Sketchup is Inacurrate???
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Wow... just... wow! Twenty-one pages of discussion on the vagaries of SketchUp's curve interpretation. Jeff: we've discussed this before, on Reddit. As you may remember from that thread, I shared your opinion that SketchUp would be improved with the addition of 'true arcs.'
@unknownuser said:
I agree- that would be a nice thing to have in SketchUp. When we designed SketchUp's object model in the early days, the thought was to keep it as simple as possible- more native entities required more tools to create them. And the interactions between different entities (arc-line, for example) are more complex to manage. So we came up with the solution that is in place today.
In another life, I was a student of Descriptive Geometry, and I learned that there is nothing that can't be drawn with a compass and straightedge. SketchUp only has the straightedge... and I can see how that is a hindrance. There's no way to construct, for example, an arc-arc intersection in SketchUp.
I agree this would be a useful thing to add in SketchUp, but it isn't something that we could add quickly or easily. I'm keen to see how other folks here would use "true arcs" if we had them in SketchUp, too.
Over the years there have been numerous ruby scripts built that add to the utility of SketchUp arc and circle entities- "point at center" was one of the first scripts I remember seeing. There are, however, limits to what can be implemented through our API without native arc support. I hope we're able to address that one day, but such low level modifications to SketchUp's object model are not to be made lightly.
john
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@dale said:
complexities of curves
fwiw, that's what i've mostly been talking about all along but got lost along the way being stuck on arcs..
offset tool is the easiest to talk about.. so is an arcif sketchup suddenly dealt with arcs properly but curves reverted to the old (now) behavior then really.. not much has improved.. because arcs are the easiest of curves to draw manually.. so yes, as long as things are straight or arcs in sketchup, it would be a great improvement..
but what happens if you have an ellipse? then it reverts to the old behavior? an ellipse which shows this behavior is worse then an arc doing it for two reasons.. it's harder to draw manually and the error is greater..
a few pages back with the ugly J part of the conversation, there was inadvertently an example of this.. when a circle exhibits this behavior, it's not obvious to the eye, you have to get in there and measure it.. with other types of curves which change in radius along their length, the error becomes noticeable to the eye alone because the error is so big..
when i said a while ago about which point the developers would need to draw the line as to when the behavior reverts, i wasn't saying stop at arcs and everything else is the same.. in my head, i was thinking about the issue of the endpoints of the curves.. so i don't know how they would/could figure out the ends but all the right info is in the middle parts of a curve..
[edit-- oh.. hi john.. so can you make that ^ happen real quick like?
]and i'm not saying that in a lame sort of way.. i just mean.. is that crossing the line for sketchup/trimble/you guys?
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@jbacus said:
As you may remember from that thread,
.ouch.
that's the smack in the face that i deserve from this thread.
John, i don't know what to say as far as me talking down on the suteam regarding not talking about this. i'm sorryi completely forgot about that live chat day and i do sincerely apologize.
(but the rest still stands though
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@unknownuser said:
[–]jbacusProduct Manager http://www.reddit.com/r/IAmA/comments/u18do/wera_bunch_of_sketchup_developer_types/
"I agree- that would be a nice thing to have in SketchUp. When we designed SketchUp's object model in the early days, the thought was to keep it as simple as possible- more native entities required more tools to create them. And the interactions between different entities (arc-line, for example) are more complex to manage. So we came up with the solution that is in place today.
**In another life, I was a student of Descriptive Geometry, and I learned that there is nothing that can't be drawn with a compass and straightedge. SketchUp only has the straightedge... and I can see how that is a hindrance.**There's no way to construct, for example, an arc-arc intersection in SketchUp.
I agree this would be a useful thing to add in SketchUp, but it isn't something that we could add quickly or easily. I'm keen to see how other folks here would use "true arcs" if we had them in SketchUp, too.I learnt this too, in this life years ago.
