HED MATLAB tools

The HED MATLAB tools allow validation, summary, search, factorization, data epoching and other HED processing in MATLAB by providing MATLAB wrappers for the Python HEDTools. These MATLAB wrappers allow MATLAB users to use HED without learning Python.

Tool overview

The HED MATLAB tools provide the following interface to HEDTools as explained in more detail in the following sections. The MATLAB HEDTools package provides two interchangeable implementations of these functions – calling the HED Python tools through a web service or directly calling the Python HEDTools.

HED MATLAB tools.

Both approaches take MATLAB data as input and translate these values as needed to access the HEDTools. After making the call, the implementation translates the results back into MATLAB data types.

Calling the HED MATLAB tools using web services requires no installation beyond downloading the HED MATLAB package, but its use requires Internet access.

Using direct calls to the Python HEDTools from MATLAB is more efficient and provides additional functionality not available through the web service interface. However, direct calls require a one-time setup of Python for your MATLAB installation. See MATLAB Python install for additional setup information.

Another option is to use the EEGLAB HEDTools plug-in integration for HED. The EEGLAB plug-ins provide easy access through the EEGLAB GUI interface

The HED MATLAB interface

The HED MATLAB tools provide the following routines for accessing HED:

MATLAB HEDTools interface (* indicates optional Name-Value parameter)

MATLAB

Purpose

Parameters

Returns

getHedAnnotations

Assemble a cell array of HED string
annotations from data such as events.

events
sidecar
removeTypesOn
includeContext
replaceDefs

cell array of char

searchHed

Given a cell array of n HED string
annotations and a cell array of
m of HED queries return an
n x m array indicating whether
each annotation satisfied the
respective query.

annotations
queries

n x m array of 1’s and 0’s.

validateEvents

Validate HED events data in various
format sand return a printable
string with HED issues.

events
sidecar
checkWarnings*

char

validateSidecar

Validate HED in a BIDS sidecar
(in various formats) and return a
printable string with HED issues.

sidecar
checkWarnings*

char

validateTags

Validate a HED annotation
(in various formats) and return a
printable string with HED issues.

hedTags
checkWarnings*

char

The parameters are defined as follows:

Parameters for MATLAB HED tools (* indicates optional Name-Value parameter).

Parameter

Description

MATLAB type

annotations

Cell array with hed annotations.

cell array of char or string,
(or of HedString if Python)

events

Tabular data with HED tags
ex: EEGLAB EEG.events or BIDS _events.tsv.

char, string, struct
(or TabularInput if Python)

sidecar

Data representing a BIDS-type sidecar
containing metadata for tabular data.

char, string, struct
(or Sidecar if Python)

hedtags

A HED annotation string.

char, string
(or HedString if Python)

queries

Valid HED query strings.

Cell array of char or string

checkWarnings*

Optional flag indicating whether to check
for warnings during validation.

true or false

includeContext*

Optional flag indicating whether to
include the event context in assembling HED tags.

true or false

removeTypesOn*

Optional flag indicating whether to
remove Condition-variable and Task tags.

true or false

replaceDefs*

Optional flag indicataing whether to
eplace Def tags with contents
of their definitions.

true or false

What to download

The HED MATLAB tools can be downloaded from the hed-matlab GitHub repository. Add the hedmat directory and all of its subdirectories to your path.

The following table describes the directories of this repository:

Directories in hed-matlab GitHub repo.

Directory

Description

Directory

Description

data

Data used for the demos and tests.

docs

Source code for the documentation.

hedmat/hedtools

MATLAB interface for the HED tools.

hedmat/remodeling_demos

Demos of calling the HED remodeling tools.

hedmat/utilities

General purpose utilities.

hedmat/web_services_demos

Demos of directly using the HED web services (without hedtools).

tests

Unit tests for MATLAB. (Execute run_tests.m to run all unit tests.)

Using MATLAB HEDTools

This section gives some examples of using the HED MATLAB tools.

