A SimObject can implement a range of methods. Some of these are optional, some are required. Their calling sequence is given in the figure below.

The FhSim call sequence.

Constructor

 SimObject(const string& simObjectName);

Parameters

[in] simObjectName -> The name of the simobject

The constructor performs all initial setup for a SimObject. It is responsible for reading the parameters belonging to the SimObject from the input file and defining the interface of the SimObject, in terms of:

Input ports
Output ports
States It should also perform any additional 'one time only' resource setup.

A list of methods used in the constructor is found in SimObject construction methods.

InitialConditionSetup

 void InitialConditionSetup(const double T, const double* const currentIC, double* const updatedIC, ISimObjectCreator* const creator);

Parameters

[in]	T	The time at the beginning of the simulation. Usually but not necessarily zero
[in]	currentIC	Array of initial conditions. If the value is QNAN, the value is not set yet and can be set by the SimObject, otherwise the value is set, and can not be changed. Trying to change an already set value will trigger an error
[out]	updatedIC	Array of ONAN. Any initial condition that are ready to be set, should be written to this array.
[in]	creator	Pointer to creator class

Method for SimObjects that wants to set it's own initial conditions based on the value of it's input ports. SimObjects that wants to use this feature overrides the method, and it will be called if the SimObject has unspecified initial conditions after the values from the SimObject constructor, and the input file are set.

The following service contract applies to SimObjects that wishes to override this method:

All or some of the input ports may not be ready yet, for instance if other SimObjects also has unfinished states. In the case where a port is not yet ready, it will return an array of QNAN. In the case where a state is not ready it will contain a QNAN. The method should detect this and act appropriately. See also the is_qNaN function in SimObject
The method are expected to set as many states as it can based on which input port are ready or not.
If due to unfinished states, the SimObject itself has output ports that are not yet ready to return a legal value, these ports must be reported as unfinished. Note that if an output port is dependent on an unfinished input port, this in itself does not warrant a report. Only dependence on unfinished states are grounds for report.
If, after the method returns, the SimObject were unable to set all of it's states, the method will be called again until all states are set.
The solution is always growing. This entails that states that are once set, cannot be unset or changed. Input ports that were once ready, will continue to be ready. Output ports that during previous calls were not reported as unfinished are assumed ready and cannot be reported as unfinished later.
In all these matters the SimObject are expected to tell the truth, the whole truth, and nothing but the truth.
Some or all states that were suggested by the SimObject in the constructor, may have been overwritten by values from input files or other.
The SimObject may assume that multi element states and input ports are either set completely, or not at all. This means that if one element is QNAN the rest of the elements will be QNAN, and if one element is set all elements will be set. The exceptions from this rule are when the SimObject itself, either in the constructor or the InitialConditionSetup- method, specified the state with incomplete elements. In this case the SimObject are expected to know about it and take the proper precautions.

FinalSetup

 virtual void FinalSetup(double T, const double* X, ISimObjectCreator* creator);

Parameters

[in]	T	-> Current simulation time
[in]	X	-> Current simulation state
[in]	creator	-> Retrieve shared resources

Make final preparations before simulation starts. Mainly for retrieving shared resources registered during construction phase. Note: Called before RenderInit. Simobjects that wish to exchange shared resources through GetSharedResource/SetSharedResource may not retrieve said resources inside the constructor due to unpredictable order of construction.

RenderInit

 void RenderInit(Ogre::Root* ogreRoot, ISimObjectCreator* creator);

Parameters

[in]	ogreRoot	-> Register visualization resources
[in]	creator	-> Retrieve parameters.

Sets up visualization resources.

PreOdeFcn

 void PreOdeFcn(double T, const double* X, IStateUpdater* updater);

Parameters

[in]	T	-> Current simulation time
[in]	X	-> Current simulation state
[in]	updater	-> Callback class for updating states

Pre ode-function pass for handling discontinuous/discrete states. Called once before the start of every major timestep.

OdeFcn

 void OdeFcn(double T, const double* X, double* XDot) const = 0;

Parameters

[in]	T	-> Current simulation time
[in]	X	-> Current simulation state
[out]	XDot	-> State derivatives

This method is responsible for calculating the derivative of the states of the SimObject as a function of time, states and input ports.

