[All] Introduce integration schemes by bakpaul · Pull Request #6117 · sofa-framework/sofa

bakpaul · 2026-05-20T15:38:10Z

This PR replaces the ODE solvers by Integration schemes. It enriches the API of Implicit integration schemes by exploding the timestep solving in sub phases useful for Newton applications

Now instead of single monolithic way of expressing an integration scheme, two families of integration schemes are introduced the Implicit and Explicit ones.

Explicit integration schemes get no improvement from being run into a Newton solver, thus their implementation remain unchanged and they can still live in a one solve function. Enabling local optimization such as lumping the mass matrix to compute acceleration.

Implicit solvers are divided here in to three main families : Velocity-based, acceleration-based and the rest. By doing so, the velocity and acceleration based integration schemes share a common implementation enabling a clear API to introduce new ones. As it can be seen in the PR, for any new Velocity based IS we need to implement 4 simple methods. For acceleration based it is 5. But the implementation sticks to the basics of what makes an IS : the position update, the velocity update, the acceleration computation and some derivative terms. A full documentation + scientific explanation will be added using the following this custom note https://typst.app/project/rVNQw8slVmqX7aHTCUrewe

To list them, the current velocity based IS are

Linear multistep famillies
Implicit Euler

And the acceleration based :

Newmark Beta

And the others are :

StaticEquilibrium
Simplectic (energy-based)

This is a first required step to implement a global Newton on the simulation.

There is still a TODO list :

Investigate the reduced stability in example scenes
~~(re) implement the symplectic IS~~
Re-implement a newton for the static solver (replaced in future PR)
Implement trapezoidal rule
Implement first order for implicit solver
Fix all scenes
Add a compatibility layer ?
Build a doc using the scientific notes
Decide where to put the rayleigh parameters

PS: I am sorry for the history being croped to 11 commits. There should be way more but it has been lost because I started with a much complex PR including the work on the Newton solver which in the end was to intricated, so I started fresh with only the modifications on the ODE...

About first order :
I've only implemented is for velocity-based IS, I don't feel that it makes sense in acceleration based one. For instance, in Acceleration based one we could also think of second order approximation. But for instance for the Newmark solver it would react as an explicit IS or as if beta = gamma = 0. For me this approximation makes sense for second order IS, but if you use a higher order one, you just want every order. Actually I have the implementation somewhere but it makes the implementation much more complexe for no real use IMO

[with-all-tests]
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[ci-depends-on https://github.com/sofa-framework/SofaPython3/pull/615]
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[ci-depends-on https://github.com/sofa-framework/CSparseSolvers/pull/8]
[ci-depends-on https://github.com/sofa-framework/ManifoldTopologies/pull/16]
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Reviewers will merge this pull-request only if

it builds with SUCCESS for all platforms on the CI.
it does not generate new warnings.
it does not generate new unit test failures.
it does not generate new scene test failures.
it does not break API compatibility.
it is more than 1 week old (or has fast-merge label).

…based IS and some implementaiton of final IS. Still need to make the solve visitor use these IS

…ons cheme

Co-authored-by: Alex Bilger <alxbilger@users.noreply.github.com>

bakpaul · 2026-06-11T07:52:40Z

[ci-build][with-all-tests]

sofabot · 2026-06-11T08:31:35Z

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alxbilger · 2026-06-12T14:12:47Z

+/**
+ *  \brief Component responsible for timestep integration, i.e. advancing the state from time t to t+dt.
+ *
+ *  This class currently control both the integration scheme (explicit,


There is something wrong in this sentence.

alxbilger · 2026-06-12T14:14:32Z

+ *  (some computations can be executed in parallel).
+ *
+ */
+class SOFA_CORE_API IntegrationScheme : public virtual objectmodel::BaseComponent


Usually the abstract base classes in Core are prefixed with Base. I recommand BaseIntegrationScheme for consistency.

alxbilger · 2026-06-12T14:15:00Z

+
+public:
+
+    virtual void integrate(const core::ExecParams* params, SReal dt, sofa::core::MultiVecCoordId xResult, sofa::core::MultiVecDerivId vResult) = 0;


Could you add a documentation for this function?

alxbilger · 2026-06-12T14:16:01Z

+    /// Given the solution dx of the linear system inversion, how much will it affect the velocity
+    ///
+    /// This method is used to compute the compliance for contact corrections


Sorry, but the current documentation does not tell me what this function returns. Same for getPositionIntegrationFactor

alxbilger · 2026-06-12T14:17:05Z

+     */
+    virtual void updateStatesFromLinearSolution(SReal alpha, bool firstIteration = false) = 0;
+
+    virtual void integrate(const core::ExecParams* params, SReal dt, sofa::core::MultiVecCoordId xResult, sofa::core::MultiVecDerivId vResult) ;


