How do you generate a bottom TIN surface from three-dimensional modeled object?

[bobwoodruff4]

I received a dxf file of a modeled contamination plume containing 3d face objects (See 1st image below). This "plume" was modeled using C Tech's Earth Volumetric Studio (EVS). I now need to create a TIN surface of the bottom of the plume. I've tried doing by "Adding Object Data To A Surface", but because the 3d face objects are essentially a three-dimensional object, the TIN surface created is not accurate (see 2nd image below).

Does anyone know how to generate a bottom surface from three-dimensional object?

I've attached the DWG file for reference.

Image 1 = modeled contamination plume

Object Viewer.PNG

Image 2 = TIN Surface Generated Using 3D Faces

Object Viewer 2.png

[brian.strandberg]

Normally, I would do this sort of thing by layer, top being one layer bottom being another.  Taking a look at your drawing, it looks to me like it is all one layer. 

I think you will need to isolate which points represent which bit of data you need to model.  If you know you can exclude elevations above or below a certain value, you can try specifying these in the surface build options.  I think the best way will be to separate the data into two chunks.

brianstrandberg_0-1680742074157.png

[JamesMaeding]

@bobwoodruff4 

You need to separate the faces into an upper and lower layer.

There are no undercuts or caves allowed in a TIN, so you have to watch that.

Don't forget in acad you can select with polygonal windows using WP and CP options when selecting.

Use 3d orbit a lot.

One approach rather than layers is to move the bottom items lower in space.

I have a "LF" (short for lift) command we use for this a lot. Paste this on command line to make available:

(DEFUN C:LF   () (ECHOOFF)(princ "\nLift or Lower by Delta")(OR LF:AMT (SETQ LF:AMT 1.0))(IF (SETQ TMP (GETREAL (STRCAT "\nDelta <" (RTOS LF:AMT 2 3) ">:")))(SETQ LF:AMT TMP))(COMMAND "_MOVE" (SSGET) "" (LIST 0.0 0.0 0.0) (LIST 0.0 0.0 LF:AMT)) (PRINC))

 Then change the layer and move back. This helps see things more clearly sometimes.

I just looked at your drawing, it has faces "crossing" the upper/lower edge.

That is a problem, as very few people have toolsets for trimming 3d faces.

But.....I happen to have good tools for this. Its still tricky though, as drawing the 3d boundary along the edge is like trying to hand pick the top of a circle. I'll see what I can do.

thx

[AllenJessup]

A lot depends on what you mean by bottom.  I set the view to look at it from between the right and back. Flat in the X,Y plane. Then selected by eye.

AllenJessup_0-1680810862867.png

[JamesMaeding]

@bobwoodruff4 

I messed with this more, as a challenge.

Separating out the completely top and completely bottom parts left the faces that needed to be trimmed.

I drew 3d lines from nose to nose to form a trimming plane edge, then assumed that plane was flat as you went away from the edge. Kind of odd to explain but it makes sense for how you would line up a knife to cut this if were real.

My 3d face trimming tools were useless on this, as I expected, so learned how to trim them another way.

You can convert them to planar surfaces using CONVTOSURFACE. Then draw a 3d box which will be subtracted from them. To do that, use UCS, pick an end of the blue line, then the other end to set X axis, then enter to accept.

Then use BOX command to draw the box, extruding it up enough to envelop the faces.

Then use subtract on the surface items, and subtract the box. Explode the surfaces to regions, and then to 3d lines at some point. I did not get that far. I had never used the UCS command and X axis like that before, I always say Face, and use some 3d object's face to set the ucs.

Once I did all that, the edges have a problem, looks like:

JamesMaeding_0-1680814955211.png

What is going on is your faces along the edge are like a deck of cards that has been fanned like a dealer would do.

They partially overlap. So even though I cut them using a plane that separates upper from lower along the noses, the in between is a bunch of under and overs, like twisted window blinds a bit.

That is a problem for the end goal.

To do a tin surface, you basically have to draw 3d lines instead of those overlapping faces.

The lines form breaklines for the surface.

I attached the drawing so you can take a shot at it. You can add the 3d faces and 3d lines to your c3d surface once done.

[JamesMaeding]

@AllenJessup 

Exactly. A lot is obvious, but still takes careful selection.

The overlapping faces are then a nightmare.

IMO, the maker of the plume software should have done better. The 3d triangulation engines I have seen allow splitting of triangles to some minimum angle, so no long thin ones.

Also, the software needs to interpolate between known data better, to add in between data to control its triangulation.

Then you don't get "fanfold" type tris reaching between wireframe linework like we see.

Just like how civils add breaklines on pavement at intersections to control the warp between EP and centerlines.

That is a compliment to that software, it does its main job well, so worth the extra attention to surface output.

thx