On 4/10/2016 2:28 PM, Bald Eagle wrote:
> Stephen <mca### [at] aolcom> wrote:
>> On 4/10/2016 1:37 PM, Bald Eagle wrote:
>
>> Just one thing to mention. There are two coordinate systems*, Y up and Z
>> up.
>
> Right.  I've use right-handed, z-up, and it did, for whatever strange human
> reason, take a bit of getting used to when I started using POV's left-handed
> y-up system.
>

When I used to work for a living. I've used both for different parts of 
the job and polar coords as well. People just assume that everyone uses 
the one they're using.

Using Moray (Z-up) as a modeller then modifying the code in PovRay 
showed me a number of ways how to mess up.


> But some days (nights) _nothing_ makes sense, and that's why drawing things out,
> and checking both the math and _the assumptions_ you are progressing from, are
> always a good idea.
> I use lots of paper, Excel spreadsheets, unit circle diagrams, 3D space models
> or diagrams, test renders, etc.
>

I thought a "corner of a cardboard box" was a good idea. And a pencil 
and rubber comes in handy too.
I think Excel spreadsheets are the best thing since sliced bread. I 
start a new one for each project.



>> One of the reasons I use a modeller I cannot visualise scenes well
>> enough to be a true Pover.
>
>> * Not counting all the fancy ones like Polar, Plücker, cylindrical etc.
>
> I wouldn't really call polar "fancy" - especially since I often find it easier
> on the brain to "be lazy" and do the ole' define an object at the origin,
> translate, then rotate.   Saves doing a lot of trig to get the coordinates,
> especially if I'm not just rotating in one plane, but then tilting up into "3D".
>     Cylindrical is just polar with an "altitude" (z) thrown in.
>

Sorry, I was being flippant. Don't take everything I say seriously. 
Polar is very useful and you need to be able to switch between them when 
it is easier.

And that is not laziness it is common sense. I create things in unions 
at the origin. As I don't have direct access to the code until its 
exported. Having an extra union gives me a local set of axles.



> Plucker looks like something clipka might be well familiar with.   I have a
> feeling he's got a way better handle on all of that affine geometric stuff than
> I've got.
>
> But yes, SecondCup - you should take Mr. McAvoy's

No need to be so formal. :-)


>advice and make sure that your
> assumptions about your axes are correct.   That has caused me, and many other
> POV-ers way too many unnecessary headaches.   I've even done it when I KNEW I
> wanted to be avoiding it.   So write it down, calculate it, and graph it out.
> Don't just let your code fly into uncharted territory - nail down your FACTS,
> and double and triple check what you're doing until you get it to work out
> correctly.   Then - just for fun, and as a test of how robust your code is,
> change some things around a bit.   Add in things like negative values, zero,
> very large values, etc. to see how robust your system is, and to make sure that
> you're not just in some little coincidental mathematical bubble.
>

Oo! I don't know. I would say that once you get it working. Leave it 
alone. If it breaks next year, fix it then. :-)

I am  only joking, I think. But then I can't code so maybe I'm not. :-)



> “What are the facts? Again and again and again – what are the facts? Shun
> wishful thinking, ignore divine revelation, forget what “the stars foretell,”
> avoid opinion, care not what the neighbors think, never mind the unguessable
> “verdict of history” – what are the facts, and to how many decimal places? You
> pilot always into an unknown future; facts are your single clue. Get the facts!”
>
> ― Robert A. Heinlein
>
>
TO WOUND THE AUTUMNAL city.
So howled out for the world to give him a name.
The in-dark answered with wind.
All you know I know: careening astronauts and bank clerks glancing at 
the clock before lunch; actresses cowling at light-ringed mirrors and 
freight elevator operators grinding a thumbful of grease on a steel 
handle; student riots; know that dark women in bodegas shook their heads 
last week because in six months prices have risen outlandishly; how 
coffee tastes after you’ve held it in your mouth, cold, a whole minute.

Dhalgren

Samuel Delany

-- 

Regards
     Stephen

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