Programming Reference:MatlabFilter

The MatlabFilter calls the Matlab engine to act upon signals, parameters, and states. It provides the full BCI2000 filter interface to a Matlab filter implementation.

Filter Interface

For each BCI2000 filter member function, there is a corresponding Matlab function as follows:

GenericFilter member      Matlab function syntax
====================      ======================
Constructor               [parameters, states] = bci_Construct
Destructor                bci_Destruct
Preflight                 out_signal_dim = bci_Preflight( in_signal_dim )
Initialize                bci_Initialize( in_signal_dim, out_signal_dim )
Process                   out_signal = bci_Process( in_signal )
StartRun                  bci_StartRun
StopRun                   bci_StopRun
Resting                   bci_Resting
Halt                      bci_Halt

Existence of the above-listed Matlab functions is not mandatory. The MatlabFilter uses the Matlab exist command to determine whether a given function is available, and will not try to call that function when this is not the case.

If either of the bci_Preflight, bci_Initialize, or bci_Process functions is not available, a warning will be displayed to the user.

Accessing Parameters and States

Parameters and states are accessible via global Matlab structs called bci_Parameters and bci_States. In Matlab, write

 global bci_Parameters bci_States;
 my_sampling_rate = bci_Parameters.SamplingRate;

Parameter values may be changed from inside the bci_StopRun script, and will automatically be propagated to the other modules.

State values may be modified from the bci_Process function.

Declaring Parameters and States

To add parameters and states to the BCI2000 list of states, the bci_Construct function may return non-empty cell arrays of strings in its parameters and states return values. The strings constituting these cell arrays are parameter and state definitions.

Your added parameters will appear in the operator module's parameter configuration dialog. Your parameter definition's section name will be used to direct its position in the configuration dialog's register cards:

MyFilter int MyParam= 2 0 0 2 // ...

will display the MyParam parameter on separate register card named MyFilter.

Filtering:MyFilter int MyParam= 2 0 0 2 // ...

will display the parameter on the Filtering register card, inside a group called MyFilter.

Signal Format

BCI2000 signals are mapped to Matlab matrices with channel index first, and sample (element) index second. Signal dimension arguments of bci_Preflight and bci_Initialize are vectors of integers (1x2 matrices) as in [n_channels n_elements].

Error Reporting

To report errors from Matlab functions, use Matlab's error command. Your error messages will be displayed to the user from the operator module.

Combining with BCI2000 Signal Processing Filters

In the BCI2000 binary distribution, the MatlabFilter is shipped inside the MatlabSignalProcessing executable. There, the signal processing chain consists of the SpatialFilter and the MatlabFilter. However, by editing src/core/SignalProcessing/Matlab/PipeDefinition.cpp, you may add as many signal processing filters as you wish. (See Programming Reference:Filter Chain for information about defining a filter chain.)

This modification requires access to the BCI2000 source code. You will need to rebuild the MatlabSignalProcessing executable.

Command Line Options

MatlabWD

By default, the Matlab engine's working directory is set to the signal processing module's working directory at startup. When using the Matlab filter for signal processing, your filter's code will be contained in the bci_Process and associated Matlab functions, which will typically be located inside a dedicated directory. Thus, you may switch between Matlab-based filter implementations by selecting a Matlab working directory at startup. This may be done by specifying the MatlabWD parameter from the command line when starting the MatlabSignalProcessing module:

start MatlabSignalProcessing.exe --MatlabWD="./matlab" 127.0.0.1

will change Matlab's working directory to a directory called "matlab" inside the BCI2000 "prog" directory at startup, and execute bci_Process and associated functions from there.

This parameter must be set from the command line; later changes will have no effect.

Using the --MatlabWD command line option, you may easily switch between multiple Matlab-based BCI2000 configurations. Simply create a separate startup batch file for each configuration, and specify the respective configuration's path on the command line that starts up the MatlabFilter.

MatlabStayOpen

By default, the Matlab instance associated with the Matlab engine will be closed when BCI2000 quits. Generally, this behavior is desired to ensure a well-defined BCI2000 system state after each startup. For debugging purposes, though, it is sometimes useful to have the engine's command window available even after BCI2000 has been quit. This behavior is controlled by another command-line parameter, MatlabStayOpen, which may take the following values:

0 is the default (no change in behavior),
1 keeps the Matlab engine's command window open but clears variables used to transmit information forth and back between BCI2000 and Matlab,
2 also preserves intermediate variables (such as bci_Parameters and bci_States) in the Matlab engine's workspace.

To set this parameter to 2, use this command line to start up the signal processing module:

start MatlabSignalProcessing.exe --MatlabStayOpen=2 127.0.0.1

Once the MatlabStayOpen parameter has been set from the command line, it will be listed on the Operator Module's parameter configuration dialog's "system" tab, and may be modified from there. The change will be applied when clicking Set Config.

Troubleshooting

Debugging Matlab functions

Start BCI2000. The MatlabFilter will open a Matlab instance in its own window in minimized state.
Switch to the minimized Matlab instance.
Type, e.g.,
edit bci_Process.m
to open a Matlab editor window connected to the Matlab interactive window.
In the editor window, set breakpoints as you would normally.
Execution will be paused, and you may examine variables moving the mouse over them.

Matlab doesn't find your functions

Make sure to

either set Matlab's working directory to the directory containing your functions, using the --MatlabWD command line option described above,
or add the respective directory to your Matlab path.

Generally, the first option is recommended over the second one.

There is no Matlab engine started up

Execute
matlab /regserver
from the command line when logged in with administrative privileges.

Changes to M-files appear to have no effect

Switch to the Matlab engine's command window, and enter
clear;
or
clear scriptname;
each time you change a Matlab function.

You get linker errors when rebuilding MatlabSignalProcessing

If you get linker errors after editing PipeDefinition.cpp, make sure that all filters' cpp files are part of the MatlabSignalProcessing project.