[Therion] 3d model generation

[Steve Clark]

I understand there are some limitations in how the current 3d model 
generation works in therion, particularly in 'vertical' cave. I would 
like to try and understand how the current 3d engine/algorithm works so 
we can make best use of it to produce a good 3d model (ideally for .dxf 
output to 3dstudio for fly-by animations). I have attempted to read the 
source code, but this is way beyond me!

We have had lots of success with therion generally and have used it to 
process the data from our 3 year long project to survey the St Georges 
cave in the Lot region of France (diving cave). We will be publishing 
the 2d maps for this in the next few days. See 
https://www.facebook.com/groups/835902993089768/ for further details.

I have a few questions and would welcome any advice based on the 
experience people have gained from generating 3d models, particularly in 
part submerged/part dry caves :

1. The 'dimensions' point is used to define the height above and below 
the centreline plane. What is the 'centreline plane'? How is this 
defined in the model and is it horizontal either side of the centreline? 
It appears different in adjacent scraps at the same station and results 
in steps at joins. Even with no 'dimensions' points in the model, the 
cave is not always square just based on LRUD data alone.

2. How does 'passage-height' work? Is this just a label or is it used in 
3d model generation? If so, where is the origin? Is it height above 
centreline or does it use LRUD 'down' as the starting point.

3. If I put lots of 'dimensions' points in a scrap, will therion use all 
of them and increase the complexity of the model or does it take an average?

4. Is there a good way of drawing vertical cave using the current 
algorithm (or a cheat/work-around)? We have a large aven surveyed with 
hundreds of shots (20m dia, 50m high). When viewed in Loch, this 
actually looks ok. Every line has a passage around it and the ends of 
the passages actually form a shape that looks similar to the true shape. 
These doesn't appear in the .dxf output, just a set of polylines lines 
on the CENTRELINE layer.

5. We have generally found '-wall-source maps' gives a better model in 
.dxf format for our cave. We have a lot of twists in small passage and 
there are gaps at corners in the model if we use '-wall-source 
centreline'. Can we use lots of 'dimensions' points to create accurate 
cross-sections in 'maps' mode or does it just use a couple of points to 
define to shape?

6. The 'dimensions' point does not seem to accept a negative number for 
the down dimension. Say we have a cross-section in the cave where the 
centreline is low down on the left and the floor is above the centerline 
on the right hand side of the passage. The down dimension on the right 
hand side is negative and it results in an error. Is there a reason why 
these negative values are rejected?

Lots of questions I know. If there is some more information about this 
elsewhere in the archive I'm happy to go look for it.

Regards,

Steve Clark