How can one imagine to make a soft like SU without circle and arc? The easy way or the lazy one?@unknownuser said:
Over the years there have been numerous ruby scripts built that add to the utility of SketchUp arc and circle entities- "point at center" was one of the first scripts I remember seeing. There are, however, limits to what can be implemented through our API without native arc support. I hope we're able to address that one day, but such low level modifications to SketchUp's object model are not to be made lightly.
john
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"...exaggerate the essential and leave the obvious unclear." --Vincent Van GoghJohn Bacus
Make me smile
So John Bacus, what's cooking now?
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@jbacus said:
Wow... just... wow! Twenty-one pages of discussion on the vagaries of SketchUp's curve interpretation.
Got to do something while "it's rendering". 
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Another of the quirks of the Arc tool...which is why I tend to use circles for laying out stuff like this. I reserve the Arc Tool for rounding corners, turning block ends into bullnose or rectangles into lozenges....or when 'looking good' is good enough.
The accuracy is actually pretty impressive, but I find it very odd that you get an utterly different result depending on whether you type the radius into the VCB (Measurements) or Entity Info. It would be really useful if you could inference off an established centre...something it seems to recognise if you go the Entity Info route.
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@jbacus said:
@thomthom said:
@jbacus said:
Wow... just... wow! Twenty-one pages of discussion on the vagaries of SketchUp's curve interpretation.
Got to do something while "it's rendering". heh. +1
+21 more pages on the same (ok..not exactly the same) thing..
(still about circles, or lack of)
http://sketchucation.com/forums/viewtopic.php?f=15&t=44972.
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@unknownuser said:
but what happens if you have an ellipse? then it reverts to the old behavior? an ellipse which shows this behavior is worse than an arc doing it for two reasons.. it's harder to draw manually and the error is greater..
here's what happens with an ellipse..
![the left one is a correct offset.. the right one is what sketchup gives upon offsetting 2'..
i'm out an 1 3/4" over a distance of 24".. i don't think anyone, even nick, can scoff at that error.. ;)](/uploads/imported_attachments/85qH_lips.jpg "the left one is a correct offset.. the right one is what sketchup gives upon offsetting 2'..
i'm out an 1 3/4" over a distance of 24".. i don't think anyone, even nick, can scoff at that error.. ;)")
[edit- and the same thing stands as previously mentioned in this thread.. if the user desires the results on the right (i.e.- consistent distance between the segments, then they would just make sure they're offsetting a series of edges as opposed to a single curve)]
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@alan fraser said:
Another of the quirks of the Arc tool...which is why I tend to use circles for laying out stuff like this. I reserve the Arc Tool for rounding corners, turning block ends into bullnose or rectangles into lozenges....or when 'looking good' is good enough.
The accuracy is actually pretty impressive, but I find it very odd that you get an utterly different result depending on whether you type the radius into the VCB (Measurements) or Entity Info. It would be really useful if you could inference off an established centre...something it seems to recognise if you go the Entity Info route.Alan, I'm not quite sure what you are trying to explain.
First of all I can't get Entity Info to show the (any) arc's length with that number of digits in decimals. Probably my fault, I was pretty sure it could be done.
When I scale up by 100x and once again by 100x, then I get the value ~ 14398966,3mm which is (obviously) 100x100 times your values.
Not scaled I get an arc length of ~ 1439,9mmI draw a circle, second click (cardinal point on red). Then I take out a 165 degrees part. Result is arc A with incorrect chord length and correct arc length.
After rotating a copy of the circle by 0.5 degrees I'll get two endpoints lined up on same red value, to form the endpoints of the correct chord.If I then copy this chord to the side, I can use the arc tool directly to this edge to create a correct segmented arc: click first > click second point > either type the correct radius [Enter] or click third point and type the correct radius [Enter]
Like other dimension input you can override the current units. So (since I usually use mm) I can type 50cmr. The resulting arc when scaled up is ~ 14398966,3mmUsing the incorrect chord (from circles segments midpoints) would lead to an incorrect arc.
edit: Notice that you can input either the bulge or the radius (include the r)!
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I was telling me we should stop arguing about arcs, circles and curves in SU as they do not exist... just polylines and polygones, kind of arcs, circles and curves tells entity info.
