I guess there aren't to many Dead Heads in the forum.

Here is my stab at this.

Is there a known way to take AutoCad drawings (.dwg or .dxf) files and convert them to .thr files?

I think this would look better using a table with minimal sand.

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This is superb!

And yes - there is a direct way to go from pline to thr file (I am a long (long) time user of ACAD :).

Thank you for sharing.

Bruce

sadly, the AutoLISP file I attached is not (yet) allowed - coming soon...

Can you email it to me or use a dropbox?

I am dying to create!

Thank you,

Can you give me the name of the lisp or any information on it?

There is a good chance I would be able to find it. Lisp routines are shared openly across the web.

Sorry for the delay - instead of waiting for my web-devs to add *.lsp to the list of attachments, I just changed the extension (change back to .lsp after downloading). Since I wrote these routines back in ~2000 using much older ACAD versions, and I was very into debugging the conversion, there are two routines involved and some couple caveats:

1- This is a two step process: First, create a circle with center at 0,0 and radius of 1 (this is the just to remind you of the boundary of your drawing canvas). Within the circle, put/draw your artwork consisting of a single polyline composed only of straight segments (no arcs). I think line width is ignored, but set it to 0 if it complains (doesn't work). Make sure the polyline is the old style "lightweight". On newer versions, this is not the default polyline type. To change to this type, enter "plinetype" at the command line, and enter "0".  Load polarxy2c.lsp. Execute it by typing "makepolar" at the command line. It will prompt you to choose a pline. Do so and <enter>. You then should see a deformed version of the original. The routine has changed the XY positions of each vertex of the original to polar coordinates (numerically), and plotted them as new positions (as if they were still XY coords, hence the distortion). If that worked, you are ready for the next step (next post).

Step 2:

Load PL2thr1.lsp (attached extension mod below). Run by entering "pl2thr" at the cmd line. Choose your now distorted polyline and hit <enter>. It will prompt your for a file name and location, and create the thr file there.

Some caveats:

• Yes this could be a single step by combining the routines, but I was too lazy then, and I don't use it very often now (plus, I think the intermediate distortion looks interesting at times).
• Just vanilla plines (type 0). No arcs, splines, etc.
• Start and end your tracks either at the center or the edge of the circle. Check the resulting thr file. The rho value of the first and last vertex in the file should be either 0 or 1 (start and end can be the same rho). Rho = 1 is anywhere on your boundary circle.
• Remember, in the polar world, the path between 2 points is a spiral (Archimedes). Erase.thr is a 100-rev spiral having only two vertices - starting at 0 0, and ending at 200*Pi 1. If you want a straight line in your artwork to appear straight on the sand, it must contain points along its span - these "tack it down" during the conversion. The number of these tack-down vertices you need to add is up to you - more looks better, but makes the file longer (and at a certain point just slows things down and doesn't produce better output on the sand).

Let me know how it goes :).

AWESOME!

Thank you for taking the time to type all that up.

I'm looking forward to hammering more patterns out!

I'm sure I'll be back with questions after I put some time in.

Again, Thank You very much! 

When making the PLine, is it ok to back track along the same PLine?

example: drawing a capital T you would have to start at the upper left of the top of the T then go across to the far right, then back half way then down for the vertical section.

Yes. That does make it a lot harder to edit the vertices though. I often make the back-tracking slightly offset so I can easily grab its vertices if I want to edit them.