Sketchup is Inacurrate???
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@unknownuser said:
ok.. ..... .. what i've done in this .skp, is manually draw a letter J (step-by-step.. real basic stuff and i know your skill level is way beyond this but i'm just trying to be clear in the example).. then, after i draw it manually, i do the same thing with the follow-me tool then show how the results are different..
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the point of the drawing is that it shows two different results.. so either my manually drawn version is wrong -OR- the follow-me version is wrong.. they're not 'both wrong' and they're not 'both right'.. ONE is right - ONE is wrong.. you decide which one that is...Thanks for making it so easy for me. Luckily I don't need decide anything. For your manually drawn J isn't according to what 'Offset' is about. The meaning of offset in SU (and in other programs that I know)is offsetting edges, not vertices. You offset to the point of where you offset infinitly short edges 0.0000000000000mm and "beyond" (true curves), but always edges. Now you introduce a totally different meaning of the operation, even unknown in other applications, for all I know.
Doing it manually means that you first decide the number of segments > the rotation angle per segment > construct the inner segmented line ("arc") > offset all segments outwards > extend the outer segments to intersect adjacent segments, all forming the outer "arc" > also intersection of first segment with the long edge > last segment has a perpendicular end (due to no connected edges)
If you fit this one on SU's offset, then it will fit. -
@alan fraser said:
If you draw a default arc then chop a little off each end segment, it's still obviously a 12 segment arc...but with two of those segments shorter than the others. Now use Entity Info to change the resolution to...say...24 segments, then back again to 12.
You find that what you have now is a redrawn 12 segment arc with all its segments now all the same length...but neither end point actually at the stated radius, only the middle ones.
You can play with this to get some interesting insights into the way that SU deals with curves. Afraid I'm not on my work machine, so no pics - but here's a little run-down of an experiment you might like to try.
- Draw an arc with a nice grand curve - a half circle, say, with 12 segments.
- Bisect one of the segments near the middle - chop it right in half at the point where the 'radius' is smallest.
- Select one half of the chopped arc, and choose a small number of segments
- And a large number for the other half.
- Erase the stumps of the line that you used to chop the arc.
- You now have a single arc with very uneven segments.
- Look closely at the bit where the two sections join - eeek.
- See how far you can shrink it with the offset tool before it gets, err, odd.
Similarly, you can show that if you change the segment count to even up the segment length after chopping a curve mid-segment, you'll still have 'off-radius' end points - and if you explode the arc and measure, you'll find that the end segment is not quite the same length as all the others
Just a rather contrived illustration that the lines are being offset, not the arc's 'parameters', of course - I offer it more as a curiosity than a judgement of SU.
I do think, though, that Jeff made a good point about this being a problem of expectations. We may know that it is the lines that get offset, but once you decide to call something an 'arc' or a 'circle', it is not surprising that users expect them to exhibit the behaviour associated with those categories of objects.
The "line offset" tool is a very good, and accurate, "line offset" tool - what is really being requested is a different tool, not a more accurate one.
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And Jeff, even the 'Follow Me' tool is consistant when applied on a series of connected coplanar edges. It's just a different, now 3D way of creating an offset. If you delete the third dimension's geometry, it will fit on what the offset does.
b.t.w. You could do your J with the arc tool in a few seconds.
The arc tool acceps (during the operation) changing the number of segments (s), the chord length ((units)) and the bulge ((units)) or the radius ((units)r). (Units) is optional, needed when not according to current settings.
So what IS the problem.