X and XDot span the whole model, not this SimObject, and are indexed by global state index. A SimObject must therefore offset every access by the index ISimObjectCreator::AddState returned to it in the constructor:

 XDot[m_stateIndex + i] = ...;   // correct
 XDot[i]                = ...;   // wrong unless this object owns the model's first states

Indexing from zero reads and writes some other SimObject's states. It appears to work in a model with a single stateful SimObject, because that object's block starts at index 0.

OdeFcn is const and should contain derivative computations only. Use AcceptedStep() for once-per-accepted-step side effects.

AcceptedStep

 void AcceptedStep(double T, const double* X);

Parameters

[in]	T	-> Accepted simulation time
[in]	X	-> Accepted simulation state

Called once after each accepted integration step.

Typical uses:

Once-per-step bookkeeping
Event/logging hooks
Side effects that previously lived in if (isMajorTimeStep) branches

Default implementation is empty.

HasJacobians

 bool HasJacobians() const;

Returns: true if this SimObject provides an analytical Jacobian via OdeJacobian()

Returns whether this SimObject can provide analytical Jacobian information. The default implementation returns false. Override this method to return true if you have implemented OdeJacobian().

This method is used by implicit integration methods (e.g., SUNDIALS CVODES with BDF, ARKODE with DIRK, or the Engine's Euler1imp method) to determine whether to use analytical or numerically approximated Jacobians. Providing analytical Jacobians improves both performance and numerical stability, especially for stiff systems.

Note: If any SimObject with states returns false from HasJacobians(), the simulation will fall back to hybrid or numerical Jacobian approximation depending on the <Jacobian Policy> setting. See Implicit Integration with Analytical Jacobians for details.

See also: OdeJacobian(), GetJacobianSparsity(), HasPortJacobians(), Implicit Integration with Analytical Jacobians

OdeJacobian

 void OdeJacobian(double T, const double* X, double* J, int nStates);

Parameters

[in]	T	-> Current simulation time
[in]	X	-> Current simulation state
[out]	J	-> Jacobian matrix stored in row-major order (nStates x nStates)
[in]	nStates	-> Number of states (matrix dimension)

Computes the analytical Jacobian matrix J = dF/dX at the given time and state, where F is the right-hand side function defined by OdeFcn().

The Jacobian matrix is stored in row-major order, meaning element J[i][j] (the partial derivative of state derivative i with respect to state j) is stored at index i * nStates + j in the array.

This method is only called if HasJacobians() returns true. The default implementation does nothing.

Example

For a 2-state system with:

dX[0]/dt = -2*X[0] + X[1]

dX[1]/dt = X[0] - 3*X[1]

The Jacobian is constant:

J = [ -2 1 ]

[ 1 -3 ]

Implementation:

void MyObject::OdeJacobian(double T, const double* X, double* J, int nStates)
{
    J[0] = -2.0;  // dF[0]/dX[0]
    J[1] =  1.0;  // dF[0]/dX[1]
    J[2] =  1.0;  // dF[1]/dX[0]
    J[3] = -3.0;  // dF[1]/dX[1]
}

See also: HasJacobians(), GetJacobianSparsity(), Implicit Integration with Analytical Jacobians

GetJacobianSparsity

 int GetJacobianSparsity(int nStates, int* rowPtr, int* colIdx);

Parameters

[in]	nStates	-> Number of states
[out]	rowPtr	-> Row pointer array (size nStates+1), may be nullptr for query
[out]	colIdx	-> Column index array, may be nullptr for query

Returns: Number of non-zero elements, or -1 if the Jacobian is dense

Returns the sparsity pattern of the Jacobian matrix in Compressed Sparse Row (CSR) format. This is used by SUNDIALS to construct sparse Jacobian matrices for large systems, reducing memory usage and computation time.

The CSR format consists of two arrays:

rowPtr[i] gives the starting index in colIdx for row i
colIdx[p] gives the column index of the p-th non-zero element

Return -1 to indicate a dense Jacobian (the default). For small systems (typically fewer than ~50 states), dense Jacobians are usually more efficient than sparse ones.

