[PATCH] Show negative ceilings

Dirk Hohndel dirk at hohndel.org
Thu Jun 12 07:11:14 PDT 2014

On Thu, Jun 12, 2014 at 12:55:02PM +0200, Robert Helling wrote:
> Hi,
> it was suggested by Very Important People to show tissue loadings like the Petrel. I tried to argue that such a display is qualitative at best given that the scale for “tissue loading” is pretty much arbitrary (could be partial pressure but what does it mean that “inert gas loading in tissue 4 is 1.4711 bar”?) or it could be normalized to the value that would make the current depth or the surface the ceiling. Still, given that even without a diving the initial values are 79% of ambient pressure and thus non-zero (and thus some arbitrary number of your chosen scale) the visual impression does not mean much.
> But there is a very simple rescaling of the axis that makes immediate sense: That is translate that partial pressure to an M-value (or ceiling in our parlance). Then the unit is unit of depth. Problem is: We plot this already, nothing to be done.
> OK, then it was argued “I want to see how far from deco I am”. And fair enough we don’t display that. We only display the ceiling if it is actually below the surface (I thought this is where this program’s name comes from). And if you are still in NDL, there is no such ceiling. 
> But of course, numerically, there is a ceiling, it’s just at negative depth. So this patch enables negative values for the ceiling and thus shows as well how far away from deco you are. Note this is more a proof of concept as the rendering is pretty ugly. 

All good points.
But it is indeed ugly and not very intuitive.
I posted the link to the Petrel video for a reason. Most people who have
seen this in real life seem to consider it very intuitive and useful.
Actually, I really like it on my Petrel - the OSTC does something very
similar (and possibly Matthias will tell us that they started it and
others copied the idea - I really don't know who did it first).

Anyway, I really want that graph :-)

And I don't think it's that hard to do.


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