You want a tool that lets you "offset" a series of edges and "arcs" (in one go) where edges and "arcs" act differently in the same operation. This requires a different tool that spits the entities by property. It could be integrated but IS a special, not according to offset's rules, operation. I remember your previous example with the first "curve"-stud parallel to the last "edge"-stud and a consistant shell width of the bottom plate. Special cases! -
@wo3dan said:
For your manually drawn J isn't according to what 'Offset' is about. The meaning of offset in SU (and in other programs that I know)is offsetting edges, not vertices.
gerrit
it sounds as if you've learned geometry from sketchup... instead of in school (or where_ever) then bringing that knowledge into sketchup..(on a side note, this is a point where i really (really) wish jbacus or another su official* would comment on the topic.. like i've been implying, he knows exactly what i'm talking about.. he absolutely has to.. much of the thread is simply trying to point out that there is an error occurring and i think if someone more official than me gave a statement, then more people (everybody?) would be less likely to defend the app and re_go through some of the example files with a fresh mind to see and understand the error themselves..) (*or some other definitive source where this part of the discussion can be cleared up)
regarding "The meaning of offset in SU is offsetting edges, not vertices."..... i'm no programer but there are some in this thread (tt?) that could clear this up.. when moving geometry around etc, i'm willing to bet at the root, sketchup actually never moves edges around.. it moves vertices.. if it moves an line, it's in fact moving the two end points of that line..
if you draw an edge, you click on the start and endpoints of that line.. it's ALL about the vertices.. when sketchup offsets a square, it boils down to moving the cornerpoints around.. (maybe not the case with guidelines which are infinitive? but even then, points and not lines are being used to define it's placement)
and, believe it or not, the math behind measuring/transforming/etc the points defining an arc are different than the points defining straight edges.. in many ways, sketchup recognizes the real math of an arc (bullet list a few pages back outlining some of these ways).. it's not totally dumb and it doesn't always treat the points defining an arc in the same manner it treats the points defining a collection of edges.. but, sometimes, it does.. which is a pretty major error..
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Jeff, I'll get back to you tomorrow. Disagreeing with you.
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Gerrit is correct, Jeff...you haven't given SU a fair crack of the whip. In your hand-drawn version you have the final radial section running horizontally along the red axis. Yet in the prepared path for Follow Me, the final segment of the path is still at a slight angle. You can't possibly expect Follow Me to produce a squared-off final cross section under those circumstances.
In situations like that (or in more practical circumstances like a pipe bending through 90 degrees before disappearing into a wall) you have to add a final 'follow-through' segment to get Follow Me to terminate properly. In the case of the pipe and wall, that would be a segment perpendicular to the wall (which a 90 degree arc doesn't give you). In the case of the J, it's a final segment running along the Green axis. The length doesn't matter...because after you've extruded, you just Push/Pull it back to the intended end point. If you do it that way, the two versions seem to match exactly. I tried it...they do. -
@wo3dan said:
So what IS the problem.
for absolute starters.. i can't even do this in sketchup:
and that's a super basic example.. there are potentially hundreds of other things that need to happen after a step like this but this step breaks any possibility of doing much more afterwards.. it's crap geometry..
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@alan fraser said:
Gerrit is correct, Jeff...you haven't given SU a fair crack of the whip. In your hand-drawn version you have the final radial section running horizontally along the red axis. Yet in the prepared path for Follow Me, the final segment of the path is still at a slight angle. You can't possibly expect Follow Me to produce a squared-off final cross section under those circumstances.
ok alan.. to turn the tables a bit..
let's see you draw that J using the follow-me tool.. you can arrange your profile anyway you wish. and you can draw the arc (and/or it's segments) in any way you wish..
now look at the results.. measurements matter here. (i mean, that's basically the entire premise of the discussion- accuracy)
your idea of a 'fix' does nothing other than correcting one area an putting error in a different place.
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@unknownuser said:
...
gerrit
it sounds as if you've learned geometry from sketchup... instead of in school (or where_ever) then bringing that knowledge into sketchup..