Getting a HEDTools object

The HED MATLAB tools are all called by getting a HedTools object and then making the calls through this object. Use the getHedTools function to get a HedTools object.

The following example gets a HedTools object using version 8.2.0 of the HED schema (standard vocabulary) and the webservice available at https://hedtools.org/hed.

Access HED tools through web services.

hed = getHedTools('8.2.0', 'https://hedtools.org/hed');

The first parameter is number of the HED version to use, and the second parameter is the URL of the web service. The hed returned by this call is HedToolsService, which implements the interface by calls to HED web services. The https://hedtools.org/hed is the primary server for the HED online tools. An alternative server for the web services is https://hedtools.org/hed_dev. This is the HED development server, which deploys the latest features.

If you have installed the HED Python tools, you can access the HED MATLAB interface using direct calls to Python.

Access HED tools through direct Python calls.

hed = getHedTools('8.2.0');

If you call the getHedTools with only the HED version number parameter, getHedTools assumes you are using direct calls to Python and returns a HedToolsPython object. The HED MATLAB interface calls and behavior are identical whether you use the services or direct calls. You must have the HED Python tools installed to use direct calls. See MATLAB Python install.

Calling a tool

Once you have the HED tools object, you can use it to call the tools listed above as illustrated in the following example:

Validate a string containing HED tags.

issues = validateTags(hed, 'Sensory-event,Red,Blech,(Image, Banana)');

The issues is a printable char array. The HED tags string in the above example has two unrecognized tags: Blech and Banana. The call to validateTags produces the following issues message:

Validate a string containing HED tags.

   TAG_INVALID: 'Blech' in Blech is not a valid base hed tag.
   TAG_INVALID: 'Banana' in Banana is not a valid base hed tag.

Input of events

Events are markers on the experimental timeline that indicate something of interest. One of HED’s primary purposes is to annotation event markers with usable metadata to assist in analysis.

Events are generally represented in a tabular input form with column names. Each row in such a table corresponds to information associated with one event marker.

Example events input

row #

onset

latency

type

modifier

1

8.50

851

show

A2

2

8.90

891

show

A5

3

9.00

901

resp

key1

4

onset

1001

resp

key2

HED expects that event files have an onset column that provides the time in seconds of the event marker relative to the experimental timeline.

When this data is stored in a tab-separated value file called test.tsv, it can be read as:

Read a tab-separated value file into a char array.

events = fileread('test.tsv')

The events is a char array:

    'onset	latency	type	modifier
     8.50	851	show	A2
     8.90	891	show	A5
     9.00	901	resp	but1
     10.00	1001	resp	but2'

The same data can also be stored in MATLAB struct array:

Read a tab-separated value file into a char array.

events(1) = struct('onset', 8.5, 'latency', 851, 'type', 'show', 'modifier', 'A2');
events(2) = struct('onset', 8.9, 'latency', 891, 'type', 'show', 'modifier', 'A5');
events(3) = struct('onset', 9, 'latency', 901, 'type', 'resp', 'modifier', 'but1');
events(4) = struct('onset', 10, 'latency', 1001, 'type', 'resp', 'modifier', 'but2');

Displaying events on the MATLAB command line just gives the overall structure:

  1×4 struct array with fields:

    onset
    latency
    type
    modifier

but the MATLAB workspace editor provides a more informative view: ExampeMATLABStruct.

Once you have events data, it is easy to validate the HED associated with this data as shown by the following example:

Validate events data.

issues = hed.validateEvents(events, sidecar);

Input of sidecars

A tabular dataset may have a HED column whose entries provide HED annotations for the individual event markers represented by the rows. However, a more typical approach to annotation is to provide an additional dictionary, often called a sidecar. In BIDS the sidecar dictionaries are represented in JSON format. The MATLAB jsonencode and jsondecode translate various MATLAB data types into a JSON-compliant char value.

