Real-Time Physics Simulation Forum

Blender 2.42 release physics bug report

2006-07-19T17:54:51+00:00

- Is moment of inertia properly calculated for convex polyhedra?

It is approximated, convex polyhedra, cylinders and cones are currently using an approximation of the local bounding box for inertia.

Ah, that's why the spinning top looks kind of wierd.

You know about this, right? Free code for moment of inertia of a polyhedron.
http://www.cs.berkeley.edu/~jfc/mirtich/massProps.html

Statistics: Posted by John Nagle — Wed Jul 19, 2006 5:54 pm

Blender 2.42 release physics bug report

2006-07-19T06:33:54+00:00

Some minor questions:
- Where do you set coefficient of friction in the Blender interface?

Good question, the Blender UI is good in hiding things like this. I added an entry in the Blender Physics Tips in the Wiki:
http://www.continuousphysics.com/mediaw ... ysics_Tips

- Is moment of inertia properly calculated for convex polyhedra?

It is approximated, convex polyhedra, cylinders and cones are currently using an approximation of the local bounding box for inertia.

- What are the units of mass and torque? (Rather than setting mass, with a default of 1, how about setting density? A good default would be 1 gm/cc, or 1000kg/cubic meter, which is a reasonable value for water and most living things. The system knows the object volume (if you use Mirtich's moment of inertia calculation, that's one of the outputs), but the user probably doesn't)

The constraint solver doesn't handle mass ratios very well, and it's safest to keep masses all in the same order. Using density would create big mass ratios, which would be a burden for most blender users. However it would be nice to add some 'expert mode' with more accuracy.

Further question: How do you make a union of two convex objects without them having collision problems with each other? I tried making them a group, and that didn't work,and I tried making one a parent of the other, which didn't work either. Does that work yet?

Compounds are work in progress, and it's not possible yet to disable collisions on a pair basis yet in the Blender UI. Compounds are on the todo for 2.43. It is possible to 'fix' to 2 objects using constraints, but they need to be non-overlapping (see fixed_constraint.blend demo in http://download.blender.org/demo/test/e ... view34.zip ).

Erwin

Statistics: Posted by Erwin Coumans — Wed Jul 19, 2006 6:33 am

Blender 2.42 release physics bug report

2006-07-19T01:42:10+00:00

Further question: How do you make a union of two convex objects without them having collision problems with each other? I tried making them a group, and that didn't work,and I tried making one a parent of the other, which didn't work either. Does that work yet?

Statistics: Posted by John Nagle — Wed Jul 19, 2006 1:42 am

Blender 2.42 release physics bug report

2006-07-18T22:49:52+00:00

The spinning top test is working nicely. It's spinning, precessing, slowing down, falling down, rolling, reversing due to friction, and coming to a stop. I'm impressed. Few real-time physics engines get that right. Thanks.

Some minor questions:
- Where do you set coefficient of friction in the Blender interface?
- Is moment of inertia properly calculated for convex polyhedra?
- What are the units of mass and torque? (Rather than setting mass, with a default of 1, how about setting density? A good default would be 1 gm/cc, or 1000kg/cubic meter, which is a reasonable value for water and most living things. The system knows the object volume (if you use Mirtich's moment of inertia calculation, that's one of the outputs), but the user probably doesn't)

I'm really interested in friction working right, because I want to move my legged locomotion work into Blender. Foot/ground contact really matters.

Statistics: Posted by John Nagle — Tue Jul 18, 2006 10:49 pm

Blender 2.42 release physics bug report

2006-07-17T07:13:55+00:00

The reference says that no object should be bigger than 50 units, so I reduced the big static convex to 45 x 45 x 1. With that, the objects stabilized properly.

Now, on to the harder cases.

(I understand you're trying to do this in single precision. Unless you really need to be on the PS2, don't do that. You spend forever fighting scaling issues like this and explaining why it doesn't work. And you get wierd problems like physics working differently far from the origin.)

Statistics: Posted by John Nagle — Mon Jul 17, 2006 7:13 am

Blender 2.42 release physics bug report

2006-07-17T03:43:39+00:00

A quick check of the file shows that the test is using a huge static convex. It's best to keep object sizes smaller then 10 units (meter). However, Blender is using an older version of Bullet, so the bug is likely already fixed, see the discussion here:
http://www.continuousphysics.com/Bullet ... .php?t=346

I'll check this, when I update to the most recent version of Bullet.

Statistics: Posted by Erwin Coumans — Mon Jul 17, 2006 3:43 am

Submitted bug report with .blend file

2006-07-16T21:06:50+00:00

OK, made a bug report at http://projects.blender.org/tracker/ind ... 9&atid=306which includes a .blend file.

Statistics: Posted by John Nagle — Sun Jul 16, 2006 9:06 pm

Blender 2.42 release physics bug report

2006-07-16T20:26:01+00:00

It would be easier if you just mail the .blend file that shows the problem to bugs at continuousphysics.com

Alternatively, you can upload the .blend as part of a bugreport at the blender game engine bugtracker:

http://projects.blender.org/tracker/ind ... 9&atid=306

But this requires registration.
Thanks,
Erwin

Statistics: Posted by Erwin Coumans — Sun Jul 16, 2006 8:26 pm

No, it really doesn't work.

2006-07-16T06:15:47+00:00

Hey John,

Make sure that the right bounds are set to the objects. Press "n" in the 3d-window, once the big cube is selected, and make sure that scale is 1:1:1.
You can apply the scale and rotation of an object by hitting ctrl+a

OK, did that and fixed the scaling. Now the cone doesn't settle, it just bounces around a bit on its side, while the cube lands and then goes flying off into space.

Where can I upload the .blend file so others can try this?

Statistics: Posted by John Nagle — Sun Jul 16, 2006 6:15 am

Blender 2.42 release physics bug report

2006-07-16T04:00:24+00:00

Hey John,
You might want to check this out: Physics Tips

Make sure that the right bounds are set to the objects. Press "n" in the 3d-window, once the big cube is selected, and make sure that scale is 1:1:1.
You can apply the scale and rotation of an object by hitting ctrl+a

Peace

Statistics: Posted by pHalanx — Sun Jul 16, 2006 4:00 am