) Not Used : string

Example:

-0.1715 -0.1345 -0.012 -0.006 -0.1345 -0.012 -0.006  0.044
-0.012 -0.1715
0.044 -0.012
A-CEIL-MAIN-0  1  0.9764706  0.854902  0.508001  0.508001 ""


Note: This intermediate file format may change at some
point. We eventually 
want to be able to pass info from the CAD package (color,
texture map, physical
properties) to FDS. Essentially, the CAD program could form
the basis of a front
end for FDS. We don't want to try to replicate the
functionality of a modern CAD
package. It would be a waste of time, plus we could never
keep up with all the
innovation taking place in the field of computer aided
design. -- Kevin




One more thing: we will continue to enhance this package
dxf2fds to make it more
versatile. Like everything else, it will evolve. Your
comments and suggestions
are welcome.

Kevin







-- 
Kevin McGrattan
National Institute of Standards and Technology
100 Bureau Drive, Stop 8663
Gaithersburg MD 20899-8663

Telephone: (301) 975 2712
FAX:       (301) 975 4052