Query Pattern

The method may be called with rowPtr = nullptr and colIdx = nullptr to query the number of non-zeros. In this case, only the return value is used.

Example

For a 2x2 dense Jacobian (all 4 elements non-zero):

int MyObject::GetJacobianSparsity(int nStates, int* rowPtr, int* colIdx)
{
    if (nStates != 2) return -1;
    if (rowPtr != nullptr && colIdx != nullptr) {
        rowPtr[0] = 0; rowPtr[1] = 2; rowPtr[2] = 4;
        colIdx[0] = 0; colIdx[1] = 1;
        colIdx[2] = 0; colIdx[3] = 1;
    }
    return 4;
}

For a tridiagonal 3x3 Jacobian (7 non-zeros):

int MyObject::GetJacobianSparsity(int nStates, int* rowPtr, int* colIdx)
{
    if (rowPtr != nullptr && colIdx != nullptr) {
        rowPtr[0] = 0;
        rowPtr[1] = 2;  // row 0: cols 0,1
        rowPtr[2] = 4;  // row 1: cols 0,1,2 -> but col 0 already counted
        rowPtr[3] = 7;  // row 2: cols 1,2
        colIdx[0] = 0; colIdx[1] = 1;
        colIdx[2] = 0; colIdx[3] = 1; colIdx[4] = 2;
        colIdx[5] = 1; colIdx[6] = 2;
    }
    return 7;
}

Note: For most SimObjects, returning -1 (dense) is sufficient. Sparse Jacobians are primarily beneficial for large systems with many states.

See also: HasJacobians(), OdeJacobian(), Implicit Integration with Analytical Jacobians

HasPortJacobians

 bool HasPortJacobians() const;

Returns: true if this SimObject provides analytical port Jacobians via OutputPortJacobian() and InputPortJacobian()

Returns whether this SimObject can provide analytical Jacobian information for its input and output ports. The default implementation returns false. Override this method to return true if you have implemented both OutputPortJacobian() and InputPortJacobian().

This method is used during Jacobian assembly to determine whether off-diagonal blocks (from port connections between stateful SimObjects) can be computed analytically via the chain rule, or must be approximated numerically.

Note: Both OutputPortJacobian() and InputPortJacobian() must be implemented for this to return true. If only one is implemented, the port connection will fall back to numerical approximation.

See also: OutputPortJacobian(), InputPortJacobian(), GetPortDependencyPolicy(), Cross-Object Port Jacobians

PortDependencyPolicy

 enum class PortDependencyPolicy { All, UserSpecified };

Policy for determining which local states an output port depends on. Used to build structural sparsity patterns for off-diagonal Jacobian blocks.

Value	Description
`All`	Conservative: assumes the port depends on all local states. Safe but may result in more numerical perturbations than necessary.
`UserSpecified`	The SimObject author provides exact dependency information via `GetPortStateDependency()`.

The default policy is All. Override GetPortDependencyPolicy() to return UserSpecified and implement GetPortStateDependency() for tighter sparsity.

See also: GetPortDependencyPolicy(), GetPortStateDependency(), Structural Sparsity & Port Dependencies

GetPortDependencyPolicy

 PortDependencyPolicy GetPortDependencyPolicy() const;

Returns: The port dependency policy for this SimObject

Returns the policy used to determine which local states each output port depends on. The default implementation returns All (conservative). Override to return UserSpecified if you implement GetPortStateDependency().

See also: PortDependencyPolicy, GetPortStateDependency(), Structural Sparsity & Port Dependencies

GetPortStateDependency

 std::vector<int> GetPortStateDependency(const std::string& portName) const;

Parameters

[in] portName -> Name of the output port

Returns: Vector of local state indices this port depends on

Returns the list of local state indices that a specific output port depends on. Only called when GetPortDependencyPolicy() returns UserSpecified.

Return an empty vector to fall back to "all states" for that port.

