Sketchup is Inacurrate???

mac1 · 3 Apr 2012, 18:26

Wow! Lots of posts. Think I've read through them all but I don't think the following has been pointed out... Sketchup is based on floating point data structure. These are inherently inaccurate. Try the following out in the ruby console

(Math::PI).to_l
=> 3.14159265358979
(Math::PI * 10**13).to_l
=> 31415926535897.9

This means that the further away from the origin your entities are, the more inaccurate they are.

Floats are a lightweight data structure ideal for a program like Sketchup. There are others that are actually capable of manipulating irrational numbers (pi, square root of two...check out GMP) without ever loosing accuracy but they are very heavy weight and only really necessary if you're, I don't know, sending a rocket to the moon or something.

Sketchup overcomes this floating point inaccuracy by allowing for a little tollerance but as you can see the difference is still there

point1.to_s + point2.to_s
=> (258,878708mm, 172,933835mm, 0mm)(0mm, 0mm, 0mm)
point1.z == point2.z
=> true
point1.to_a
=> [10.1920751212053, 6.80841870273468, 1.77635683940025e-015]
point2.to_a
=> [0.0, 0.0, 0.0]

Nonetheless Sketchup is a great tool and its inaccuracy (that is also present in those other "more professional" packages!) can be overlooked 99% of the time. Hope this was of use to anyone.

See my 3-16 post above and has been addressed before. The reason some packages show more accuracy the spec allows for extened or extendable implimentations. The single float 32 is only good to 7.225 digits.

jeff hammond · 4 Apr 2012, 00:43

@unknownuser said:

Well, thank god for construction industry standards allowing actual built accuracy to be 1/16" in ten feet or greater for different trades!

that tolerance should be set aside for the builders though.. with framing, i generally give my crew an 1/8" before i start bching at them or start contemplating a redo..

but, if i do that in the drawing.. say, give myself an eight in the model, then i'm looking at 1/4" errors onsite.. or, my crew has to be 100% perfect according to the drawing (i.e.-basically impossible)

EDIT
well, i already do cut in to the tolerance a bit when handing out a cultist.. everything is rounded to the 16th" even if the drawing calls for x/32" etc.. but yeah, there's definitely some allowable play in there.. i just prefer the drawing or calculations to be as close to perfect as possible (mainly because i think it IS possible.. we are dealing with computers here ).. and as i said early in the thread, sketchup is highly accurate and 100% capable of fulfilling my needs.. the rest of my babbling about in this thread has be regarding one small situation which (i feel) is mistreated in sketchup.. and i can still draw those parts accurately in sketchup-- i just use a different approach than whats offered in the automated toolset (offset tool/followme tool)

thomthom · 4 Apr 2012, 09:28

@unknownuser said:

when handing out a cultist..

Rich O Brien · 4 Apr 2012, 09:32

You dare judge Brother Jeff?

pilou · 4 Apr 2012, 09:43

These 1/4", 1/8", 1/16", 1/32" are for me perfect obscurantist measures from occultism !

thomthom · 4 Apr 2012, 11:38

@unknownuser said:

These 1/4", 1/8", 1/16", 1/32" are for me perfect obscurantist measures from occultism !

TIG · 4 Apr 2012, 12:22

[off:2hxvcf0c]In the UK at least no one uses the arcane 'pounds, shillings and pence' any more!
The UK dragged itself into the 20th century when I was a lad...
12 pence = 1 shilling
20 shillings = 1 pound
There were even coins as ha'penny [½d] and farthing [¼d]
[somewhat illogically 'd'=pence ]
We had other coins worth 1d[penny],3d[joey],6d[tanner], 1s[bob],2s[2-bob/florin],2s/6d[half-crown] and banknotes for 10s[10-bob],£1[a quid],£5[a fiver]...
There were also olde coins for 5s [crown] and guinea [£1/1s]***
So something would be priced as "£1/10s/4½d"
Nows we have decimal pounds/pence and coins up to £2.
So it is now approx. £1.53.........

*** The 'guinea' was an interesting idea.
If you were doing a service like an auctioneer you priced/charged the buyer in guineas and paid the seller in pounds - thereby keeping the standard 5% commission [1s is 1/20=5% of £1].
Having a base-12 shilling [like the feet/inch system!] allows you to divide it up into 1/4,1/3,1/2,2/3,3/4 and 1/6ths [and with ½d you jumpp to base-24 so 1/8ths are possible - 1½d was 1/8th of a shilling; using ¼d even allowed 1/16ths !!], but not 1/10ths ! Of course we still use 12 hours, 60 minutes/seconds etc [and obscure 'degrees'] for that very reason...
[/off:2hxvcf0c]
I can still 'conjure' with the olde fractional feet-and-inches - but 'metric' IS so much easier...

jgb · 4 Apr 2012, 14:07

@thomthom said:

@unknownuser said:

when handing out a cultist..