HED expects that sidecars will follow the BIDS format for sidecars associated with tabular files. The top-level JSON is a dictionary with keys that are the column names of the associated tabular file. Each key points to a dictionary of metadata for a column. One of the keys in this second dictionary can be HED as illustrated by the following example.

JSON sidecar for the events of previous section.

{
   "type": {
      "HED": {
         "show": "Sensory-presentation, Experimental-stimulus",
         "resp": "Agent-action, Participant-response"
      }
   },
   "modifier": {
       "HED": {
          "A2": "(Face, Image)",
          "A5": "(Animal/Horse, Image)",
          "but1": "(Press, (Left, Mouse-button))",
          "but2": "(Press, (Right, Mouse-button))"
      }
   }
}

Assembling HED annotations

The getHedAnnotations tool assembles the complete HED annotation for each event marker (i.e., each row in an event structure).

Assembling HED annotations.

annotations = hed.getHedAnnotations(events, sidecar);

Here annotations is a cell array with the same number of rows as events containing the char HED annotations for the events. The HEDTools look up the HED annotation for each element in a row of an events table and concatenate those together (comma-separated) to form the HED annotation for the event marker.

For the event data and sidecar defined in the previous sections, the sidecar provides HED annotations for the type and modifier columns. The resulting HED annotations for each row are given in the following table:

Example assembled HED annotations

row

HED annotation

1

Sensory-presentation,Experimental-stimulus,(Face,Image)

2

Sensory-presentation,Experimental-stimulus,(Animal/Horse,Image)

3

Agent-action,Participant-response,(Press,(Left,Mouse-button))

4

Agent-action,Participant-response,(Press,(Right,Mouse-button))

The annotation for the first row consists of the HED tags for the show value in column type (i.e., Sensory-presentation,Experimental-stimulus) concatenated with the HED tags for the A2 value in column modifier (i.e., (Face,Image)). Note: HED annotations are unordered, so the tags may appear in any order. Use parentheses to group tags as appropriate.

The example annotations do not use any Def tags, so the replaceDefs option is not relevant. Users can give a name to particular string of HED tags using the Definition tag and then use the Def tag with that name as a shortcut in annotations. The Def tags are useful for tag strings that are frequently used and are required to define event processes. However, when assembling annotations to search for tags, you usually want to replace these shortcut Def tags with the actual HED tags they represent. Thus, the default value of replaceDefs is true. See Advanced annotation for details.

Since no special tags defining extended event processes are used (i.e., Onset, Offset, Duration) in the example annotations, the includeContext option has no effect in this example. If includeContext is true (the default) event processes contribute their tags to the Event-context of time markers that fall in their scope. See Advanced annotation for an explanation of this.

The example annotation does not contain any Condition-variable or Task tags, so removeTypesOn has no effect. Typically removeTypesOn should be true to remove the effects of these tags for ordinary assembly and searching as extraction of design matrices is a separate operation. See HED conditions and design matrices for additional information.

Searching HED annotations

The searchHed tool takes a cell array of n HED annotations and a cell array of m HED search queries and returns an n x m array of 1’s and 0’s indicating whether the annotations satisfy the queries.

Example search of HED annotations

factors = hed.searchHed(annotations, {'Sensory-event', 'Agent-action'});

The result of this query is the following:

Query result for the sample annotation.

1

0

1

0

0

1

0

1

The queries can be quite complex as described in the HED search guide.

MATLAB Python install

Although MATLAB began python support of python in 2014, you must be using MATLAB version 2020b or later with the HEDTools because the current version of the HEDTools requires Python 3.8 or later. See compatible version of Python for a listing of which Python versions are compatible with which versions of MATLAB.

Note: For your reference, the source for hedtools is the hed-python GitHub repository. The code is fully open-source with an MIT license. The actual API documentation is available here, but the tutorials and tool documentation for hedtools on HED Resources site provides more examples of use.

You will need at least MATLAB version R2020b, since the Python HEDTools require at least Python 3.8. See MathWorks Python Compatibility Docs for additional information.

