Sketchup is Inacurrate???

[jgb]

Let's keep in mind that it is called SKETCHup Not GoogleCAD.

so going off that train of thought, I should just buy AutoCad then all my drawings will be done automatically.

But only for drawing cars........

[TIG]

Going back to the FollowMe around Arcs...
There's no 'inaccuracy'.
Sketchup takes a profile Face and Extrudes it around path of Edges.
The orientation of that Face to the first Edge in the path [at its start] determines the extruded form.
If the Face is perpendicular to the first Edge in the path it's perpendicular to the last Edge in the path too.

An Arc consists of several Edges as 'Segments'.
Let's say the Arc is a semi-circle'
If the Face is perpendicular to the first Segment it will end perpendicular to the last one,
The extruded form won't therefore be a half circle of extrusion.
If you draw the Face at a Vertex it will be auto-adjusted to be projected so it is square to the first path Edge...
If you want a full half-circle there are tools like EEbyLathe [use 180 degrees], OR simply start the FollowMe at a mid-point on a Segment and end on the equivalent mid-point 180 degrees around the circle; it will then be the full half-circle form.

[Jeff Hammond]

Going back to the FollowMe around Arcs...
There's no 'inaccuracy'.
Sketchup takes a profile Face and Extrudes it around path of Edges.

the thing is, as you roundabout just said in the above three sentence, is that you can't use follow me on an arced path. (well you can, it just won't be accurate).. in the exact same way that you can't accurately offset an arc in sketchup..

sketchup ignores the 'arc' part and just treats it as a series of segments which results in an error..

this post on pg2 has an example file attached which shows the error:
viewtopic.php?p=394122#p394122

here's another way to see the error:

followme_error2.skp

dunno.. maybe i'm using the wrong word here but this is definitely inaccurate in my book..

[Alan Fraser]

Dave, I'm afraid Gerrit (Wo3dan) is correct, the start of the arc is not perpendicular. You don't even need to use a protractor to confirm this. If you go into the Styles dialog and change the edge colouring from All The Same to By Axis, then explode the arc, you will see that it isn't. If the first segment was perpendicular it would show up as green...because it ought to be running along the green axis. It doesn't, 'cos it's not.

TIG is absolutely right about SU...there is no inaccuracy when extruding around curves, just incorrect implementation. There have been other posts about SU being innacurate because of this slight adjustment when starting an extrusion along a path that is not exactly perpendicular. Some people seem to think that SU should just perform the extrusion and leave the original face where it is. However this really would be inaccurate...because it would mean that the face was being extruded obliquely along the path. This in turn would mean that at no point along the entire extrusion would the cross section be be the same as the original template.

[TIG]

It's not an error, it's not inaccurate.
As you say, Sketchup extrudes along the segment path.
There are no real Arcs in Sketchup, jut edges.
So as long as you bear that in mind, and ensure your extrusion starts on a segment and not a vertex, you can get a 'correct' extrusion around an 'Arc'

[mwm5053]

I got it dah thanks TIG

[Jeff Hammond]

It's not an error, it's not inaccurate.
As you say, Sketchup extrudes along the segment path.
There are no real Arcs in Sketchup, jut edges.
So as long as you bear that in mind, and ensure your extrusion starts on a segment and not a vertex, you can get a 'correct' extrusion around an 'Arc'

seriously, i understand that there are no 'real' arcs in sketchup.. that said, if you have a sketchup arc, all the vertices will be true distances from the centerpoint.. when using follow me, an arc's vertices and what happens at the vertices is inaccurate..

for simplifying the point i'm trying to make, let's forget about followme and look at the error in 2D using the offset tool…

offset.jpg

so you're saying this arc has been properly and accurately offset?


my whole point is that if sketchup recognizes the arc (as evident in entity info / gives proper arc length as opposed to sum of it's segments / etc.) then it should also be able to offset the arc properly.. it's an error that could (and should) be prevented through proper programming.

[TIG]

Your Arc Offset example is executed perfectly logically.

The ends are always offset at 90 degrees, making the last 'segment' longer than the rest.
This is the corollary of what you complain about with FollowMe, where the extrusion starts at 90 degrees to the path rather than being skewed.

You want the offset to be more like a Scaled down version of the parent Arc, about a common center...
However, drawing two temporary lines from the Arc center to it's ends and erasing off the offset Arc's ends and the temp-lines does the job easily enough. If you wanted a square offset then how might you do it? This way you have to do a quick trim to get a non-square end to the offset facet...

[Jeff Hammond]

Drawing in lines from arc's ends to center and trimming Offset version is the same as a scaled copy

no. it's not.. we're talking about accuracy here.. not 'well, i guess it looks similar'.. do some measuring and see if they are actually the same..

