[HN Gopher] Pixar's notes on Rigid Body Simulation (2001) [pdf]
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Pixar's notes on Rigid Body Simulation (2001) [pdf]
Author : cpp_frog
Score : 86 points
Date : 2022-05-29 14:57 UTC (8 hours ago)
(HTM) web link (graphics.pixar.com)
(TXT) w3m dump (graphics.pixar.com)
| gabereiser wrote:
| Pixar is one of those companies I admire. Not just because they
| make amazing stories, but because they make amazing tech.
|
| A long time debian user, I found renderman to be ridiculously
| awesome. The scale of which a frame can render across a network
| of random machines...
|
| Not to mention the graphics problems they have solved and
| optimized to bring to the entertainment industry. It's like if
| mathematicians and artists were combined into the head of a
| hydra.
|
| They have more papers out there.
|
| We take for granted today the problems Pixar and co faced in the
| 90s. Only now we do it at 300fps.
| djmips wrote:
| I agree with you and I feel it's remiss to not point out a lot
| of the groundwork by Baraff was done at Carnegie Mellon
| University prior to his joining Pixar. I, like many others used
| his work and papers with Witkin from CMU as the basis of rigid
| body simulation in the nineties.
| imachine1980_ wrote:
| i'm feeling that mass production of the last year is making
| this lees than was before. for example i feel the best looking
| pixar movie is toystory 4(graphic wise, and coco graphic+art
| direction), current movies like red, or souls, feel cheaper
| being a lot less "magic feeling", and look like inside out
| graphics a movie from 2015(7 years), movies like encanto feel
| more polished and movies like the spiderverse from 2018 feels
| more creative and risky.
| pfranz wrote:
| My wish/dream has been that realistic tech is so mature
| they're no longer spectacles. I felt "tentpole" movies were
| getting there before Marvel kicked off. Instead the focus is
| either on story or stylization. I love that in Turning Red
| they billowing smoke will dissipate into clumping bubbles and
| they adopted a lot of anime styles into 3d. Effects like
| smoke, water, and fire are the easiest to focus on. 2d
| animation has done a lot of this for decades; Malificent's
| dragon breath, Genie's smoke, Akira's effects.
|
| I think CG sequels, especially as far apart as Toy Story 1
| and 4 (24 years!), are a unique thing. It has to be familiar
| and consistent with previous movies that used older tech, but
| they can't make a new movie using 24 year old tech, either.
| juanre wrote:
| The simulation of collisions becomes trickier when tangential
| compliance cannot be ignored. For example, a ping-pong ball (very
| high tangential stiffness) rotating around an axis perpendicular
| to the motion falls straight on a table and bounces back up; the
| rotational velocity decreases, but it does not change sign.
| Whereas a superball (low tangential stiffness) will change the
| direction of the rotational velocity. I did my PhD on this, back
| in 2000, but haven't kept up with the field.
|
| If anyone is interested in a way to take tangential compliance
| into account my thesis may be of interest:
|
| https://s3.amazonaws.com/collisions/collision.pdf
| amelius wrote:
| For a more advanced approach, see [1], [2].
|
| [1] https://ieeexplore.ieee.org/abstract/document/6907751
|
| [2] https://mujoco.org/
| cyber_kinetist wrote:
| Note that there are multiple approaches to implementing rigid
| body simulation (especially when it comes to resolving
| collisions), and each of them all have their benefits and
| disadvantages.
|
| For example, Mujoco uses a soft contact model where
| penetrations can happen, this can be undesirable for some use
| cases but might not really matter for most people. Mujoco also
| assumes every object in the simulation to be defined beforehand
| and remain static. This is good for performance (since memory
| can be preallocated beforehand, some values can be
| precalculated, etc.) but the drawback is that it's hard to
| dynamically add/remove/change objects in your simulation. In
| overall, Mujoco is fine-tuned and intended to be used in
| robotics simulations, and might have some undesirable
| properties when used in other applications like games and 3D
| animation.
|
| Although Baraff's notes for rigid body simulation are more than
| 20 years ago, it's still roughly the same method used in most
| game physics engines (PhysX, Havok, Bullet, etc.). It struggles
| with modelling accurate friction (you can't do that much with
| LCP constraints), and the contact solver can be unstable at
| times (especially when CCD comes into the picture), but it's
| still really performant and "realistic" enough for a wide
| variety of games and animation.
| saboot wrote:
| I've been implementing a 2D physics sim myself, I'd take be
| interested to hear more about different approaches. For
| example one that can handle the creation of more objects and
| handles friction accurately.
| Animats wrote:
| I was doing spring/damper collision ragdolls in the late
| 1990s.[1] Slower, but more accurate. Friction works. Not real
| time back then, though. Not enough CPU power yet. You can be
| too early.
|
| Impulse-constraint systems have a "boink" problem. Velocities
| change instantaneously. This looks OK for small objects, but
| bad for large ones.
|
| [1] https://www.youtube.com/watch?v=5lHqEwk7YHs
| erwincoumans wrote:
| Hi John, I recall that, those ragdolls falling down stairs.
| From what I heard from a few Pixel employees, Pixar uses
| Bullet for some rigid body VFX effects, and Bullet still
| uses velocity-level LCP in combination with maximal
| coordinate constraints or with Featherstone-style reduced
| coordinate forward dynamics.
|
| The impulse/velocity-based contact model allows to model
| arbitrary soft contact (using contact force mixing (CFM)
| and error reduction parameter (ERP) terms), similar to
| spring-damper or MuJoCo-style contact (for MuJoCo see
| https://mujoco.readthedocs.io/en/latest/computation.html
| and https://github.com/deepmind/mujoco). For the last 10
| years we use this also for robotics simulation for
| quadruped robots and robot arm grasping contact. Check out
| Erin Catto's presentation about re-inventing soft springs
| in this PDF: https://box2d.org/files/ErinCatto_SoftConstrai
| nts_GDC2011.pd...
|
| Furthermore, impulse/velocity-level LCP based constraint
| solving can be combined with finite element method (FEM) or
| mass-spring based deformable contact solving if higher-
| accuracy contact and friction modeling is needed.
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