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for info with cms as Unity at the maximum of its precision for a circle of 50 cms
a nurbs program gives 143.9 896 632 cmswith mms as unity 1 439.8 966 322 mms
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@unknownuser said:
for info with cms as Unity at the maximum of its precision for a circle of 50 cms
a nurbs program gives 143.9 896 632 cmswith mms as unity 1 439.8 966 322 mms
My $1 calculator(*) reveals 1439.98966329mm.
But what about how to get Entity Info to show better than ~ ____.9mm
I mean more digits after decimal point.(*) apparently former owner had no use for the calculator anymore.
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Gerrit, you can specify the number of decimal places in Model Info > Units.
You guys might find this handy. -
@alan fraser said:
Gerrit, you can specify the number of decimal places in Model Info > Units.....
Yes, I know, and I've often set it to max decimals in mm (0.000000mm)
But it has no effect on what 'Entity Info' displays for the arc's length.
Only units does and I can't go "beyond" mm.Thank you for the link. I was just looking for this one that I don't have on the labtop that I'm working from. It's faster than setting up a program (and altering!) in Excel. Am I lazy or not?
As for your previous post:
@alan fraser said:
The accuracy is actually pretty impressive, but I find it very odd that you get an utterly different result depending on whether you type the radius into the VCB (Measurements) or Entity Info. **It would be really useful if you could inference off an established centre...**something it seems to recognise if you go the Entity Info route.
Isn't that done by entering the radius in the 'Arc' operation? Instaed of entering the bulge? If you already have the chord!
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@unknownuser said:
My $1 calculator(*) reveals 1439.98966329mm.
hum hum first number after the decimal point?
1439.8966322 mm mine
it's because the arc or any curves in this prog is "unwraped" by a plugin for have the length
So from an existing curve and not from a "calculator"Maybe it's also an error of the "calculator"! Who knows?
I will investigate !
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@alan fraser said:
@unknownuser said:
"DesertRaven"Alan, this thread is going exactly where it needs to go. If we keep saying we'll settle for "good enough", nothing will ever be gained to the better.
Where have I ever indicated that I'd settle for 'good enough'? I have pointed out several times now the shortcomings of the Offset Tool...in both exterior and more especially on interior offsets. I have also mentioned that Follow Me leaves much to be desired....
.....You can campaign for true curves from now till eternity...but you won't get them.
Alan, I have provided examples explaining to a "T" what I'm basing my criticism on, I have no problem with SU using facets vs real curves, as long as the result is what it is supposed to be and not some variation.
It is completely illogical that the end of an offset arch would result in a cut short end segment or elongated end segment - just for the sake of keeping it square.
The correct logical conclusion is:
That the offset arc follows the center to the edge in a line and all facets be consistent. Because I am starting with an equal sided arch so I expect an equal sided offset version of the arch. Plus an arch is a segment of a circle.
Why wouldn't the segments need to stay consistent in the offset version? -
@desertraven said:
The correct logical conclusion is:
i think, for the most part, we've moved past this part of the conversation.. the error is being realized by more people (and john didn't deny it
)so now.. it's- where do we go from here?
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Currently if you draw an arc, copy it, group then paste in place you can offset the arc correctly using cardinal points.
So maybe extend the move tool by adding alt+move to perform a true offset on a selection of edges.
This doesn't completely fix the issue in all circumstances but does add the required meta data to the geometry.
The move tool with cardinal points is one of SketchUp's strongest modeling features for me.
But there maybe holes in my theory.....
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right rich.. the cardinal point scales an arc and that's all that needs to happen when offsetting one..
(the resulting geometry needs to remain an arc and the central angle needs to stay the same so basically, the only thing you can do to change it's size without breaking it's inherent properties is scale it)
[EDIT] but that does bring up a good example of what should be happening..
draw a 90º arc whose endpoints are on the red axis and the green one.. using the scale tool, grab the handle opposite of the arc's center point and scale it.. that's offsetting of an arc.[/edit][edit2] oops.. cardinal points change the bulge of an arc.. it's when you put the move tool on the ends of an arc which scales it..
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