Installing Python

The greatest difficulty for users who are unfamiliar with Python is getting Python connected to MATLAB. Once that is done, many of the standard hedtools functions have MATLAB HED tool wrapper functions that take MATLAB variables as arguments and return either MATLAB variables or opaque objects that can be passed to other MATLAB functions.

Thus, once the setup is done, you don’t have to learn any Python syntax to use the tools. You should only have to do this setup once, since MATLAB retains the setup information from session to session.

Steps for setting up Python HEDtools for MATLAB.

Step 1: Find Python. If a version >= Python 3.8 is found, skip to Step 3.

Step 2: Install Python if needed .

Step 3: Connect Python to MATLAB. If already connected, skip to Step 4.

Step 4: Install HEDtools

Step 1: Find Python

Follow these steps until you find a Python executable that is version 3.8 or greater. If you can’t locate one, you will need to install it.

Does MATLAB already have a good version of Python you can use?

In your MATLAB command window execute the following function:

>> pyenv

The following example response shows that MATLAB is using Python version 3.9 with executable located at C:\Program Files\Python\Python39\python.EXE.

  PythonEnvironment with properties:

          Version: "3.9"
       Executable: "C:\Program Files\Python\Python39\python.EXE"
          Library: "C:\Program Files\Python\Python39\python39.dll"
             Home: "C:\Program Files\Python\Python39"
           Status: NotLoaded
    ExecutionMode: InProcess

If MATLAB has already knows about a suitable Python version that is at least 3.8, you are ready to go to Step 4: Install HEDTools. Keep track of the location of the Python executable.

If the pyenv did not indicate a suitable Python version, you will need to find the Python on your system (if there is one), or install your own.

There are several likely places to look for Python on your system.

For Linux users:

Likely places for system-space installation are /bin, /local/bin, /usr/bin, /usr/local/bin, or /opt/bin. User-space installations are usually your home directory in a subdirectory such as ~/bin or ~/.local/bin. The which python command will list the Python executable that is first found in your path.

For Windows users:

Likely places for system-space installation are C:\, C:\Python, C:\Program Files or C:\Program Files\Python. User-space installations default to your personal account in C:\Users\yourname\AppData\Local\Programs\Python\python39 where yourname is your Windows account name and python39 will be the particular version (in this case Python 3.9).

If you don’t have any success finding a Python executable, you will need to install Python as described in Step 2: Install Python if needed.

Otherwise, you can skip to Step 3: Connect Python to MATLAB.

Warning

You need to keep track of the path to your Python executable for Step 3.

Step 2: Install Python if needed

If you don’t have Python on your system, you will need to install it. Go to Python downloads and pick the correct installer for your operating system and version.

Depending on your OS and the installer options you selected, Python may be installed in your user space or in system space for all users.

  • You should keep track of the directory that Python was installed in.

  • You may want to add the location of the Python executable to your system or user PATH.

Warning

MATLAB installs add-ons such as the HEDTools in a specific user directory as described below. This makes user-installed Python modules available to all MATLAB projects. For this reason, we do not recommend trying to set up a virtual environment.

Step 3: Connect Python to MATLAB

Setting the Python version uses the MATLAB pyenv function with the 'Version' argument as illustrated by the following example.

Example MATLAB function call connect MATLAB to Python.

>> pyenv('Version', 'C:\Program Files\Python\Python39\python.exe')

Be sure to substitute the path of the Python that you have found.

Use the MATLAB pyenv function again without arguments to check that your installation is as expected.

Step 4: Install HEDTools

The general-purpose package manager for Python is called pip. By default, pip retrieves packages to be installed from the PyPI package repository. You will need to use the version of pip that corresponds to the version of Python that is connected to MATLAB. The right version of pip is found in the Scripts subdirectory of your Python installation.

Command to install hedtools in MATLAB.