Your Arc Offset example is executed perfectly logically.

i can't fathom how you're not seeing this, as smart as you are

if i offset an arc (say one bent to 90º), i expect it to perform exactly as what happens to offsetting 1/4 of a circle.. the newly offset arc should also be bent to 90º.. that is the logical execution.. (fwiw, you can't offset a circle accurately in sketchup either.. draw a 10'r circle then inwardly offset it by 1'.. logic tells you (well, it tells me) that the resulting circle will have a 9'r ..but it doesn't.) EDIT- sketchup *should* be moving the vertices here since it's a circle.. instead, it moves the segments.. if you have a circle/arc, it's only the vertices that matter..

The ends are always offset at 90 degrees, making the last 'segment' longer than the rest.
This is the corollary of what you complain about with FollowMe, where the extrusion starts at 90 degrees to the path rather than being skewed.

You want the offset to be more like a Scaled down version of the parent Arc, about a common center...
However, drawing two temporary lines from the Arc center to it's ends and erasing off the offset Arc's ends and the temp-lines does the job easily enough. If you wanted a square offset then how might you do it? This way you have to do a quick trim to get a non-square end to the offset facet...

if i want to offset an arc in sketchup, i definitely don't use the offset tool.. i have to draw the two arcs separately.. if i want to offset an arc in a more accurate app, i use the offset tool


there's no ifs,ands,buts about it.. you simply can not offset (or follow me) arcs in sketchup.. i've tried it a zillion times and a zillion different ways.. you can accurately offset/followme a collection of line segments but the two aren't the same thing.. there is different maths for the two situation.. if you're inwardly offsetting an arc by 1' then all of the new vertices should be exactly 1' closer to the original arc's centerpoint.. and the degree of the arc's bend should remain the same.. this doesn't happen in sketchup and you end up with an inaccurate drawing..

[andybot]

That's interesting Jeff. Even with closed shapes, SU still offsets the vertices perpendicular to the last 'arc' line segment. I was thinking maybe with a closed shape, it knows to keep the vertices correctly oriented.
Yeah, I have to agree, that's inaccurate as hell. Logical, sure, since the offset of the lines is always perpendicular, but there is no way to maintain accuracy of an arc with an offset.

[lapx]

The verticies are not acurate but the midpoints of the segment are. The vertices are not the point to be measured to. It is just the a "filler" if you will to complete the geometry. If computers drew the circle to infinity I guess you could measure from any point along the radius. As far as the entity dialogue box it shows an aproximate number which is kinda goofy to me. I think both you guys are right in some way:)

[andybot]

The verticies are not acurate but the midpoints of the segment are. The vertices are not the point to be measured to. It is just the a "filler" if you will to complete the geometry.

OK, but an arc doesn't end at a midpoint...

[Jeff Hammond]

The verticies are not acurate but the midpoints of the segment are. The vertices are not the point to be measured to. It is just the a "filler" if you will to complete the geometry.

see.. no. you have that backwards..
the segments are the "filler".. the vertices are the money

(and that's pretty simple to see in sketchup.. draw a circle to a certain radius.. now measure from the center point to one of the vertices.. it's the vertices that matter..)

[Jeff Hammond]

just being silly now with the multiple choice test but…


Q. the radius of a circle derived from offsetting a 10R circle by a unit of 1 =


a. 9
b. 8.99137

[Wo3Dan]

just being silly now with the multiple choice test but…
Q. the radius of a circle derived from offsetting a 10R circle by a unit of 1 =

a. 9
b. 8.99137

Wait, I think I know this one. It's ......hm.... It's 9 (by inferencing)
Unless you use input "1" when offsetting.

[lapx]

I think sketchup has it backwards. Like I said kinda goofy if you ask me. The "one's and zero's need to be reordered .

[Jeff Hammond]

Wait, I think I know this one. It's ......hm.... It's 9 (by inferencing)
Unless you use input "1" when offsetting.

ha Gerrit.. you're killing me here.. (but i suppose this is what TIG was talking about doing in his earlier example.. he had 'desired offset marker' in there so he was using that as an inference point)

so go ahead and inference it in 3D.. with the follow me tool

[TIG]

Scaling an Arc anchored at its center and through a 'mark' [as in my example] produces a new Arc with equal segments and vertices...
A line drawn through the center and both arc's vertices shows this.
There is no argument that this is a truly offset arc ???

Offsetting an Arc through the equivalent 'mark' extends the new arc's first/last segments so that if you were to draw a line across the two arcs' starts/ends it'd be square.
If you draw a longer line from the center to the original Arc's start/end it will cut the offset arc and if you then erase the extra bits off the start/end [and these temporary lines], then the resulting offset arc is exactly like the scaled version, with equal segments and vertex-spacing...
So it is possible to use Offset on an Arc and get a true arc result - albeit that you need to edit start/end segment to make it 'true'.

