# Transformations

Transformations are used to translate data from a cluttered or technical raw value to a processed or human-readable representation. They are often useful, to interpret received Item values, like sensor readings or state variables, and to translate them into a human-readable or better processable format.


  • Translation of a technical Binding output, e.g., "CLOSED" can be translated to the Spanish translation "cerrado"
  • Processing of a raw Item value, e.g., Parsing a number from a JSON string, like { "temperature": 23.2 }
  • Conversion of sensor readings, e.g., temperature in degree Celsius can be converted to degree Fahrenheit

# Usage

Transformations are applicable in Item and Sitemap element labels and inside DSL rules. The relevant transformation service needs to be installed via the Main UI before use. Be aware, that some Transformation services rely on transformation files, while others work by directly providing the transformation logic. Transformation files need to be placed in the directory $OPENHAB_CONF/transform.

  1. Item and Sitemap Labels

    Transformations used in the state/value part of labels are applied on the fly. While the transformed value will (for example) be visible on a Sitemap, the original value is stored in the Item.

    The following example shows a Map transformation (see below) used in the State part of an Item's label. The technical state of a Contact Item (e.g. "CLOSED") is translated into a human readable representation in Spanish ("cerrado").

    Contact Livingroom_Window        "Window [MAP(window_esp.map):%s]"             {/*Some Binding*/}
    Number  Kitchen_Temperature_C    "Temperature [JSONPATH($.temperature):%s °C]" {/*Some Binding*/}
    Number  Livingroom_Temperature_F "Temperature [JS(convert-C-to-F.js):%s °F]"   {/*Some Binding*/}

    Usage of Transformations in the label parameter of Sitemap elements works the same way.

  2. Rules

    Transformations can also be used in rules to transform/translate/convert data. The following shows three examples:

    var condition = transform("MAP", "window_esp.map", "CLOSED")
    var temperature = transform("JSONPATH", "$.temperature", jsonstring)
    var fahrenheit = transform("JS", "convert-C-to-F.js", temperature)
  3. Bindings

    Transformations can sometimes be used in binding add-ons. For example, transforming an openHAB ON command into "action=powerup" for sending to a device. If, and how, this use may be available is described in individual binding documentation.

  4. Profiles

    Transformations can be associated with channels, working on data being passed between bindings and Items. See profile documentation for more detail.

To keep these examples simple, the contents of the referenced files window_esp.map and convert-C-to-F.js were left out.

# Script Transformation

The script transformation is available from the framework and needs no additional installation. It allows transforming values using any of the available scripting languages in openHAB (JSR-223 or DSL).

The script needs to be placed in the $OPENHAB_CONF/transform folder with the native extension for the chosen language, for example stringlength.js for a transformation using JS Scripting. The script file name here acts as the script identifier for the script transformation. A script identifier can also refer to a transformation script created in the UI.

The input value is injected into the script context as a string variable input. Additional parameters can be injected into the script by adding them to the script identifier in URL style (scale.js?correctionFactor=1.1&divider=10 would also inject correctionFactor and divider). These additional parameters are also injected into the script context as string variables. The result of the transformation is provided by the script as its return value. It can be null, a string, or a value of a type that properly implements .toString().

The examples show a simple transformation with the same functionality for some languages. It takes the length of the input string and e.g. returns String has 5 characters.

    # Inline Script Transformation

    A simple transformation rule can also be given as an inline script. The inline script should start with the | character. Quotes within the script may need to be escaped with a \ when used within another quoted string such as in sitemaps or text configurations.


      # Script Transformation Profile

      The script transformation is also available as profile. When acting as transformation profile:

      • A string return value of "UNDEF" or "NULL" will be translated to UnDefType.UNDEF or UnDefType.NULL correspondingly.
      • A null return value will cause the input to be discarded and not propagated to the destination.

      # Script Transformation Profile Configurations

      Parameter Name Description
      toItemScript The script identifier for performing transformations from the Thing handler to the item.
      toHandlerScript The script identifier for performing transformations from the item to the Thing handler.

      Example usage in an .items file:

      Number <itemName> { channel="<channelUID>"[profile="transform:JS", toItemScript="decode_json.js", toHandlerScript="encode_json.js" ] }

      Here, additional parameters can also be injected into the script using the URL style syntax, e.g.:

      Number <itemName> { channel="<channelUID>"[profile="transform:RB", toItemScript="multiply.rb?factor=10", toHandlerScript="multiply.rb?factor=0.1" ] }

      Inline script is also supported in the profile syntax.

      Number <itemName> { channel="<channelUID>"[profile="transform:RB", toItemScript="| input.to_f * 10", toHandlerScript="| input.to_f * 0.1" ] }