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This is a reasonably old topic on Archicad-Talk, but I thought I’d bring it up here.
When the axis of a TUBE path changes, there are twists that occur in the profile. The specific case I have encountered was when scripting for a bent spring (such as wraps around the hose on a commercial kitchen tap). The maths works fine (and the same maths works without issue in SketchUP using Ruby with the FollowMe command), but the TUBE wants to twist. No amount of playing with the angle setting will fix this either.
I have also tested the new TUBE{2} command, but the same issue occurs.
Is it possible to fix this?
The original post: https://archicad-talk.graphisoft.com/viewtopic.php?f=6&t=51395&p=277977
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www.brucepwalker.com
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Hi,
The TUBE commands were designed for objects with much less twist, where some auto-correction of the profile rotation comes handy. We don’t want to fix that in the TUBE commands, that would break compatibility. There is a solution however: in a library update in version 21, the “gravity_tube” macro was added.
As you can read in the documentation, the W axis of a cross-section at each particular node of TUBE is tied to the Z axis. With a spatial helix, this makes no sense. Using “gravity_tube”, you can set this W axis freely. In your spatial helix, this gravity vector would be pointing from the path point to the axis of the helix.
Based on your example object, here is a sample code that models a bent helix nicely:
resol 16 ! model a helix bent 180 mat = 45 ! glass rWire = 0.002 rCoil = 0.011 nCoil = 18 ! number of turns around axis nTurn = 36 ! number of points in one turn R = 0.1 ! radius of bending dFi = 180 / (nCoil * nTurn) dim axis[][3] dim helix[][3] dim gravity[][3] n = 0 for fi = 0 to 180 step dFi n = n + 1 ! compute points as a helix on a straight cylinder, then bend it aCoil = fi / dFi * (360 / nTurn) ! angle around axis xCoil = cos(aCoil) * rCoil ! x, y on cylinder yCoil = sin(aCoil) * rCoil rTurn = R - yCoil ! turn radius of vertical line on cylinder ! helix axis point axis[n][1] = 0 axis[n][2] = cos(fi) * R axis[n][3] = sin(fi) * R ! helix point helix[n][1] = xCoil helix[n][2] = cos(fi) * rTurn helix[n][3] = sin(fi) * rTurn ! vector from helix to axis gravity[n][1] = axis[n][1] - helix[n][1] gravity[n][2] = axis[n][2] - helix[n][2] gravity[n][3] = axis[n][3] - helix[n][3] next fi ! show helix for i = 1 to n - 1 lin_ helix[i][1], helix[i][2], helix[i][3], helix[i + 1][1], helix[i + 1][2], helix[i + 1][3] next i ! show helix axis and put helix as tube path for i = 1 to n hotspot axis[i][1], axis[i][2], axis[i][3] put helix[i][1], helix[i][2], helix[i][3], 0 next i ! try TUBE TUBE 2, n, 1 + 2 + 16 + 32, 0, 0, 901, rWire, 360, 4001, GET(NSP) ! try "gravity_tube" add 4 * rCoil, 0, 0 ! parameters for gravity_tube (derived from TUBE{2}): ! top_material, bottom_material, cut_material, ! n, m, mask, ! u1, w1, s1, mat1, ! ... ! un, wn, sn, matn, ! x1, y1, z1, angle1, gravity1_x, gravity1_y, gravity1_z, ! ... ! xm, ym, zm, anglem, gravitym_x, gravitym_y, gravitym_z put mat, mat, mat, 2, n, 1 + 2 + 16 + 32, 0, 0, 901, mat, rWire, 360, 4001, mat for i = 1 to n put helix[i][1], helix[i][2], helix[i][3], 0, gravity[i][1], gravity[i][2], gravity[i][3] next i call "gravity_tube" del 1PS. “gravity_tube” might give some warnings with strict error checking options, that will be corrected in the next main release.
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE -
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Hi Peter,
Finally got around to trying this out. It works…mostly. There is still one twist near the apex (see attached image). I haven’t delved into the code to figure out why. A promising result, though.
BTW – two corrections to your code: there are two ‘next i’ nestings, and the ‘n’ in the first TUBE command doesn’t work (I replaced with NSP/4).
Attachments:
www.brucepwalker.com
https://www.mindmeister.com/65450406AC20 6005 AUS Full & AC 21 3009 NZE Full | Win 10 Pro 64 | Intel i7-7700HQ 2.80GHz | 16 Gb RAM
NVIDIA GeForce GTX 1060 8Mb -
Hi,
The forum engine got rid of [i] even in the code, so we turned off the italic BBCode feature. Please try the code again, now it should work.
If you still see the twist at the apex, please write back the library version you are using. (gravity_tube has been corrected in a library update, it’s possible that your version doesn’t contain that update yet.)Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE -
Thanks Peter. The code works fine now, but there is still the twist at the apex. My library version (which says it’s up to date) is 4550.
www.brucepwalker.com
https://www.mindmeister.com/65450406AC20 6005 AUS Full & AC 21 3009 NZE Full | Win 10 Pro 64 | Intel i7-7700HQ 2.80GHz | 16 Gb RAM
NVIDIA GeForce GTX 1060 8Mb -
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