Look Along Curve Normal



I want to move an objekt with “Look Along Curve Normal” along a long hole-path. The prolem is that it is always jumping to the startpoint of the given profil-line. How can i hold the position at the start of the animation? All together i have more than 80 peaces of this conveyor chain. If it is possible at all, is it nessesary to do this for each one?

What can i do?

conveyor chain.rar (4.9 MB)

(Marika Almgren) #2


Bongo starts the calculation from the curve seam. You can use the command CrvSeam to change the position of the curve seam to change the starting position of where Bongo starts the counting from.

If you have a lot of those pieces I’d set the rest up using the IK. So that one piece is pushing all the other ones.


Hallo Marika!

Thank you very much for reply!
I’m in the very beginning of learning Bongo, so i have also a lot of problems with understanding the rules of IK. Another thing is that the objects are flying apart when i am clicking on the curve to show the begining of the path, last step of the command “Look Along Curve Normal”. It does not matter if I have created a group or a herachie. Look please at the screenshots. The animation without the normal alignment works problem-free if i generate for each peace on special curve with “crvseam”. But this not the thing i need. Could you please helpme once again? Thanks.

Dear Peter

(Luc Adriaenssen) #4


First your initial questions: the jumping to the start of the path. When a Simple constraint is initiated the parameters at start and end keyframe are set to 0 and 1 (or vice versa depending on the direction of the path). This brings each object at the start (the seam) of the curve regardless its initial position. The position on the path can be altered by adjusting the value of the parameters as I did in "conveyor chain 001.3dm"

The mathematics are simple: since there are 80 objects on your path the offset is 1 div 80 or 0.0125. Because the curve path is “closed” a parameter values higher than 1 (e.g 1.0125) will push the object past the seam. It is indeed inevitable to set the parameters of each individual object separately.

In the model above I used the ‘To path’ simple constraint because indeed (as you noticed) a “Look Along Curve Normal” doesn’t give any control over the 2 non-constraint directions which makes objects twirl in space unwanted an uncontrollable. The “Look Along Z Up” constraint will make the objects pointing upwards whenever possible. In your model this is useless at the bottom side of the track.

The IK option Marika suggested would be valid when you didn’t have so many objects. In "conveyor chain 002.3dm" I made a chain of 10 clasps in accordance with to the roler-coaster model (http://download.mcneel.com/s3/mcneel/bongo/2.0/samples/en/BongoIK_03.3dm). Theoretically it should work for the whole chain of 80 links but practically you’ll see that when you add more links the IK solver cannot cope with a vast number of linked constraints. This leads to chaos and even apparent ‘hanging’ of the system. So I’m afraid IK isn’t an option either.

In the Train composition animation topic Micha proudly presents his trick using Rhino’s History and Flow command. This technique is set forth in the tutorial-video http://bongo.rhino3d.com/video/the-power-of-history at minute 6. In "conveyor chain 003.3dm" you can see what it would look like for your model. You must press and hold the left/right arrow on the timelineslider to get a Preview of History based animation.

This technique may suffice depending on what the goal of your quest is. In the hidden world of your model I can see that the platforms are driven by a genuine chain with 160 links. Making this work, I feel, would be very very hard. This model is not ideal for starting to learn Bongo!

conveyor chain 000.rar (14.2 MB)



Hallo Luc!

Thank you very much for your detailed description! That helped me alot!!!

You are right, that is nothing for the first hour in Bongo.
I have solved the problem with this approach:

  1. Group the objects which i want to animate.
  2. Draw a line with the length of the long hole starting with the intersectionpoint between Group and long hole
    in +X-axis direction.
  3. With “CrvSeam” move the closing point of the long hole to the intersectionpoint.
  4. With “Flow” and active “History” connect the group from the line to the long hole (RIGID!)
  5. Move the line with Bongo in -X-axis direction with the value of the line length.
  6. Animate by pulling the Ticker.

conveyor chain_result.rar (8.9 MB)

Thanks, Peter