Accessing OTSLM from LabVIEW

It is possible to use functionality from OTSLM in LabVIEW. This can be useful for implementing user interfaces which can be easily customised or for integrating OTSLM with existing code.

To run MATLAB scripts from LabVIEW, you will need to use MathScript or Interface for MATLAB depending on the version of LabVIEW you are using. We have provided an example package using LabVIEW NXG 3.1 (using LabVIEW Interface for MATLAB) in the examples/labview folder of the toolbox.

This section provides an overview of the LabVIEW example package and the PrismsAndLenses example LabVIEW application. The example package only provides the features needed for the PrismsAndLenese example. We welcome contributions from LabVIEW users to improve this package to provide better coverage of the OTSLM functionality.

Creating an otslm.simple function interface

LabVIEW Interface for MATLAB (LIFM) provides a system for defining different interfaces to matlab functions. Using LIFM, you can specify the input parameter names and types in order to create an object that can be imported into a VI. The current version of LabVIEW Interface for MATLAB doesn’t provide a good method for dealing directly with string constants or named parameters, instead it is better to create a separate VI which wraps the LIFM interface.

In this section we take you through defining a interface for the otslm.simple.lienar() function. This function takes two required inputs (the size of the pattern and spacing of the grating) and outputs a 2D array of doubles for the pattern. The complete example can be found in otslm/examples/labview/OtslmMatlabInterface/otslm.gcomp/simple/linear.mli and otslm/examples/labview/OtslmMatlabInterface/otslm.gcomp/simple/linear.gvi.

Start by creating a new Interface for Matlab, go to: File > New > Interface for MATLAB. In the box marked Select a MATLAB program file or enter a MATLAB function name enter otslm.simple.linear and click Add interface node. Click Add parameter five times and name the parameters im, sz, spacing, centre_str and centre as shown in Fig. 26.

Fig. 26 Setting up the parameters for the Matlab interface to otslm.simple.linear().

To change the data types and the input/output mode for the parameters, click on the parameter and change the corresponding settings in the Item panel, as shown in Fig. 27.

Fig. 27 Screenshot of the item configuration panel.

In order to use this interface the OTSLM path must be added to the Matlab path. You can do this either by adding the OTSLM path in the Startup.m script, as described on the getting started page, or you can run a script which adds OTSLM to the Matlab path. In the example package, we run a script to add OTSLM to the path. The initOtslm.m script, located in the examples/labview directory contains the following code:

function initOtslm()

  fname = mfilename('fullpath');
  [fpath, ~, ~] = fileparts(fname);
  fparts = split(fpath, filesep);

  % Add current path
  addpath(fpath);

  % Add toolbox path
  toolbox_path = fullfile(fparts{1:end-2});
  addpath(toolbox_path);

end

The script first finds the path for the mfile, adds the examples/labview directory to the path and adds the relative path for the otslm directory to the path. To call this script, you will need to create another Matlab interface and specify the file path to this script. The interface for this script doesn’t need any parameters. This script should be run at the start of each LabVIEW session or at the start of each LabVIEW application.

The Matlab interface created for otslm.simple.linear() can now be included in LabVIEW applications or VIs. To use the interface, you must connect values to each of the input and output parameters and optionally the input/output error connectors. However, most of the time you will not need to change all of the parameters, for instance, the centre_str parameter will always be the string 'centre'. To simplify the interface and allow customisation of the icon we can create a wrapper VI for the Matlab interface. To create a new VI, click File > New > VI. Add the Matlab Interface VI you just created to the centre of the diagram and connect nodes to the terminals as shown in Fig. 28.

Fig. 28 Wrapper for LabVIEW interface for matlab.

This interface can be further improved, for instance, making the centre array optional and checking the length of the array is correct. For example code, see linear.gvi.

Once the diagram has been configured, you can create a front panel to test the interface and configure the icon.

Calling a function with a cell array

The otslm.tools.combine() function takes as input a cell array of patterns to combine and returns a single pattern as the result. LabVIEW doesn’t currently provide a mechanism for calling a function with a cell array, however we can work around this by writing a wrapper function which takes a 3D array of images and converts them to a cell array of 2D images. The unpackCombine.m function in examples/labview does exactly this:

function varargout = unpackCombine(input3, varargin)

  input = mat2cell(input3, size(input3, 1), size(input3, 2), ...
    ones(1, size(input3, 3)));

  input = squeeze(input);

  [varargout{1:nargout}] = otslm.tools.combine(input, varargin{:});

end

It is now possible to create an LabVIEW Interface for Matlab using this function as described in the previous section.