Example

std::vector<int> MyObject::GetPortStateDependency(const std::string& portName) const override
{
    if (portName == "force") {
        return {0, 1};  // Force output depends only on states 0 and 1
    }
    if (portName == "velocity") {
        return {2, 3, 4};  // Velocity depends on states 2, 3, 4
    }
    return {};  // Unknown port: fall back to all states
}

See also: GetPortDependencyPolicy(), Structural Sparsity & Port Dependencies

OutputPortJacobian

 void OutputPortJacobian(const std::string& portName, double T, const double* X,
                         double* dPort_dX, int portSize, int nStates, int index = -1);

Parameters

[in]	portName	-> Name of the output port
[in]	T	-> Current simulation time
[in]	X	-> Current simulation state (local to this SimObject)
[out]	dPort_dX	-> Jacobian matrix (row-major, portSize x nStates)
[in]	portSize	-> Size of the port signal
[in]	nStates	-> Number of local states
[in]	index	-> Port index for indexed ports, -1 for standard ports

Computes the Jacobian of an output port with respect to the SimObject's local states: \( \frac{\partial (\text{port\_value})}{\partial X_{local}} \).

The result is stored in row-major order with dimensions [portSize x nStates]. Element (i, j) at index i * nStates + j represents the partial derivative of the i-th port element with respect to the j-th local state.

Note: If the port output depends on CommonComputations, ensure they are executed before calling this method. The caller is responsible for invoking CommonComputations beforehand.; This method provides the C matrix (d(output)/dX) used in directional derivative computation. In forward mode, C is applied directly. In adjoint mode, its transpose Cᵀ is used to propagate sensitivities backwards.

Example

For a 2-state system with a scalar output port force = 3*X[0] + 2*X[1]:

void MyObject::OutputPortJacobian(const std::string& portName, double T,
    const double* X, double* dPort_dX, int portSize, int nStates, int index)
{
    if (portName == "force") {
        dPort_dX[0] = 3.0;  // d(force)/dX[0]
        dPort_dX[1] = 2.0;  // d(force)/dX[1]
    }
}

See also: HasPortJacobians(), InputPortJacobian(), Cross-Object Port Jacobians

InputPortJacobian

 void InputPortJacobian(const std::string& portName, double T, const double* X,
                      double* dF_dInput, int portSize, int nStates);

Parameters

[in]	portName	-> Name of the input port
[in]	T	-> Current simulation time
[in]	X	-> Current simulation state (local to this SimObject)
[out]	dF_dInput	-> Sensitivity matrix (row-major, nStates x portSize)
[in]	portSize	-> Size of the port signal
[in]	nStates	-> Number of local states

Computes the sensitivity of the SimObject's ODE derivatives with respect to an input port value: \( \frac{\partial F}{\partial (\text{input\_port})} \).

The result is stored in row-major order with dimensions [nStates x portSize]. Element (i, j) at index i * portSize + j represents the partial derivative of the i-th state derivative with respect to the j-th input port element.

Example

For a 2-state system with a scalar input port u:

dX[0]/dt = ... + 5*u

dX[1]/dt = ... - 1*u

void MyObject::InputPortJacobian(const std::string& portName, double T,
    const double* X, double* dF_dInput, int portSize, int nStates)
{
    if (portName == "force") {
        dF_dInput[0] =  5.0;  // dF[0]/du
        dF_dInput[1] = -1.0;  // dF[1]/du
    }
}

See also: HasPortJacobians(), OutputPortJacobian(), Cross-Object Port Jacobians

Note: This method provides the B matrix (dF/d(input)) used in directional derivative computation. In forward mode, B is applied to input perturbations. In adjoint mode, its transpose Bᵀ is used to map adjoint sensitivities to inputs.