To install the latest released version of hedtools type a pip command such as the following in your MATLAB command window.

system('"C:\Program Files\Python\Python39\Scripts\pip" install hedtools')

Use the full path of the pip associated with the Python that your MATLAB is using.

Giving the full path to correct version of pip ensures that MATLAB knows about HEDtools. (The version of Python that MATLAB is using may not be the same as the Python in the system PATH.)

Also watch the resulting messages in the MATLAB command window to make sure that HEDtools was successfully installed. In the above example, the Python being used is in system space, which requires administrator privileges. The first line of the output from installation was:

   Defaulting to user installation because normal site-packages is not writeable

On Windows these packages will be found in a site-packages directory such as:

C:\Users\username\AppData\Roaming\Python\Python39\site-packages

On Linux these packages might be found in directory such as:

/home/username/.local/lib/python3.9/site-packages/

Warning

If your system had a Python 2 installed at some point, your Python 3 executable might be named python3 rather than python.

Similarly, the pip package manager might be named pip3 instead of pip.

You will need to install the Python HEDTools.

Command to install HEDTools in MATLAB.

To install the latest released version of hedtools type a pip command such as the following in your MATLAB command window:

system('"C:\Program Files\Python\Python39\Scripts\pip" install hedtools')

The above command retrieves the latest version of HEDTools from the PyPI archive. If you want the newest features of the Python HEDTools, you will need to install directly from the GitHub repository as shown in the following example.

Command to install HEDTools from the GitHub repository.

system('"C:\Program Files\Python\Python39\Scripts\pip" install git+https://github.com/hed-standard/hed-python/@develop')

This installs the version of HEDTools on the develop branch. Be sure to replace the path to pip, with the one for your Python installation.

Warning

If you have an existing version of HEDTools installed and you want to install a new version, you must uninstall the HEDTools first:

system('"C:\Program Files\Python\Python39\Scripts\pip" uninstall hedtools')

Be sure to replace the path to pip, with the one for your Python installation.

The following MATLAB statement can be used to test that everything was installed correctly.

Test that everything is installed.

pyrun("from hed import _version as vr; print(f'Using HEDTOOLS version: {str(vr.get_versions())}')")

If everything installed correctly, the output will be something like

Using HEDTOOLS version: {'date': '2022-06-20T14:40:24-0500', 'dirty': False, 'error': None, 'full-revisionid': 'c4ecd1834cd31a05ebad3e97dc57e537550da044', 'version': '0.1.0'}

MATLAB functions for Python

The following table lists the relevant MATLAB functions that are available. You should refer to the help facility for your version of MATLAB to get the details of what is supported for your version of MATLAB.

MATLAB command

Purpose

pyenv

Setup your Python environment in MATLAB.
Without arguments outputs information about your current Python environment.

pyrun

Run a Python statement and return results.

pyargs

A recent addition for more advanced argument handling.

pyrunfile

Run a Python script from MATLAB.

The MATLAB matlab.exception.PyException captures error information generated during Python execution.

Additional demos

Calling HED remodeling tools

Many of the most useful HEDTools functions are packaged in the HED remodeling tool suite. These tools allow operations such as creating summaries, validating the dataset, and transforming event files to be run on an entire dataset.

The following example illustrates a call that creates a summary of the experimental conditions for a HED-tagged dataset.

A MATLAB wrapper function for a remodeling operation to create a summary.

function runRemodel(remodel_args)
    py.importlib.import_module('hed');
    py.hed.tools.remodeling.cli.run_remodel.main(remodel_args);

Example MATLAB calling code for this function:

dataPath = 'G:\ds003645';
remodelFile = 'G:\summarize_hed_types_rmdl.json';
remodel_args = {dataPath, remodelFile, '-b', '-x', 'stimuli', 'derivatives'};
runRemodel(remodel_args);

The command line arguments to the various remodeling functions are given in a cell array, rather than a regular MATLAB array. For the remodeling operations, first and second operation must be the dataset root directory and the remodeling file name, respectively. In this example, dataset ds003645 has been downloaded from openNeuro to the G:\ drive. The remodeling file used in this example can be found at See File remodeling quickstart and File remodeling tools for additional information. The wrapper functions are available on GitHub in the hedtools_wrappers directory.