Yeah, I saw that too. You posted faster than I could.

Handing out cultists.... what a concept.

jeff hammond · 4 Apr 2012, 14:12

@thomthom said:

@unknownuser said:

when handing out a cultist..

haha. good catch!

(I think i need a context checker on my computer as opposed to a spell checker )

TIG · 4 Apr 2012, 14:18

At least you weren't handing out a cutlass [pirate theme...]

jeff hammond · 4 Apr 2012, 14:31

.

ok, so here's a real world scenario showing why the offset tool w/ arcs is a no go… inferencing doesn't work because if i inference for the arc, everything else goes sour & viceversa..

wall_offset_error_.skp

…and that's a basic slice of a real world drawing.. this perimeter wall would actually be a lot bigger and possibly more complex in a full drawing.. the whole process has to be done manually where as if the offset tool worked properly, it'd be a big timesaver..

EDIT ugh.. that uploaded version of my skp didn't have any of my notes on there.. ?? i'll sort it out soon..

EDIT #2 -- ok.. fixed

jeff hammond · 4 Apr 2012, 14:58

@tig said:

I can still 'conjure' with the olde fractional feet-and-inches - but 'metric' IS so much easier...

my daughter is learning length & volume in school right now..
her homework the past two weeks has been all metric stuff.. (you know.. how to move a decimal point around )

i just wish they were teaching her metric as 'the way things are' as opposed to 'here, you might see this stuff occasionally'.

Rich O Brien · 4 Apr 2012, 15:06

That's a very good example Jeff

TIG · 4 Apr 2012, 15:36

The USA is already quite 'metric'...

You have had 'decimal' money forever.
You buy coke/pop in 'liters'.
Your cars have 'cc' engine data.
The US Army measure horizontal distances in 'meters' [but then heights in 'feet' - as do aircraft the world over!].
Most complex science in conducted in 'metric'.
So I expect that much of the US space-program is 'metric' too...

Now if only we can get you into using a few 'metric lengths'...

I find it surprising that given the US's break with the UK just as the French were pushing their newfangled 'metric' system, that the US didn't adopt it too, much of the rest of the world did...
We are not trying to get you to adopt a decimal time or calendar system

pilou · 4 Apr 2012, 15:55

Remember the Mars Climat Orbiter
Some millions $ in smoking vapors for error between unities in translation!

jeff hammond · 4 Apr 2012, 16:08

@tig said:

So I expect that much of the US space-program is 'metric' too...
:

right. much of it is. unfortunately, not all of it.
we crashed a probe into mars by accident due to some imperial/metric conflicts

re:length.
all of our official land surveys etc are metric as well.. it's just the tradesmen that won't switch. I think most of us would be into switching but most of our materials are still sold imperial only.

it's pretty much the govt and manufacturers of building materials that need to lead the way.

Edit. well, I see frenchy has beaten me to the mars thing

jeff hammond · 4 Apr 2012, 17:37

@unknownuser said:

That's a very good example Jeff

maybe one of these days I'll get the point across that I've been trying to

jeff hammond · 4 Apr 2012, 18:39

@arcad-uk said:

I can see the argument from both sides for arc offsets because in the past I've wanted both results. Ideally SU will be given true arcs at some point. But from this thread I now understand why things are the way they are and on balance from an architectural perspective I prefer to have any offset from an arc create a consistent (wall) thickness which is what happens now rather than tapering the arc section to maintain a vertex offset.

I guess I'm now flogging a dead horse but the way it works now doesn't give a consistent wall thickness. see my last upload in this thread which shows an example using an actual wall.

andybot · 4 Apr 2012, 19:41

Hey, what's better than a dead horse to beat! Anyhoo, wouldn't you use follow-me in a case such as the one you show? That way you are certain to have a consistent profile along the path.

jeff hammond · 4 Apr 2012, 20:01

@andybot said:

Hey, what's better than a dead horse to beat! Anyhoo, wouldn't you use follow-me in a case such as the one you show? That way you are certain to have a consistent profile along the path.

it would be nice if I could. but if I were to put a 2x6 profile at the end of one of those perimeter edges (in my latest example skp) then run follow me, it would produce the exact same (bad) geometry as offsetting the perimeter then push/pulling the face upwards.

these two tools (offset/followme)use the same brain..