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.

Example / template plugin

  1# -*- coding: utf-8 -*-
  2# imageio is distributed under the terms of the (new) BSD License.
  3
  4""" Example plugin. You can use this as a template for your own plugin.
  5"""
  6
  7import numpy as np
  8
  9from .. import formats
 10from ..core import Format
 11
 12
 13class DummyFormat(Format):
 14    """The dummy format is an example format that does nothing.
 15    It will never indicate that it can read or write a file. When
 16    explicitly asked to read, it will simply read the bytes. When
 17    explicitly asked to write, it will raise an error.
 18
 19    This documentation is shown when the user does ``help('thisformat')``.
 20
 21    Parameters for reading
 22    ----------------------
 23    Specify arguments in numpy doc style here.
 24
 25    Parameters for saving
 26    ---------------------
 27    Specify arguments in numpy doc style here.
 28
 29    """
 30
 31    def _can_read(self, request):
 32        # This method is called when the format manager is searching
 33        # for a format to read a certain image. Return True if this format
 34        # can do it.
 35        #
 36        # The format manager is aware of the extensions and the modes
 37        # that each format can handle. It will first ask all formats
 38        # that *seem* to be able to read it whether they can. If none
 39        # can, it will ask the remaining formats if they can: the
 40        # extension might be missing, and this allows formats to provide
 41        # functionality for certain extensions, while giving preference
 42        # to other plugins.
 43        #
 44        # If a format says it can, it should live up to it. The format
 45        # would ideally check the request.firstbytes and look for a
 46        # header of some kind.
 47        #
 48        # The request object has:
 49        # request.filename: a representation of the source (only for reporting)
 50        # request.firstbytes: the first 256 bytes of the file.
 51        # request.mode[0]: read or write mode
 52        # request.mode[1]: what kind of data the user expects: one of 'iIvV?'
 53
 54        if request.mode[1] in (self.modes + "?"):
 55            if request.extension in self.extensions:
 56                return True
 57
 58    def _can_write(self, request):
 59        # This method is called when the format manager is searching
 60        # for a format to write a certain image. It will first ask all
 61        # formats that *seem* to be able to write it whether they can.
 62        # If none can, it will ask the remaining formats if they can.
 63        #
 64        # Return True if the format can do it.
 65
 66        # In most cases, this code does suffice:
 67        if request.mode[1] in (self.modes + "?"):
 68            if request.extension in self.extensions:
 69                return True
 70
 71    # -- reader
 72
 73    class Reader(Format.Reader):
 74        def _open(self, some_option=False, length=1):
 75            # Specify kwargs here. Optionally, the user-specified kwargs
 76            # can also be accessed via the request.kwargs object.
 77            #
 78            # The request object provides two ways to get access to the
 79            # data. Use just one:
 80            #  - Use request.get_file() for a file object (preferred)
 81            #  - Use request.get_local_filename() for a file on the system
 82            self._fp = self.request.get_file()
 83            self._length = length  # passed as an arg in this case for testing
 84            self._data = None
 85
 86        def _close(self):
 87            # Close the reader.
 88            # Note that the request object will close self._fp
 89            pass
 90
 91        def _get_length(self):
 92            # Return the number of images. Can be np.inf
 93            return self._length
 94
 95        def _get_data(self, index):
 96            # Return the data and meta data for the given index
 97            if index >= self._length:
 98                raise IndexError("Image index %i > %i" % (index, self._length))
 99            # Read all bytes
100            if self._data is None:
101                self._data = self._fp.read()
102            # Put in a numpy array
103            im = np.frombuffer(self._data, "uint8")
104            im.shape = len(im), 1
105            # Return array and dummy meta data
106            return im, {}
107
108        def _get_meta_data(self, index):
109            # Get the meta data for the given index. If index is None, it
110            # should return the global meta data.
111            return {}  # This format does not support meta data
112
113    # -- writer
114
115    class Writer(Format.Writer):
116        def _open(self, flags=0):
117            # Specify kwargs here. Optionally, the user-specified kwargs
118            # can also be accessed via the request.kwargs object.
119            #
120            # The request object provides two ways to write the data.
121            # Use just one:
122            #  - Use request.get_file() for a file object (preferred)
123            #  - Use request.get_local_filename() for a file on the system
124            self._fp = self.request.get_file()
125
126        def _close(self):
127            # Close the reader.
128            # Note that the request object will close self._fp
129            pass
130
131        def _append_data(self, im, meta):
132            # Process the given data and meta data.
133            raise RuntimeError("The dummy format cannot write image data.")
134
135        def set_meta_data(self, meta):
136            # Process the given meta data (global for all images)
137            # It is not mandatory to support this.
138            raise RuntimeError("The dummy format cannot write meta data.")
139
140
141# Register. You register an *instance* of a Format class. Here specify:
142format = DummyFormat(
143    "dummy",  # short name
144    "An example format that does nothing.",  # one line descr.
145    ".foobar .nonexistentext",  # list of extensions
146    "iI",  # modes, characters in iIvV
147)
148formats.add_format(format)

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