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Jeff, I've been selfemployed for about 26 years, created many constructions (drawings+calculations), mostly buildings (from small up to some up to 50m50m8m / factory+office etc, total package) with and without architects, and also steel constructions. Some big projects for one single person...., me. I'm not bragging about this. Point is, in those early days there was no sketchup, nor drafting programs for us poor souls. I had to solve many geometrical challenges with nothing but a calculator and what I know/had learned. Maybe that's why I can appreciate so much how SU works. Clean yet powerful. It performs how I think in 3D, but much faster. -
@wo3dan said:
And Jeff, even the 'Follow Me' tool is consistant when applied on a series of connected coplanar edges. It's just a different, now 3D way of creating an offset. If you delete the third dimension's geometry, it will fit on what the offset does.
youre exactly right.. ive been trying to say that over and over again in this thread..
the offset tool is used (by me at least) in the discussion because it's the least confusing to talk about.. but the same exact error that occurs in the offset tool occurs in the follow me tool (only it's now multiplied the amount of bad offsets)... and then- all those plugins i listed a few pages back do the same_exact_thing.. the offset error is like a virus and it infiltrates many other areas of the app..
sketchup does not handle arcs properly.. end of story.
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@wo3dan said:
Point is, in those early days there was no sketchup, nor drafting programs for us poor souls. I had to solve many geometrical challenges with nothing but a calculator and what I know/had learned.
then dust off that old compass and drafting table.. draw the J.. measure it.. draw it in sketchup using any of the problem tools ive mentions.. it's not going to happen..
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oh. and hey.. with those last jpgs i posted, delete the profile shape.. take followme out of it..
select the surface and offset inwardly the same distance as the corner arc's radius..what the heck is that? and if you must, use the smart offset plugin since it's smarter..
what the heck is that? -
@unknownuser said:
...and that's a super basic example.. there are potentially hundreds of other things that need to happen after a step like this but this step breaks any possibility of doing much more afterwards.. it's crap geometry..
Jeff, it's an example still according to "offset" rules. It may not look nice and needs fixing. But it's a cosistant way of offsetting an SU arc. What you are looking for is a special case as mentioned before: split edges and "arcs" in the selection prior to operating the offset, ignore the "arc's" first segment's direction, Offset edges perpendicular at the ends, draw the inner (or outer) "arc" with radius R-offset (resp. with R+offset) between offsetted (perpendicular) last edges's ends, to fill in the gaps.
(now it's time to Zzzzzzzzzzzzz / see you tomorrow)
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Here you go, Jeff. I think this is what you asked for, As you can see, the two Js superimpose almost exactly. I say 'almost' because the Follow me version is ever-so-slightly broader at the extremity of the curve.
It would be difficult to say either version is wrong...because what is being exhibited are the compromises made when a curve is approximated by straight-line segments. Your version maintains a 1cm thickness at the vertices, the Follow Me version maintains a 1cm thickness perpendicularly, edge to edge (which is how it starts out, after all)
This means that it's slightly wider at each vertex...which explains the broader curve. Your J, however, gets slightly thinner along each segment (0,991445cm it would seem)...thinner than the 1cm it is down the stem. Like I said. It's hard to say either is wrong. It depends what your priorities are.
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@alan fraser said:
Here you go, Jeff. I think this is what you asked for, As you can see, the two Js superimpose almost exactly. I say 'almost' because the Follow me version is ever-so-slightly broader at the extremity of the curve.
It would be difficult to say either version is wrong...because what is being exhibited are the compromises made when a curve is approximated by straight-line segments. Your version maintains a 1cm thickness at the vertices, the Follow Me version maintains a 1cm thickness perpendicularly, edge to edge (which is how it starts out, after all)
This means that it's slightly wider at each vertex...which explains the broader curve. Your J, however, gets slightly thinner along each segment (0,991445cm it would seem)...thinner than the 1cm it is down the stem. Like I said. It's hard to say either is wrong. It depends what your priorities are.lofl.. this is turning into twilight zone for me
you just freaking showed everything i'm trying to say here..
in the situation of the J... both the inner and outer edges should be a radius.. the inside radius should be 5cm and the outsideshould be 6cm radius..
as in.. you just showed that sketchup can not offset arcs accurately but managed to brush it off as "Follow me version is ever-so-slightly broader"
of course it's broader (in sketchup).. because it's broken.
