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Building aperatie, physics
Posted: 22 March 2013 08:06 AM   [ Ignore ]
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I love playing with the bullet physic in carrara, I can spend hours just playing around with it. I have seen some of the great vids done with it, but how do you build all the aperatie? The slides, spinners, stacks of cubes and so on used in the vids.

My last play time lol http://carraracafe.com/forum-2/physics/lets-get-physical/

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Posted: 22 March 2013 09:37 AM   [ Ignore ]   [ # 1 ]
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I use Carrara’s modeling tools. The animation I’m currently working on was no particular effort put into creating the models, but that isn’t to say I didn’t lean some things doing it. I’ve used primitives (primarily sphere, cube and plane), splines and vertex models. The chutes were quickly roughed out in the spline modeler, then converted to mesh and modified from there.

The platforms were mostly done in the vertex modeler using a cube.  Because I need thickness but no other detail from the height I tended to leave them at 3x3x3, loop select the ones around the waist and delete them. Extrusion to raise walls (like on the ramp) and tesselation to add density to the mesh to avoid dropping through.

The turnstile was a mesh cylinder that I added loops around the waist to, then extruded arms for the balls to strike. I also extruded rings above and below how it sets in the platform to keep it in place. Having a very low friction was necessary to avoid it being pushed through the platform. It was sized a bit smaller than the cylinder that I added to the platform for the hole. Still had problems trying to start it at any rotation other than zero.

The rods are just cylinders. Probably primitives, I don’t actually remember. What I call the ‘pit’ at the end was a 3x3x3 vertex cube that used extrusion to build. The exit in the middle pit was created by tilting the floor, deleting a wall, and linking vertices. At the moment head hurts too much to remember how I built the funnel—but I’ve really gotten to love the ‘move along’ function (IIRC the name).

If I was going to build a stack of cubes I’d consider doing a replicator then converting to objects. However, even with very low collision threshold I’m not sure you can get a wall to stand with bullet physics. Too jittery.

edited to say: “extract along” was what I meant

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Posted: 22 March 2013 11:26 PM   [ Ignore ]   [ # 2 ]
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Hi ManStan.

I use the vertex modeler mostly. Path sweeps for slides. Torque Force for spinning objects. ‘Use Collision Detection’ on in Assemble for stacking blocks and then duplicate rows etc.(I have a YT video with 1000 blocks created this way). Point Force for pulling/pushing objects. Tip:...for fastest simulations, use Spline objects for physics mehtod objects when possible.

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Posted: 23 March 2013 07:08 AM   [ Ignore ]   [ # 3 ]
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Here’s a good question, why wont a stack of blocks stand? Really, try making a stack of blocks that doesn’t collapse on it’s self.

I was working with a stack of bowels being dropped on a table. I found even with a collision distance of .01 the bowels still wouldn’t set on the surface of the table. So when I set the bowels on the table with collision on, then clicked to start the calculations, the bowls dropped that last little bit, bounced, and blew my set up blank stare

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Posted: 23 March 2013 07:33 AM   [ Ignore ]   [ # 4 ]
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ManStan - 23 March 2013 07:08 AM

Here’s a good question, why wont a stack of blocks stand? Really, try making a stack of blocks that doesn’t collapse on it’s self.

I was working with a stack of bowels being dropped on a table. I found even with a collision distance of .01 the bowels still wouldn’t set on the surface of the table. So when I set the bowels on the table with collision on, then clicked to start the calculations, the bowls dropped that last little bit, bounced, and blew my set up blank stare

To much jitter in the resolver. It might actually work any given time, but mostly it won’t. In my current project there’s a “structure”—a base, rods across that, vertical block supports at either end with a long horizontal block set across them and a ball on top of that. That *used* to be stable for at least 14 seconds of calculation and is sort of equivalent to a stack of blocks.

Now it collapses entirely in less than 3 seconds and it takes less than a second for the inevitable to be obvious. If the ball doesn’t spontaneously decide to roll in some direction the horizontal block will start to shimmy. If the horizontal block doesn’t shimmy one of the vertical support blocks will shimmy or bounce. If neither horizontal block shimmies or bounces then one of the rods will skitter out from underneath.

Bottom line is, the more objects in contact the more likely that at least one of them will start moving. You might be able to affect the probability a bit by playing with density and friction, but it is just a matter of time.

I tried a few approaches:

Accommodate: because (at least for the times I ran physics calculations) one of the balls always took off in approximately the same direction (albeit at different speeds) I moved it so that it would be in about the right spot when events reached it. This of course introduced variability into later events.

Modify: Another ball (the last one) had to remain in place for a considerable length of time. I couldn’t just place it differently and hope it was in the right spot when the time came. I did some tessellation and put a dimple exactly underneath it. This isn’t ever visible and keeps it completely in place as long as I needed it to be.

Block: In the structure I described above neither of those techniques was workable. What I ended up doing was adding some extra non-physics blocks to prevent the objects from moving until shortly before they were going to be hit. This of course required doing a calculation to find what time that was going to be. Because the calculations vary each time it is only approximate but I had ~0.25 seconds before the shimmy would become apparent and possibly even ~0.5 seconds before it would be noticeable in the animation (given view points, etc.). I went with .5 seconds and it worked out.

Note that re-running physics can introduce even greater variance in the calculations (at least in my experience). The method I worked out was to load, make previously noted alterations, save, calculate, close, load, ...

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Posted: 23 March 2013 07:34 AM   [ Ignore ]   [ # 5 ]
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oh, I also found it helpful to set the bounce of surface objects (platforms, etc.) to zero.

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Posted: 23 March 2013 10:47 AM   [ Ignore ]   [ # 6 ]
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The only way I’ve found so far to generally stop unwanted movements on multiple stacked physics objects is to disable the default gravity and setup a Directional Force. I setup the blocks/cubes stacks with a zero gravity directional force, then keyframe a directional force positive value(or turn on gravity) right before an object collides with it.

Carrara has issues with resting contact physics(like a stack of cubes) no matter what the physics accuracy(etc.) in my experience. Have not found the exact reason why yet. Waiting for the final release of Bullet for Carrara.

Side note; Bullet seems to have different friction properties than Standard. Place a box on a moving conveyor belt and Bullet will slide the box.

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