POV-Ray : Newsgroups : povray.beta-test : v3.8b2. height_field input values at 0.0 not clean. : Re: v3.8b2. height_field input values at 0.0 not clean. Server Time
2 Mar 2024 01:50:41 EST (-0500)
  Re: v3.8b2. height_field input values at 0.0 not clean.  
From: Kenneth
Date: 17 Feb 2023 14:10:00
Message: <web.63efce3b1428d28b9b4924336e066e29@news.povray.org>
William F Pokorny <ano### [at] anonymousorg> wrote:
>
> I'm going to re-state things because what showed up where I
> have ???? characters above looks different on the web than what I see in
> Thunderbird(a).

No problem at my end; I see the correct syntax in the web portal.

Sorry for my longer-than-usual-delay in responding; I had to think hard about
this 'negative option being ignored' thing, and the difference between a 3-part
vs. 4-part select(). I was still thinking about it when I feel asleep last
night! William P's function construct makes better sense to me now; I finally
had the 'flashing insight'.

docs for the 4-part select:
When used with four parameters, if A < 0 it will return B. If A = 0 it will
return C. Else it will return D (A > 0).

I had to work out my own rather pedantic 'truth table' of sorts, as I see it --
hopefully corrected this time--

-------
Given  ((x<0.0) | (x>1.0))  as the first argument 'A':
The boolean results in parentheses are only ever 0 or 1 (true or false).

if x is indeed less than 0.0 OR greater than 1.0, that produces boolean TRUE (or
1) in the parentheses. Compared against select's 'zero by definition' for A,  1
is 'greater than zero'.  So according to the rules, the outcome of select() will
be argument D-- the f_boom macro. Effectively, it doesn't matter if x exceeds
either limit; the result will be TRUE in either case.

If x is exactly 0.0  *or inside the given range*, the boolean operation in
parentheses produces FALSE (or 0). But, for select's argument A
comparison, 'zero equals zero' -- and argument C is chosen, which is 0.0 in
William P's code and 0.3 in mine.
-------

So far, argument B is never chosen; C and D take care of every possible outcome
of    ((x<0.0) | (x>1.0))

HOWEVER, I see now that a simpler 3-part select() would not work when using
argument A as written-- because of the 3-part rules: Argument B is only chosen
when argument A is *less than* 0, which never occurs with  ((x<0.0) | (x>1.0))


Post a reply to this message

Copyright 2003-2023 Persistence of Vision Raytracer Pty. Ltd.