RenderUpdate

 void RenderUpdate(double T, const double* X);

Parameters

[in]	T	-> Current simulation time
[in]	X	-> Current simulation state

Updates visualization resources to reflect current state

Showing text on the 3D view

Text a SimObject wants on the 3D view goes on the HUD (fhsim::Hud, <fhsim/visual/renderer/Hud.h>, visualisation builds only). The framework owns the font, the colours, the size and the placement, so lines from different SimObjects never overlap; a SimObject names only a corner, a meaning and a priority. What the viewer sees is described in Text on the 3D view: the HUD. Fetch the HUD in RenderInit and keep the handles it hands out:

#ifdef FH_VISUALIZATION
#include <fhsim/visual/renderer/Hud.h>
#endif
 
class MyVessel : public SimObject
{
    // ...
#ifdef FH_VISUALIZATION
    fhsim::HudLine m_speedLine;
    fhsim::HudLine m_alarmLine;
#endif
};
 
#ifdef FH_VISUALIZATION
void MyVessel::RenderInit(Ogre::Root* ogreRoot, ISimObjectCreator* creator)
{
    auto* hud = static_cast<fhsim::Hud*>(creator->GetSharedResource(fhsim::kSimMgrKeyHud));
    if (hud == nullptr) {
        return; // no renderer, or one that has no HUD
    }
    m_speedLine = hud->AddValueLine(fhsim::HudCorner::TopRight, "Speed:", "%.2f", "m/s");
    m_alarmLine = hud->AddTextLine(fhsim::HudCorner::BottomRight, "", fhsim::HudStyle::Warning);
}
 
void MyVessel::RenderUpdate(double T, const double* X)
{
    m_speedLine.SetValue(Speed(T, X));
    m_alarmLine.SetText(IsOverloaded(T, X) ? "Winch overloaded" : "");
}
#endif

Corners. HudCorner::TopLeft, TopRight, BottomLeft and BottomRight. Top regions stack downward, bottom regions upward. The first top-left line is always the simulation time. Nothing is placed at the bottom centre, where FhSimUI's progress bar is, and in FhSimUI the regions move clear of the menu bar, the orientation gizmo and the logo.
Priority. The last argument of AddTextLine/AddValueLine, default fhsim::Hud::kDefaultPriority (1000). A lower number sits closer to the corner's edge; lines with the same priority keep the order they were added. The lowest int is the framework's.
Styles. HudStyle::Normal, Highlight, Warning and Error (white, cyan, yellow, red). Change it with SetStyle.
Updates. SetText does nothing when the text is unchanged. A value line shows "label value unit"; SetValue formats only a changed value, at most ten times a second, and the latest value is always shown in the end, so calling it every frame is cheap. The format must consume exactly one double.
Background. Each corner region with visible text has one semi-transparent box behind its lines, a quarter of a text height larger than the text on every side. Its height follows the visible lines; its width is the widest text the region has shown since the scene was loaded, so changing digits do not make it jitter. The default colour is FhSimUI's window background at half opacity (fhsim::Hud::kDefaultBackgroundColour). hud->SetBackgroundColour(fhsim::HudColour{0.2f, 0.0f, 0.0f, 0.6f}) changes it for all four regions; there is one colour for the whole HUD, so if several SimObjects set it, the last call wins. Tearing the scene down puts the default back, so set it again in RenderInit in every scene.
Space. Text is 2.5 % of the window height. A line wider than half the window is cut off with "…"; lines that do not fit in their region are not drawn, and the log gets one warning per region.
Lifetime. A HudLine is movable and not copyable. Destroying it or calling Remove() removes the line. Lines may be added and removed in RenderUpdate as well as in RenderInit. When the scene is torn down the HUD drops every line, and any handle still alive stops doing anything (IsActive() returns false), so a SimObject destroyed after the renderer needs no teardown hook.
Threads. Rendering, and so every call into the HUD, runs on the simulation thread; the HUD does no locking.

TextRenderer::addTextBox, removeTextBox, setText and printf still work but are deprecated in favour of the HUD; define FHSIM_SILENCE_TEXTRENDERER_DEPRECATION before including TextRenderer.h to silence the warning while a library moves over.

Table of Contents

Constructor

InitialConditionSetup

FinalSetup

RenderInit

PreOdeFcn

OdeFcn

AcceptedStep

HasJacobians

OdeJacobian

Example

GetJacobianSparsity

Query Pattern

Example

HasPortJacobians

PortDependencyPolicy

GetPortDependencyPolicy

GetPortStateDependency

Example

OutputPortJacobian

Example

InputPortJacobian

Example

RenderUpdate

Showing text on the 3D view