[Therion] Adjust opacity value in transparent color

[Robert Staven]

Hi,

I'm using Therion to map waterfilled caves, and some systems have 
'air-locks' (air chambers), so it's necessary to distinguish between air 
filled cave, waterfilled cave and water surfaces. But the waterfilled 
area should show details (be transparent) in same way as dry cave.

Is there any way to adjust the opacity value (in meta code?) when 
setting a transparent color?


I have made a test to show my issues:
The files can be found on http://www.staven.no/cavetest/


The cave (ca 100m long)
plan:
-> http://www.staven.no/cavetest/testcave_plan.pdf
elevation:
-> http://www.staven.no/cavetest/testcave_elevation.pdf

This doesn't work to well draw underwater maps.


*** Changing the water color:

layout test
   code metapost

     def a_water (expr p) =
       T:=identity;
       thfill p withcolor (0.8, 0.8, 1.0);
     enddef;

   endcode
endlayout

-> http://www.staven.no/cavetest/testcave_elevation_2.pdf

Looks better but the cave and line 'behind' doesn't show thru the water.


*** Try with transparent water color:

     def_transparent_rgb(tr_lblue, 0.80, 0.80, 1.00);

     def a_water (expr p) =
       T:=identity;
       thfill p withtransparentcolor tr_lblue;
     enddef;

-> http://www.staven.no/cavetest/testcave_elevation_3.pdf


*** Almost perfect, but I like to adjust the opacity value so it looks 
like this:

     def a_water (expr p) =
       T:=identity;
     enddef;
     # no drawing of water

   color map-fg [80 80 100]

-> http://www.staven.no/cavetest/testcave_elevation_4.pdf
(NB You won't see the difference in Adobe Reader, but it shows up in 
SumatraPDF)


Is there any way to adjust the opacity value when setting a transparent 
color?

Or should I do this another way?