Web service MATLAB demos

MATLAB HED web services allow MATLAB programs to request the same services that are available through the online tools. These services are available through the https://hedtools.org/hed server. Alternatively, these services can be accessed through a locally-deployed docker module. See the hed-web GitHub repository documentation for additional information on the docker deployment. The MATLAB code to run demos of these services is available on the hed-matlab GitHub repository.

The following MATLAB code demos are available to show how to access HED web services.

Target

MATLAB source

Purpose

Overall

runAllDemos.m

Harness for running all demos.

Overall

demoGetServices.m

List available services.

Events

demoEventServices.m

Validation, conversion, sidecar generation.

Events

demoEventSearchServices.m

Search, assembly.

Schema

in progress

For schema library developers.

Sidecars

demoSidecarServices.m

Validation, conversion, extraction, merging.

Spreadsheets

demoSpreadsheetServices.m

Validation, conversion.

Strings

demoStringServices.m

Validation, conversion.

The runAllDemos.m script runs all the demo code and reports whether the demos run successfully. Before using the HED web services from MATLAB, you should run this script to make sure everything is working on your system, that you have Internet access, and that the HED services are available.

This script also demonstrates how to call the individual demo functions. Each demo function takes a host URL as a parameter and returns a list of errors. The demos all use demo data read by the getDemoData function, which returns a MATLAB struct containing all needed test data. The individual demo scripts illustrate how to call each type of available web service.

Overview of service requests

Calling HED services from MATLAB requires the following steps:

  1. Set up a session.

  2. Create a request structure.

  3. Make a request.

  4. Decode the response.

Usually, you will do the first step (the session setup) once at the beginning of your script to construct a fixed session header that can be used in subsequent requests in your script.

Setting up a session from MATLAB

The goal of the session setup is to construct a header that can be used in subsequent web requests. The first step is to call the getHostOptions.m.

Establish a session.

host = 'https://hedtools.ucsd.edu/hed';
[servicesUrl, options] = getHostOptions(host)

The host should be set to the URL of the webserver that you are using. The call to getHostOptions, only needs to be done once at the beginning of your session. The servicesURL and the options can be used for all of your subsequent requests.

This function constructs the services URL from the host URL. The function also makes a service request to obtain a CSRF token and a cookie using getSessionInfo. The function then constructs a header and calls the MATLAB weboptions function to get an options object suitable for use with the MATLAB webwrite function use in all of our examples.

The getHostOptions does all the setup for using the services. As indicated by the code below, all communication is done in JSON. However, as demonstrated below, the MATLAB webwrite function takes a MATLAB struct as its request parameter and internally converts to the format specified in the header before making the request.

Source for getHostOptions.

function [servicesUrl, options] = getHostOptions(host)
    csrfUrl = [host '/services']; 
    servicesUrl = [host '/services_submit'];
    [cookie, csrftoken] = getSessionInfo(csrfUrl);
    header = ["Content-Type" "application/json"; ...
              "Accept" "application/json"; ...
              "X-CSRFToken" csrftoken; "Cookie" cookie];

    options = weboptions('MediaType', 'application/json', ...
                         'Timeout', 120, 'HeaderFields', header);

The Timeout parameter indicates how many seconds MATLAB will wait for a response before returning as a failed operation. The timeout value of 120 seconds is sufficient for most situations. However, this can be adjusted upward or downward to suit the user. The HeaderFields sets the parameters of HTTP request.

Creating a request structure

The request structure is a MATLAB struct which must have a service field and can have an arbitrary number of fields depending on which service is being requested.

The simplest service is get_services, which returns a string containing information about the available services. This service requires no additional parameters. In this and other examples, we assume that getHostOptions has been called to retrieve servicesUrl and options for use in the session.

