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blender, path animations, and scale


Rhys Goode
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I've been working on learning Blender to make SL mesh objects.  In particular, lately I am trying to make a necklace, by laying out a Nurbs path, making it the parent object for a bead, and then duplicating the bead to follow the path.  Lots of tutorials show how to do this on the web, the Blender manual does it with the monkey, seems nice and straightforward.

After a lot of messing around, it looks like there is a scale issue I don't understand.  

If I lay out a nurbs path at the default length of 4 m (path animation checkes, frames to 15, check follow), and then make it the parent of a 25 cm cube, the cube gets duplicated along the path, no problems. Each copy of the cube is just like the other, the orientation and position shift as they move along the path.  I can change the size and orientation of the parent cube, all the rest mimic it nicely.   After I get the shape I want, I can shrink the whole path down to the size I actually want, no distortions.

However, if I lay out a 40 cm path (the size I actually want), and make it the parent of a 2.5 cm cube, (all the same steps in the same order as using the 4 m default nurbs path, just the scale changed),  then all the copies of the cube are distorted, with the distortion getting worse the further along the path one goes. I end up with a grotesquely distorted bunch of child cubes that look nothing like I want.

I am hoping someone can explain what is going on, and maybe tell me if there is a setting that I can change to get better behavior.  While "make it big, then shrink it" seems to be a fine work around, I usually prefer working to my finished scale.

 

 

 

 

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Sounds like you need to apply the transformations before you do the other operations.  I'm not enough of a Blender expert to confirm that with absolute certainty, but I can say I'm 90% certain that that's the problem, since that's how it tends to go in other modeling programs that work along similar lines to how Blender does certain things.

Until transformations are applied, they are merely mathematically active. The effects they produce are not 'real' yet.  The equation is happening, and what you see on screen is just the end result of the equation.

When you bring your animation path into the mix, there's now an extra step in the equation.  Two sets of mathematics are applied to the object's parameters instead of just one.  So, of course, as you step further and further along the path, the combined effects become more and more severe. 

The reason you didn't see this behavior when you used default values was because those values are mathematically neutral in the equation.  It's like multiplying by 1.  You just get the same result.

When you changed the scaling, the values became no longer mathematically neutral. So, the end results could no longer be identical to the original values.  When you apply the transformations, you'll reset the factors, so that they once again act as ones in the equation.

In other words, remove the extra math, and you'll get the behavior you were expecting.  Make sense?

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Follow these steps and see if you get a better result:

 

  1. Open a default scene.
  2. Select the default cube if it isn't already selected and Tab into Edit Mode.
  3. Scale the cube down to 25cm
  4. Add an array modifier
  5. In the array modifier's properties, set the fit type to "Fit Curve" and leave the curve target blank for now.
  6. Add a curve nurbs path to your scene.
  7. Size it down until it is just a little longer than your cube.
  8. Tab into edit mode and remove all but the 2 end points if you want.
  9. Select the cube again, and in the array modifier's properties set the curve target to the nurbs path you just created.
  10. Change the relative offset in the x axis to something reasonable to you.
  11. Play around with the control points of your curve, noting that it now adds new iterations of your cube as you make the nurbs path longer.
  12. Add a curve modifier to your cube and set the target to your nurbs path as well. You shouldn't need to change anything else.
  13. Extrude, subdivide and shape your nurbs path as you like to get your necklace shape. Note that you want to fine-tune the control points so they don't fall within one of the cubes, or else the cube on that point will become distorted.
  14. You can adjust the bevel depth and resolution of your curve if you want to give it physical shape. I am assuming you already know how that works.

I think Chosen's explaination was basically correct, though I'm no expert. You were probably performing your scale operations in Object mode instead of edit mode. That does not actually apply real transformations to the object's mesh data. That can be problematic down the line if you never apply your transformations -- particularly when you are using modifiers to do work for you.

Let me know if those steps work for you or if you need clarification.

 

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Thanks Chosen,  Rahkis  

I was indeed scaling the nurbs path in object mode.  If I scale it in edit mode, everything works fine, my beads follow the path with no distortion.  

I fiddled with the curve modifier, adjusting to minimize the distortion, but I really do want exact copies.  And it looks like the key for me is simple, just adjust the length of the path in edit mode.

I love it when people give answers that get to whats happening underneath the hood, thanks again!

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