[Therion] Two scraps in the same .TH2

[Tarquin Wilton-Jones]

Hi Rodrigo,


> A general question: can someone point me a reasonable situation where 
> having 2 scraps in the same TH2 file makes sense/is important?

It is exceedingly rare for me to have a .th2 with just one scrap.

If one passage overlaps another, then you need two scraps do they can be 
drawn below above/below each other properly (eg. with translucency, or 
just hiding the lower passage, or with background colours). That could 
just be a short roof tube, or a fork where one climbs over the other. It 
would be crazy to have another .th2 file for each of them, since you 
probably surveyed them at the same time. So instead, you have two scraps 
in the one .th2 file.

Back before you had smooth-shading, you needed to break your survey into 
several scraps just so it could colour them differently by altitude as 
the passage sloped uphill (eg. one scrap for every 5 metres altitude 
change). You still need to do that if you have abrupt changes in height, 
since smooth-shading will not change colour nicely at the edge of the 
drop, but it will change colour nicely at the edge of a scrap, as long 
as it has the correct stations marked.

If you have a high level passage that intersects a lower level passage 
(so you have a pitch down into it), then continues at high level on the 
other side, and you surveyed the upper level stuff all in one survey, 
you would have to end the first scrap where it joins the passage, then 
create a new scrap on the other side. This is because a scrap cannot 
have multiple outlines. Although Therion might sometimes understand it 
if you do have 2 outlines in one scrap, it cannot export it properly in 
KML format, and struggles with some other outputs too. So you are not 
supposed to do it. You are supposed to use two scraps.

I also have my ropes on a rigging topo in separate scraps from the cave, 
so we can use maps to select the rendering with and without some of the 
ropes (where a layout can only disable them completely).

Sometimes, I have another scrap just for adding another layer of 
information on a survey, such as hardware serial number labels for fixed 
aid maintenance. I don't want them on the main survey since regular 
cavers won't know why XY123 is written next to an anchor, but I want 
them for when I create a special version of the survey for the hardware 
maintainer.

Lots and lots of reasons :)


Cheers,

Tarquin