Creating imageio plugins

Imagio is plugin-based. Every supported format is provided with a plugin. You can write your own plugins to make imageio support additional formats. And we would be interested in adding such code to the imageio codebase!

What is a plugin

In imageio, a plugin provides one or more Format objects, and corresponding Reader and Writer classes. Each Format object represents an implementation to read/write a particular file format. Its Reader and Writer classes do the actual reading/saving.

The reader and writer objects have a request attribute that can be used to obtain information about the read or write Request, such as user-provided keyword arguments, as well get access to the raw image data.

Registering

Strictly speaking a format can be used stand alone. However, to allow imageio to automatically select it for a specific file, the format must be registered using imageio.formats.add_format().

Note that a plugin is not required to be part of the imageio package; as long as a format is registered, imageio can use it. This makes imageio very easy to extend.

What methods to implement

Imageio is designed such that plugins only need to implement a few private methods. The public API is implemented by the base classes. In effect, the public methods can be given a descent docstring which does not have to be repeated at the plugins.

For the Format class, the following needs to be implemented/specified:

  • The format needs a short name, a description, and a list of file extensions that are common for the file-format in question. These ase set when instantiation the Format object.
  • Use a docstring to provide more detailed information about the format/plugin, such as parameters for reading and saving that the user can supply via keyword arguments.
  • Implement _can_read(request), return a bool. See also the Request class.
  • Implement _can_write(request), dito.

For the Format.Reader class:

  • Implement _open(**kwargs) to initialize the reader. Deal with the user-provided keyword arguments here.
  • Implement _close() to clean up.
  • Implement _get_length() to provide a suitable length based on what the user expects. Can be inf for streaming data.
  • Implement _get_data(index) to return an array and a meta-data dict.
  • Implement _get_meta_data(index) to return a meta-data dict. If index is None, it should return the ‘global’ meta-data.

For the Format.Writer class:

  • Implement _open(**kwargs) to initialize the writer. Deal with the user-provided keyword arguments here.
  • Implement _close() to clean up.
  • Implement _append_data(im, meta) to add data (and meta-data).
  • Implement _set_meta_data(meta) to set the global meta-data.

If the plugin requires a binary download from the imageio-binaries repository, implement the download method (see e.g. the ffmpeg plugin). Make sure that the download directory base name matches the plugin name. Otherwise, the download and removal command line scripts (see __main__.py) might not work.

Example / template plugin

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# -*- coding: utf-8 -*-
# Copyright (c) 2015, imageio contributors
# imageio is distributed under the terms of the (new) BSD License.

""" Example plugin. You can use this as a template for your own plugin.
"""

from __future__ import absolute_import, print_function, division

import numpy as np

from .. import formats
from ..core import Format


class DummyFormat(Format):
    """ The dummy format is an example format that does nothing.
    It will never indicate that it can read or write a file. When
    explicitly asked to read, it will simply read the bytes. When
    explicitly asked to write, it will raise an error.

    This documentation is shown when the user does ``help('thisformat')``.

    Parameters for reading
    ----------------------
    Specify arguments in numpy doc style here.

    Parameters for saving
    ---------------------
    Specify arguments in numpy doc style here.

    """

    def _can_read(self, request):
        # This method is called when the format manager is searching
        # for a format to read a certain image. Return True if this format
        # can do it.
        #
        # The format manager is aware of the extensions and the modes
        # that each format can handle. It will first ask all formats
        # that *seem* to be able to read it whether they can. If none
        # can, it will ask the remaining formats if they can: the
        # extension might be missing, and this allows formats to provide
        # functionality for certain extensions, while giving preference
        # to other plugins.
        #
        # If a format says it can, it should live up to it. The format
        # would ideally check the request.firstbytes and look for a
        # header of some kind.
        #
        # The request object has:
        # request.filename: a representation of the source (only for reporting)
        # request.firstbytes: the first 256 bytes of the file.
        # request.mode[0]: read or write mode
        # request.mode[1]: what kind of data the user expects: one of 'iIvV?'

        if request.mode[1] in (self.modes + '?'):
            if request.filename.lower().endswith(self.extensions):
                return True

    def _can_write(self, request):
        # This method is called when the format manager is searching
        # for a format to write a certain image. It will first ask all
        # formats that *seem* to be able to write it whether they can.
        # If none can, it will ask the remaining formats if they can.
        #
        # Return True if the format can do it.

        # In most cases, this code does suffice:
        if request.mode[1] in (self.modes + '?'):
            if request.filename.lower().endswith(self.extensions):
                return True

    # -- reader

    class Reader(Format.Reader):

        def _open(self, some_option=False, length=1):
            # Specify kwargs here. Optionally, the user-specified kwargs
            # can also be accessed via the request.kwargs object.
            #
            # The request object provides two ways to get access to the
            # data. Use just one:
            #  - Use request.get_file() for a file object (preferred)
            #  - Use request.get_local_filename() for a file on the system
            self._fp = self.request.get_file()
            self._length = length  # passed as an arg in this case for testing
            self._data = None

        def _close(self):
            # Close the reader.
            # Note that the request object will close self._fp
            pass

        def _get_length(self):
            # Return the number of images. Can be np.inf
            return self._length

        def _get_data(self, index):
            # Return the data and meta data for the given index
            if index >= self._length:
                raise IndexError('Image index %i > %i' % (index, self._length))
            # Read all bytes
            if self._data is None:
                self._data = self._fp.read()
            # Put in a numpy array
            im = np.frombuffer(self._data, 'uint8')
            im.shape = len(im), 1
            # Return array and dummy meta data
            return im, {}

        def _get_meta_data(self, index):
            # Get the meta data for the given index. If index is None, it
            # should return the global meta data.
            return {}  # This format does not support meta data

    # -- writer

    class Writer(Format.Writer):

        def _open(self, flags=0):
            # Specify kwargs here. Optionally, the user-specified kwargs
            # can also be accessed via the request.kwargs object.
            #
            # The request object provides two ways to write the data.
            # Use just one:
            #  - Use request.get_file() for a file object (preferred)
            #  - Use request.get_local_filename() for a file on the system
            self._fp = self.request.get_file()

        def _close(self):
            # Close the reader.
            # Note that the request object will close self._fp
            pass

        def _append_data(self, im, meta):
            # Process the given data and meta data.
            raise RuntimeError('The dummy format cannot write image data.')

        def set_meta_data(self, meta):
            # Process the given meta data (global for all images)
            # It is not mandatory to support this.
            raise RuntimeError('The dummy format cannot write meta data.')


# Register. You register an *instance* of a Format class. Here specify:
format = DummyFormat('dummy',  # short name
                     'An example format that does nothing.',  # one line descr.
                     '.foobar .nonexistentext',  # list of extensions
                     'iI'  # modes, characters in iIvV
                     )
formats.add_format(format)