You should do a pass on the whole PR checking missing override keyword.

alxbilger · 2026-06-12T14:18:43Z

+    /**
+     * Returns the evaluation of the residue
+     */
+    virtual SReal evaluateResidue() = 0;


I think it should be residual instead of residue.

alxbilger · 2026-06-12T14:20:55Z

+    // WARNING we expect the linear integrator to initialize the working vecs. Meaning that if we
+    // work in FreeMotion, the xResult should already be equal to the actual position.
+    // Same for the velocity. This is expected when updating the position, only a += will be done.
+    virtual void setupIntegrationStep(const core::ExecParams* params, SReal dt, sofa::core::MultiVecCoordId xResult, sofa::core::MultiVecDerivId vResult);


Is it necessary to make it virtual since you use the template method pattern?

alxbilger · 2026-06-12T14:21:09Z

+    // WARNING we expect the linear integrator to initialize the working vecs. Meaning that if we
+    // work in FreeMotion, the xResult should already be equal to the actual position.
+    // Same for the velocity. This is expected when updating the position, only a += will be done.


Could you clarify a bit?

alxbilger · 2026-06-12T14:25:14Z

+namespace sofa::simulation::integrationschemes
+{
+
+class SOFA_SIMULATION_CORE_API AccelerationBasedImplicitIntegrationScheme :


Can you document this class?

alxbilger · 2026-06-12T14:28:24Z

+    const core::ExecParams* m_params;
+    sofa::core::MultiVecCoordId m_xResult;
+    sofa::core::MultiVecDerivId m_vResult;
+    sofa::core::MultiVecDerivId m_unknown;


I was confused by the name of this variable. I recommend m_systemUnknown

bakpaul added this to Integration of the method Fast but Accurate May 20, 2026

bakpaul and others added 25 commits June 10, 2026 15:51

Introduce base integration schemes + acceleration based and velocity …

9e4057b

…based IS and some implementaiton of final IS. Still need to make the solve visitor use these IS

Remove ODESolvers

9fa72d2

Fix last remnence of ODE + add BDF that doesn't work

b236424

Fix EulerImplicit + partially fix BDF, at least it matches Alex implem

dee22fc

Update BDFIntegrationScheme.cpp

cdd48b5

Added one more level of inheritance whith implicit/explicit integrati…

01b779d

…ons cheme

Add Static solver

d1ccd7d

Add linear ùmultistep methods

12b5711

Bring back forward solvers

a6f8e5f

Add explicit solvers

dbba380

Rename implicit solvers using IntegrationScheme

ef23a02

Fix part of instability in freemotion + remove unnecessary clear

831c64d

Better usage of vop and mop

10bdc8d

Clean and add comments

e22642d

Fix instability by computing RHS before LHS

eb146a7

Add first order capability to velocity-based IS

2723715

changed iteration parameter into a bool instead of a number

123c461

Fix condition for rayleigh damping in the RHS

8420e1f

Implement the trapezoid scheme

3dd2ed8

Refactor IS order getter to explicit the time characteristic

62313d8

Use right mop and vop in static solver

94214e8

Rename residue estimation method

14481b9

Fix compilation

87add8b

Add Newton solver to StaticEquilibriumIS

4aedea8

fix typo in comment

f14bd54

Apply changes from sofa-framework#6087

16b6e0f

Co-authored-by: Alex Bilger <alxbilger@users.noreply.github.com>

bakpaul force-pushed the 26_05_introduce_integration_schemes branch from 0aba213 to 16b6e0f Compare June 10, 2026 14:46

bakpaul marked this pull request as ready for review June 10, 2026 14:49

bakpaul added pr: status to review To notify reviewers to review this pull-request and removed pr: status wip Development in the pull-request is still in progress labels Jun 10, 2026

Fix compilation

20184c0

bakpaul mentioned this pull request Jun 11, 2026

Apply changes from SOFA PR #6117 sofa-framework/Regression#114

Open

fix more scenes

17153f8

sofa-framework deleted a comment from sofabot Jun 11, 2026

Fix compilation

0fe9025

sofa-framework deleted a comment from sofabot Jun 11, 2026

Fix requiredPlugins

ff4dc57

hugtalbot added this to the v26.12 milestone Jun 12, 2026

alxbilger reviewed Jun 12, 2026

View reviewed changes


		public:

		virtual void integrate(const core::ExecParams* params, SReal dt, sofa::core::MultiVecCoordId xResult, sofa::core::MultiVecDerivId vResult) = 0;

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bakpaul commented May 20, 2026 •

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