Sketchup is Inacurrate???
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Accuracy and precision are related concepts. When you model something with no particular units in mind, and a whole bunch of people say hey it looks just like the real thing--that is accuracy. When they say hey it's exactly the same size as the real thing--that is precision. Both terms are relative, or can have a range of values.
I think folks are interchanging one term for another in their arguments. If you work with Sketchup, mindful of its "limitations", you can achieve your purpose. -
I've seen complaints about models distorted and skewed when imported to Max. These are usually from the 3d snobs using pirated software. I calmly explain to them that if they have a licensed subscription version of max that a sketch up importer is available and works fine. I've never used max personally but i've seen people dragging geometry randomly about, and of course i've received some really horrific auto cad files. It seems to me that autocad makes you sloppy ?
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Is SU inaccurate. The answer is YES. IT is a digital machine and they all have an native inaccuracy and then quite often some program design "quirks". It is up to the user then to understand and decide if what is provided is adequate for their intended application. If one is really interested in the native accuracy look up IEEE 754 Single Precision 32 bit and you will find numerous papers on the subject. There are pathological cases where large errors can occur.
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@chedda said:
I've seen complaints about models distorted and skewed when imported to Max. These are usually from the 3d snobs using pirated software. I calmly explain to them that if they have a licensed subscription version of max that a sketch up importer is available and works fine. I've never used max personally but i've seen people dragging geometry randomly about, and of course i've received some really horrific auto cad files. It seems to me that autocad makes you sloppy ?
Probably not AutoCAD. Some people probably feel if it represents the object in print, that's enough. They didn't draw to be accurate to the computer.
One thing about SU--if you are drawing buildings (for which it was designed) it gives you cues that your line work is lined up and connected, in fact you can't really model well if it isn't. On the other hand AutoCAD will let you draw whatever and you might not pick up your tiny mistakes.
You could say it's a matter of precision, not accuracy, that SU has 6 decimal places for inches, and my CAD can go up to 12. SU has 3 decimal places for degrees and my CAD has 5. If you are drawing a survey, SU won't give you the precision expected in a computer file, but I guess it can be said, within its own range of precision, it seems to be accurate.
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@pbacot said:
that SU has 6 decimal places for inches
The developers has said that SketchUp's internal precision is one thousandth of an inch. What you see in the UI is one thing - when SketchUp does 3D calculation its to 1/1000".
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I have searched more than once for Google backup for all these number limits thrown around.
Have you ever seen any?
The floating point has a accuracy of 7.+ digits so I don't know where the developers get the 1/ 1000 value and does that not mena it is not a 32 bit application as stated? You will see more digits reported (like the value I listed above form a float 32 online IEEE 754 Single precision 32 bit conversion ) and there was a post not long ago about Google changing the conversion in dae for inch to cm to 2. 53999969330360 which causes then the file to use a more storage and considerable larger file size.I don't think that change was necessary , but again could find zero SU8M2 release notes on that issue.
In fact some processors ( non 64 machines aka my P4 CPU ) will use 64 bits but then gets reduced to 32 bits when the value is stored. -
a couple of decades ago solar energy had a bad reputation in brasil as being inefficient. it turned out it did not work because the people who introduced it to us had no idea as to how to make it work: panels very seldom faced the correct direction and the height relationship among the parts was wrong. as soon as cleverer people started dealing in it solar energy become known for what it is.
could something similar happen at times with sketchup users? I remember hearing a colleague at my university say it was a pity sketchup could not be used to produce precise models...
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I imagine these naysayers negativity comes from a possible perception that 'free' can never be 'good'. On the other hand I've yet to read a negative SketchUp assessment from a knowledgeable source. I think those are the ONLY sources that we should be listening to, not the, I'm sorry to call them, SketchUp ignorant begrudgers
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Sure it is : a circle is not a circle, it's a polygon!
But we love it anyway!
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But when you think about what a 'circle' really is, it can only be a polygon, all be it, in some programs, one with extremely short 'line' segments, a position beside a position, so to speak.
But of course, anyone that knows about CNC work also knows that SU is not the app for this. But I imagine certain commentators have grasped this idiosyncrasy of SU and jumped on the old band wagon
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sometimes it's innacurrate.. the vast majority of the times-- it's fully accurate.
knowing what the sometimes are and how to avoid them is key.
[for instance-- draw an arc at 90° and try offsetting it]
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@mike lucey said:
But when you think about what a 'circle' really is, it can only be a polygon, all be it, in some programs, one with extremely short 'line' segments, a position beside a position, so to speak.
But of course, anyone that knows about CNC work also knows that SU is not the app for this. But I imagine certain commentators have grasped this idiosyncrasy of SU and jumped on the old band wagon
I'm not sold on that. I work directly with a contractor with both a CNC and robotic plasma cutter with my sketchup modeling, and we make some pretty complex structural knife plates for wood to wood and wood to concrete joinery. The end result has been pretty cool.