Creating an otslm class interface

In order to use OTSLM classes, such as otslm.utils.ScreenDevice we need to construct and instance of the object, call its methods and clean up the instance once we are done. LabVIEW only supports creating function and script interfaces for Matlab. In order to work around this, we can write a dispatch method which creates the class instance and handles calls to the function methods. The following is an example of a dispatch method:

function varargout = callClassMethod(varname, classname, methodname, varargin)

assert(~isempty(varname), 'varname must be supplied');

tmpvarname = 'ourargs';

if isempty(methodname) && ~isempty(classname)

  % Create a new instance of the class
  assignin('base', tmpvarname, varargin);
  evalin('base', [varname, ' = ', classname, '(', tmpvarname, '{:});']);

elseif isempty(classname) && ~isempty(methodname)

  % Call a class method
  assignin('base', tmpvarname, varargin);
  [varargout{1:nargout}] = evalin('base', [varname, '.', methodname, '(', tmpvarname, '{:});']);

else
  error('Only classname or methodname must be supplied');

end

This function places the Matlab class instance in the base workspace, we keep track of the class instance using a string (varname) in LabVIEW. To use this dispatch method, we need to create a LabVIEW Interface for MATLAB for the class and add each class method we wish to use, including the constructor and destructor. For ScreenDevice, the interface might look something like the one shown in Fig. 29.

Fig. 29 An interface example for a Matlab class using the callClassMethod dispatch function.

We can then implement a wrapper VI for each of these methods as described in the previous sections. The classname and methodname arguments specify the constructor name and the class method name to be called. For the destructor, use the string 'delete' for the method name. In order to use this interface, we need to keep track of the class instance name and make sure we construct and delete the object before using other methods of the class. For example usage, see Building an application.

Building an application

This section describes building a LabVIEW application for generating a Prisms and Lenses hologram which is drawn using ScreenDevice. You can find the finished application in examples/labview/OtslmMatlabInterface/PrismsAndLenses.gcomp. This example assumes you have followed the above instructions to implement your own VIs for the spherical, linear, combine and ScreenDevice OTSLM functions/classes or you are using the examples provided in the examples/labview/OtslmMatlabInterface/otslm.gcomp package. If you use the example application/package, you will need to modify the path in otslm.gcomp/initOtslm.mli to find the correct path for the initOtslm.m file.

Create a new application in LabVIEW by going to File > New > Application. Name the application. Add a new VI to the application for the front panel (where the main user interface will be displayed): right click on the application icon in the project browser and click: New > VI, as shown in Fig. 30.

Fig. 30 Adding a new VI to an application.

Create the VI by adding the controls shown in Fig. 31.

Fig. 31 Layout of front panel.

The user interface will allow the user to specify the size and position of the window on the screen, change the number and location of spots in the Prisms and Lenses algorithm, and see a preview of what the image will look like on the screen.

To implement this, we need to initialise OTSLM, construct the screen device object for displaying the patterns, generate the array of patterns to pass to otslm.tools.combine for each spot the user requests, and display the result in the previous and on the screen.

To generate the array of patterns for each prisms and lenses spot, we will create a sub-vi which takes as input the pattern size and spot locations and generates a 3D array of patterns which we can pass to combine. Add a new vi to your application and configure it with the nodes shown in Fig. 32.

Fig. 32 Layout of generate images diagram.

To add the spherical and lenses sub-vis, either click and drag the VIs from the project file tree or add them from the Project Items menu, as shown in Fig. 33.

Fig. 33 Using the project items menu.

Connect the input and output nodes in the icon diagram as shown in Fig. 34.

Fig. 34 Layout of generate images icon.

Next, switch back to the front panel diagram and construct the program shown in Fig. 35.

Fig. 35 Layout of the front panel diagram.

In this example we use a loop to continuously update the display when the user changes inputs to the VI. The ScreenDevice is positioned and constructed outside the loop, this means that the size of the pattern and location are fixed throughout the entire run of the program. If the show display checkbox is not clicked, the ScreenDevice is asked to close, otherwise the pattern is displayed to the screen.