Odd calculated deco "ripples" (was Re: RFC: color change for calculated deco)
Dirk Hohndel
dirk at hohndel.org
Mon Jan 14 10:21:29 PST 2013
Perfect explanation. Thanks. This makes perfect sense.
I'll use this in the commit message when I'm back at my computer
/D
On January 14, 2013 10:11:57 AM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
> On Mon, Jan 14, 2013 at 9:29 AM, Dirk Hohndel <dirk at hohndel.org> wrote:
> >
> > That still changes the calculation by quite a bit if you are ascending
> > at typical 'travel speed' between deco stops - you'll move by 5 feet in
> > those ten seconds, right?
>
> Here's another way of explaining my second patch:
>
> - we calculate the ceiling at every single second, using the
> interpolated depth.
>
> - we then only *save* the ceiling at the points where we have a
> profile event (the whole deco_allowed_depth() function doesn't change
> any state, so we can just drop it entirely at points that we aren't
> going to save)
>
> So no, I do not agree with your "Right?". Because it does not change
> the calculations at all: the actual deco calculations are all using
> the exact same data.
>
> What it changes is what *ceiling* it shows based on those calculations.
>
> But that's a *visualization* thing, not a calculation thing. And it
> actually does it incorrectly.
>
> Why is it incorrect? I'll try to walk through my understanding of it,
> by switching things around a bit.
>
> - the whole "minimum tissue tolerance" thing could equally well be
> rewritten to be about "maximum ceiling". And that's easier to think
> about (since it's what we actually show), so let's do that. Agreed?
>
> - so turning "min_pressure" into "max_ceiling", doing the whole
> comparison inside the loop means is that we are calculating the
> maximum ceiling value for the duration of the last sample. And then
> instead of visualizing the ceiling AT THE TIME OF MAXIMUM CEILING, we
> visualize that maximal ceiling value AT THE TIME OF THE SAMPLE.
>
> End result: we visualize the ceiling at the wrong time. We visualize
> what was *a* ceiling somewhere in between that sample and the previous
> one, but we then assign that value to the time of the sample itself.
>
> So it ends up having random odd effects.
>
> And that also explains why you only see the effect during the ascent.
> During the descent, the max ceiling will be at the end of our
> linearization of the sampling, which is - surprise surprise - the
> position of the sample itself. So we end up seeing the right ceiling
> at the right time while descending. So the visualization matches the
> math.
>
> But during desaturation, the maximum ceiling is not at the end of the
> sample period, it's at the beginning. So the whole "max ceiling" thing
> has basically turned what should be a smooth graph into something that
> approaches being a step-wise graph at each sample. Ergo: a ripple.
>
> And doing the "max_ceiling during the sample interval" thing may sound
> like the safe thing to do, but the thing is, that really *is* a false
> sense of safety. The ceiling value is *not* what we compute. The
> ceiling value is just a visualization of what we computed. Playing
> games with it can only make the visualization of the real data worse,
> not better.
>
> Linus
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