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A good assessment Howard and good advice
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This is all true with curves (segments) but I would not call it inaccuracy but a limitation of SU (and as a matter of fact, any polygon modellers that use segments to approximate curves). SU accurately reports the volume of that multi-sided prism (not the ideal cylinder).
This is not only true for volumes but also this is the reason of that "offsetting quirk".
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If I may rephrase the question to: is SU accurate ?
IMHO....
Mostly YES, but sometimes NO (strictly speaking).
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Straight lines / Orthogonal Shapes / and any Geometry, Volumes etc created from them, no problem.
SU will be as accurate as the user wants to model / level of detail required.
Typically this will be limited by any source informatiom (site measurements, original drawings you may be working from, your original design etc) and the modeller's skill with SU (eg care with the snaps / inferencing)
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Circles, Curves, Arcs and any Geometry or Volumes derived from them - strictly speaking NO.
I consider this a limitation of SU and not an inaccuracy. Provided you are aware of this, its usually not a problem either.
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For example, circles in SU - the corners of these many sided polygons (~circles) will be coincident with a true circle eg as if you'd just drawn it in Autocad etc, but the lines between the corners are not accurate.It follows from this that if you derive a Volume and calculate a Mass eg of Concrete or Steelwork of any shape containing curves, arcs etc, then this will also be slightly out.
However, I've found that for most practical purposes the difference is negligible and can be worked around eg bump up the number of sides of any circle from say 24 to 64 etc if required.
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Some Top Tips I'd Recommend for Circles, Curves etc...- When drawing circles, pull the circle out in either the red, green or blue direction (as opposed to some random direction).
- Use a number of sides that neatly divides up your Circle or Arc. By default, a circle in SU has 24 Sides. This is good as 360/24 = 15 degrees. Adjust number of sides as required. Any setting-out on Circles, Arcs etc will typically be every 15 degrees, 30 degrees etc. The setting-out points will now be true as they will coincide with the corners of your SU "Circle".
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Hope this Helps
Howard L'
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@gaieus said:
This is all true with curves (segments) but I would not call it inaccuracy but a limitation of SU (and as a matter of fact, any polygon modellers that use segments to approximate curves). SU accurately reports the volume of that multi-sided prism (not the ideal cylinder).
This is not only true for volumes but also this is the reason of that "offsetting quirk".
i'm not quite sure i'd call this a limitation.. (well yeah, it's a limitation but it's preventable imo)..
the 'offsetting quirk' happens over and over again throughout sketchup.. it's just easiest to see it with the offset tool..
another notorious tool is follow-me..both the offset tool and follow me tool should only be used on straight lines/corners (such as a hipped roof etc.).. any attempt at using either of these tools on curves will result in an inaccurate model..
the problem, as i see it, is that even tough sketchup recognizes an arc or circle (it will give accurate arc lengths for instance instead of simply giving a sum of it's segments), it seems to ignore the fact that you're dealing with an arc prior to doing anything with it and treats it as a collection of individual segments.. (in fact, su will explode an arc if it's part of a follow me path etc..)
this (again, imo) is a fault of the program/programmers and it is preventable.. if you have a circle, arc, or even curve then sketchup should make the proper calculations and draw properly/accurately with them.. i haven't tested this out on other polygon modelers but i assume modo and the like can accurately deal with these situations..
here is an example file showing some of the problems with using the follow me tool..
it also shows the desired results / workaround using tig's lathe tool..
i'll repeat this though -- never use offset, follow me, or even rubies that are simply tying in to sketchup's core functionality (shapebender etc.) for accurate work using curved lines or surfaces..
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@unknownuser said:
i assume modo and the like can accurately deal with these situations..
Not sure of that
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@edson said:
a couple of decades ago solar energy had a bad reputation in brasil as being inefficient. it turned out it did not work because the people who introduced it to us had no idea as to how to make it work: panels very seldom faced the correct direction and the height relationship among the parts was wrong. as soon as cleverer people started dealing in it solar energy become known for what it is.
could something similar happen at times with sketchup users? I remember hearing a colleague at my university say it was a pity sketchup could not be used to produce precise models...
It was well known back then of the geometery relations the real issue was the efficiency of the cells . Look at the helio stat array
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SketchUp's internal precision when it comes to lengths is 1/1000th of an inch. You can see that in the Ruby Console:
` 1.0.to_l == 1.001.to_l
=> true
1.0.to_l == 1.002.to_l
=> false
1.0.to_l == 1.0011.to_l
=> false
1.0.to_l == 1.00101.to_l
=> false`
If the difference is less than 0.001 then it is considered equal. This kind of comparisons is normal and required when doing floating point comparisons. The difficult part is picking the actual precision - in SketchUp isn't 1/1000th of an inch (0,0254mm). Other applications might have a variables precision, configurable by the user, but SketchUp's is hard-coded.
I assume SketchUp's precision was picked with architecture in mind where 1/1000th of an inch would be more than enough precision. -
Sure but when you build the Channel Tunnel (50.5 km) you can have 128.27 mm of error !
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