[Therion] 3d model generation

[Stacho Mudrak]

Here are some answers to you questions:

1. centerline plane is a fictive plane, that is on the same level as
centerline. Obviously, for steep or vertical caves, it does not make sense.
It would make sense to create 3D model from elevation projection instead.
If there are adjacent scraps, as far as I remember, both scraps need to
have both stations of survey shot crossing them present. Otherwise there
will be difference in the height.

2. Passage height works same way as point dimensions, but the origin is in
1/3 of height (max 1.5m). LRUD is not used. I am not 100% sure, but this is
how I remember it.

3. Weighted average of point dimensions is used.

4. No. I do not think so. With exception of LRUD model made by Loch viewer.
But Loch is using different algorithm to generate 3D model from LRUD data
then therion it self. You can export 3D model from Loch only to VTK file
format. But I think, there are a lot of converters out there from VTK
format to DXF and other standard 3D formats.

5. I do not think a lot of point dimensions will help. Even I have written
this 3D generation algorithm, it was allways a "black box" for me. A new
one had to be written, but it is still on the TODO list.

6. There is no special reason for this - with exception of how algorithm
was supposed to work (obviously, it does not work like that). If you are
using self compiled therion, you may try to comment lines (1233, 1234, 1241
and 1242) in thpoint.cxx and see what happens.

HTH, S.


On 3 June 2014 21:21, Steve Clark <sbc23 at cantab.net> wrote:

> 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
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> Therion at speleo.sk
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