Pixar's notes on Rigid Body Simulation (2001) [pdf]
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Pixar's notes on Rigid Body Simulation (2001) [pdf]
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Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#2For a more advanced approach, see [1], [2].
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#3For a more advanced approach, see [1], [2]. [1] https://ieeexplore.ieee.org/abstract/document/6907751 [2] https://mujoco.org/
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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#4For a more advanced approach, see [1], [2]. [1] https://ieeexplore.ieee.org/abstract/document/6907751 [2] https://mujoco.org/
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…
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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#5For a more advanced approach, see [1], [2]. [1] https://ieeexplore.ieee.org/abstract/document/6907751 [2] https://mujoco.org/
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…
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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#6Pixar 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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#7The 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:
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#8Pixar 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…
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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#9Pixar 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…
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.
Re: Pixar's notes on Rigid Body Simulation (2001) [pdf]
#10Earlier quoted context omitted.
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…
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
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_SoftConstraints_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.