# Pixel Data Encoder Plugins

Pixel Data encoding in pydicom uses an `Encoder` instance for the specific Transfer Syntax as a manager for plugins that perform the encoding work. This guide covers the requirements for those plugins and how to add them to pydicom. For a more general introduction to compression in pydicom see the tutorial instead.

## Plugin Requirements

Each available pixel data encoder in pydicom corresponds directly to a single DICOM Transfer Syntax UID, and is intended to provide a mechanism for converting raw unencoded source data to meet the requirements of that transfer syntax. In order to do so, each encoder has at least one encoding plugin which performs the actual conversion.

An encoding plugin must implement three objects within the same module:

- A function that performs the encoding with the following function signature:

      def foo(src: bytes, **kwargs: Any) -> bytes:

  Where

  - src is the raw uncompressed data to be encoded as `bytes`. When the data in src represents multi-byte values (such as 16-bit pixels), then src will use little-endian byte ordering by default. Support for big-endian byte ordering by the encoding function is completely optional.

  - kwargs is a `dict` which at a minimum contains the following required keys:

    - `'transfer_syntax_uid'`: `UID` - the intended Transfer Syntax UID of the encoded data.

    - `'byteorder'`: `str` - the byte ordering used by src, `'<'` for little-endian (the default), `'>'` for big-endian.

    - `'rows'`: `int` - the number of rows of pixels in the src.

    - `'columns'`: `int` - the number of columns of pixels in the src.

    - `'samples_per_pixel'`: `int` - the number of samples used per pixel, e.g. 1 for grayscale images or 3 for RGB.

    - `'number_of_frames'`: `int` - the number of image frames contained in src

    - `'bits_allocated'`: `int` - the number of bits used to contain each pixel in src, should be a multiple of 8.

    - `'bits_stored'`: `int` - the number of bits actually used by each pixel in src, e.g. 12-bit pixel data (range 0 to 4095) will be contained by 16-bits (range 0 to 65535).

    - `'pixel_representation'`: `int` - the type of data in src, `0` for unsigned integers, `1` for 2’s complement (signed) integers.

    - `'photometric_interpretation'`: `str` - the intended color space of the encoded data, such as `'YBR_FULL'`

    kwargs may also contain optional parameters intended to be used with the encoder function to allow customization of the encoding process or to provide additional functionality. Support for these optional parameters is not required, however.

  At a minimum the encoding function must support the encoding of little-endian byte ordered data and should return the encoded data in a format meeting the requirements of the corresponding Transfer Syntax UID as `bytes`.

- A function named `is_available` with the following signature:

      def is_available(uid: pydicom.uid.UID) -> bool:

  Where uid is the Transfer Syntax UID for the corresponding encoder as a `UID`. If the plugin supports the uid and has its dependencies met then it should return `True`, otherwise it should return `False`.

- A `dict` named `ENCODER_DEPENDENCIES` with the type `Dict[pydicom.uid.UID, Tuple[str, ...]`, such as:

      from pydicom.uid import RLELossless, JPEG2000

      ENCODER_DEPENDENCIES = {
          RLELossless: ('numpy', 'pillow', 'imagecodecs'),
          JPEG2000: ('numpy', 'gdcm'),
      }

  This will be used to provide the user with a list of missing dependencies required by the plugin.

An example of the requirements of a plugin is available here.

## Adding Plugins to an Encoder

Additional plugins can be added to an existing encoder with the `add_plugin()` method, which takes the a unique `str` plugin_label, and a `tuple` of `('the import path to the encoder function's module', 'encoder function name')`. For example, if you’d import your encoder function my_encoder_func with `from my_package.encoders import my_encoder_func`, then you’d do the following:

    from pydicom.encoders import RLELosslessEncoder

    RLELosslessEncoder.add_plugin(
        'my_encoder',  # the plugin's label
        ('my_package.encoders', 'my_encoder_func')  # the import paths
    )

The `my_package.encoders` module must contain the encoding function and the `ENCODER_DEPENDENCIES` and `is_available` objects.