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@wo3dan said:
Jeff, it's an example still according to "offset" rules.
what offset rules are you talking about ?? (can i see them referenced or outlined elsewhere? or are these just your offset rules?)
i've never seen any mention of offset rules.. in fact, i've seen (official) mention that you can offset arcs in su (which implies they will be offset correctly)but if you're saying that you can't accurately use the offset tool (or any other tool like it) on an arc then that's good.. i mean scream and shout it at the top of your lungs that you can't do it.. that's the same thing i'm trying to do
it's a basic function necessary in any architectural modeler.. it doesn't work right in sketchup.. and it needs to be fixed..
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@alan fraser said:
Your J, however, gets slightly thinner along each segment (0,991445cm it would seem)...thinner than the 1cm it is down the stem.
who, in their right mind, would do any sort of calculation or measurement -of an arc- based off it's segments..
the segments are absolutely meaningless.
i can draw an arc with 8 segments then it's centerpoint.. every single measurement from the center to a vertex will be the exact radius.. if i move along a segment, the number is constantly changing? are you suggesting that an arc somehow has a variable radius? really?
i can then change the number of segments to 50.. every single measurement from the center to a vertex will be the exact radius.. the segments still give the variable result and the distance between it and an offset of it have changed as well..
the segments are absolutely meaningless in an arc.. agree? so quit using them to measure off of..
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@alan fraser said:
Dunno about you, but I know which one I prefer. Yours looks plain ugly.
my original design of the J used a piece of geometry known as an 'arc' (it's like this little curvy thing.. you should totally check it out)
your J design is to be constructed using straight boards mitered together..
one of them is an arc design
one of them isn'tso you wouldn't use the same logic when offsetting them..
this is exactly what sketchup does except it's the other way around..they are two separate situations therefore there are two separate ways to offset them.. sketchup only uses one way.. regardless of if the user has drawn an arc or straight lines in the design..
[EDIT] ok.. about my plain ugly J.. i guess you said that's a J shape being drawn with CURVES other than arcs.. as in, they're not to be constructed using straight boards mitered together.. if that's the situation then truth is, my ugly J is in fact the correct version (well, i didn't see the actual model to measure etc but the method you've outlined is right)... if it's a curved line, even if it's not an arc, then the segments simply don't matter so you'd shouldn't be judging how pretty or not it looks based of the segments.. export those vertex points then break out the french curve and connect the points.. that's the real shape but in sketchup, we can only use the vertices to represent them..
but i did already bring this up back on pg 12 or so..
@unknownuser said:
realistically, the only time the offset tool should work as it currently does is when all of the segments are individual edge entities.. anytime there's a 'curve' (not just 'arc') in the mix, the offsetting should calculate separately..
if i want to offset a bezier curve, it needs a new behavior too in order to really get to the bottom of this issue.. that would require sketchup to begin recognizing a relationship between each segment of a 'curve' instead of viewing it as a collection of edges..
so eventually, the devs would have to draw a line as to where this behavior stops and reverts to it's current behavior..
http://sketchucation.com/forums/viewtopic.php?p=450760#p450760
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please show me one situation where you would actually do any sort of inferencing/dimensioning*/ calculating etc using an ARC's segments.. please.. they are completely meaningless..
*dimension an arc in sketchup.. put the leader anywhere along that arc you choose.. (in an area represented by a straight line if you choose.. doesn't matter)
you see that? you see what happened there? sketchup isn't so dumb after all..
the way it deals with dimensioning an arc is the way it should always deal with an arc..
[edit] but, according to you, sketchup should be measuring the distance from a segment to its centerpoint. it's not.. it's ignoring the segments-- just like you should be doing..
therefore, is sketchup's dimensioning of an arc wrong -or- is it the offsetting of an arc that's wrong..
(and please.. pretty pretty please.. don't try to say.. "oh.. they're both right.." -
Yes / No
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