As I said before... If you wanted to offset an Arc to have 'square' ends it'd be more tricky.
Some simple editing of an offset=arc gives you both possibilities...

[Jeff Hammond]

I think sketchup has it backwards. Like I said kinda goofy if you ask me. The "one's and zero's need to be reordered .

it's not that sketchup necessarily has it backwards.. i mean, it's doing it perfectly accurate if you're offsetting straight lines (say a square for instance).. the problem is that it should be able to tell the difference between an arc and a collection of straight lines..

instead, it treats both situations the same way..


if i'm offsetting a square, the 4 sides should move the desired distance along the edges' perpendiculars… if i'm offsetting a 4 sided circle, the 4 vertices should move the desired distance along the edges' bisectors..

[Wo3Dan]

...ha Gerrit.. you're killing me here.. (but i suppose this is what TIG was talking about doing in his earlier example.. he had 'desired offset marker' in there so he was using that as an inference point)
so go ahead and inference it in 3D.. with the follow me tool

Sorry about that, Jeff, but I just can't see the inaccuracy. I do understand that you would like the offset tool on an arc to allow you to offset a certain distance to give you a new arc with predictable radius (=RADIUS-distance).
But offsetting with inferencing makes it possible. Even on an arc in 3D. That's all I wanted to say.

The point at center works on any arc.
Drawing a radius from center to arc's vertex also works in any situation. Now overrule the just drawn arc by inputting the new radius value. Offset works in the arc's plane and (apply the same vertex you drew the radius to!) inference the offsetted arc to the short radius's endpoint (opposite from the center on same edge).

As for the "Follow me" tool, it's very consistent, although the twisting can be a pain. But you can be sure that the cross section has the same dimensions on each segment troughout the path. Even with any more or less complicated 3D curve as a path.

This is not what I can say for the first example you presented a few "pages" back. The cross section is not the same.

@jgb,
Not only in engineering 0.5 degrees can be of great importance.
A collumn in a building that isn't plumb, but 0.5 degrees off blue, carrying a heavy load (9mm/m)?.... not that good.
But I only wanted to point out what caused the unwanted 'Follow Me' result.

[ArCAD-UK]

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.

[Pilou]

And about surface?
A disc of 1 m radius
12 segments is 3.00m2
24 segments is 3.11 m2
36 segments is 3.13 m2
100 segments is 3.14 m2

10 m radius
12 segments is 300 m2
24 segments is 310.58 m2
36 segments is 312.57 m2
100 segments is 313.95 m2

And I suppose that will be more dramatic with volume

So accurate for Sketch and drawing representation not for precises measures in case of Arc, Circle, Curves...

[TIG]

To stop the issue... here's an Arc_Offet_True tool viewtopic.php?p=395769#p395769
It has a few issues with Arcs with trimmed first/last segments BUT I'm working on that...
EDIT:
Version v1.1 now correctly mimics the original Arc's start/end segments if they were 'trimmed', it also stops you offsetting with a negative distance to 'zero' or beyond...
Version v1.2 now has a 'Number' option to make arrayed offsets, and a context-menu item too...

[noelwarr]

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.

[Trogluddite]

Sketchup is based on floating point data structure. These are inherently inaccurate.

Quite so, I have to deal with this often in my "alter ego" as an audio DSP designer.

The fact that us humans use decimal, and PC's use binary also has some consequences for accuracy.
Just as decimal cannot exactly represent 1/3, binary cannot exactly represent 1/10 - the value to be represented and the number base have unshared prime factors.

A lot of software will display rounded values, so that they look a little more palatable to us humans - but it is a mistake to assume that the display value is exactly the same as the underlying float number. If your display is telling you that your dimension is exactly 0.1units, it is probably lying, as a float number simply cannot have exactly that value, only an approximation.

Never occured to me before, but in this binary age, maybe the traditional "powers of two" divisions of the inch make more sense than decimal notation - those power of two factors can always be represented precisely in binary (up to the limit of the bit-depth) - so 1/16" really is 1/16, whereas 0.1m is only pretending!

[Sonder]

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

[mac1]

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.

[genma saotome]

Regarding my original assertion about the inaccuracy of the follow me tool -- I now understand I had not set up my work to be "compatible" with the way follow me works. Had I added tangents to each end of my arc and extruded the face along tangent, arc, tangent I'd get a middle section that was exactly what I wanted.

I'm not sure how to judge the implications of that fact WRT accuracy but I do know it is less work for me to do it that way than I had to deal with before and so while I'm not 100% happy about it, it is a better method. To everyone who helped to explain this, my thanks!

[Jeff Hammond]

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]

when handing out a cultist..