Request a list of available HED web services.

request = struct('service', 'get_services');
response = webwrite(servicesUrl, request, options);
response = jsondecode(response);

The MATLAB webwrite returns a JSON structure as specified in the options. The MATLAB jsondecode function returns a MATLAB struct whose format is explained below in Decoding a service response.

Except for get_services, all other services are of the form target_command where target is the primary type of data acted on (events, schema, sidecar, spreadsheet, or string). The possible values for command depend on the value of target. For example sidecar_validate requests that a JSON sidecar be validated.

The get_services command provides information about the HED services that are available and the parameters required. The get_services entry for sidecar_validate is the following:

The get_services entry for sidecar_validate.

sidecar_validate:  
	Description: Validate a BIDS JSON sidecar (in string form) and return errors.  
	Parameters:  
		json_string  
		schema_string or schema_url or schema_version  
		check_for_warnings  
	Returns: A list of errors if any.

The Parameters section indicates the fields in addition to the service that are needed in the request structure. For example, sidecar_validate requires a HED schema. One possibility is to read a schema into a string and provide this information in schema_string. Another possibility is to provide a URL for the schema. The most-commonly used option is to use schema_version to indicate one of the supported versions available in the hedxml directory of the hed-schemas repository on GitHub.

Create a request for the sidecar_validate web service.

jsonText = fileread('../../datasets/eeg_ds003645s_hed_demo/task-FacePerception_events.json');
request = struct('service', 'sidecar_validate', ...
                 'schema_version', '8.2.0', ...
                 'json_string', jsonText, ...
                 'check_for_warnings', 'on');

This example reads the JSON sidecar to be validated as a character array into the variable jsonText and makes a request for validation using HED8.2.0.xml.

The request indicates that validation warnings as well as errors should be included in the response. If you wish to exclude warnings, use off instead of on as the check_for_warnings field value.

The demoSidecarServices.m function shows complete examples of the various HED services for JSON sidecars.

Making a service request

The HED services all use the MATLAB webwrite to make HED web service requests. The following call uses the sidecar_validate request from the previous section.

Request the sidecar validation service.

response = webwrite(servicesUrl, request, options);
response = jsondecode(response);
outputReport(response, 'Example: validate a JSON sidecar');

The outputReport.m MATLAB script outputs the response in readable form with a user-provided table.

If the web server is down or times out during a request, the MATLAB web_write function throws an exception, and the script terminates without setting the response.

If the connection completes successfully, the response will set. The next section explains the response structure in more detail.

Decoding a service response

All HED web services return a response consisting of a JSON dictionary with 4 keys as summarized in this table.

Field name

Meaning

service

Name of the requested service.

results

Results of the operation.

error_type

Type of error if the service failed.

error_msg

Explanation of the message if the service failed.

The jsondecode function translates the JSON dictionary into a MATLAB structure.

The error_type indicates whether the service request completed successfully and was able to get results. The error_type does NOT indicate the nature of the results (for example whether a JSON sidecar was valid or not), but rather whether the server was able to complete the request without raising an exception. A failure error_type is highly unusual and indicates some type of unexpected internal web service error. Errors of this type should be reported using the GitHub hed-python issues mechanism.

The results structure has the actual results of the service request.

Field name

Meaning

command

Command executed in response to the service request.

command_target

Type of data on which the command was executed.

data

Data returned by the service (either processed result or a list of errors).

msg_category

Success or warning depending on the result of processing the service.

msg

Explanation of the output of the service.

output_display_name

(Optional) File name for saving return data.

schema_version

(Optional) Version of the HED schema used in the processing.

The results structure will always have command, command_target fields indicating what operation was performed on what type of data.

The msg_category will be success or warning depending on whether there were errors. The contents of the data field will contain different information depending on the msg_category. For example, if a sidecar had validation errors, results.msg_category will be warning and the results.data value should be interpreted as a list of errors. If the sidecar had no errors, results.data will be an empty string.