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

  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
  7from __future__ import absolute_import, print_function, division
  8
  9import numpy as np
 10
 11from .. import formats
 12from ..core import Format
 13
 14
 15class DummyFormat(Format):
 16    """ The dummy format is an example format that does nothing.
 17    It will never indicate that it can read or write a file. When
 18    explicitly asked to read, it will simply read the bytes. When
 19    explicitly asked to write, it will raise an error.
 20
 21    This documentation is shown when the user does ``help('thisformat')``.
 22
 23    Parameters for reading
 24    ----------------------
 25    Specify arguments in numpy doc style here.
 26
 27    Parameters for saving
 28    ---------------------
 29    Specify arguments in numpy doc style here.
 30
 31    """
 32
 33    def _can_read(self, request):
 34        # This method is called when the format manager is searching
 35        # for a format to read a certain image. Return True if this format
 36        # can do it.
 37        #
 38        # The format manager is aware of the extensions and the modes
 39        # that each format can handle. It will first ask all formats
 40        # that *seem* to be able to read it whether they can. If none
 41        # can, it will ask the remaining formats if they can: the
 42        # extension might be missing, and this allows formats to provide
 43        # functionality for certain extensions, while giving preference
 44        # to other plugins.
 45        #
 46        # If a format says it can, it should live up to it. The format
 47        # would ideally check the request.firstbytes and look for a
 48        # header of some kind.
 49        #
 50        # The request object has:
 51        # request.filename: a representation of the source (only for reporting)
 52        # request.firstbytes: the first 256 bytes of the file.
 53        # request.mode[0]: read or write mode
 54        # request.mode[1]: what kind of data the user expects: one of 'iIvV?'
 55
 56        if request.mode[1] in (self.modes + "?"):
 57            if request.extension in self.extensions:
 58                return True
 59
 60    def _can_write(self, request):
 61        # This method is called when the format manager is searching
 62        # for a format to write a certain image. It will first ask all
 63        # formats that *seem* to be able to write it whether they can.
 64        # If none can, it will ask the remaining formats if they can.
 65        #
 66        # Return True if the format can do it.
 67
 68        # In most cases, this code does suffice:
 69        if request.mode[1] in (self.modes + "?"):
 70            if request.extension in self.extensions:
 71                return True
 72
 73    # -- reader
 74
 75    class Reader(Format.Reader):
 76        def _open(self, some_option=False, length=1):
 77            # Specify kwargs here. Optionally, the user-specified kwargs
 78            # can also be accessed via the request.kwargs object.
 79            #
 80            # The request object provides two ways to get access to the
 81            # data. Use just one:
 82            #  - Use request.get_file() for a file object (preferred)
 83            #  - Use request.get_local_filename() for a file on the system
 84            self._fp = self.request.get_file()
 85            self._length = length  # passed as an arg in this case for testing
 86            self._data = None
 87
 88        def _close(self):
 89            # Close the reader.
 90            # Note that the request object will close self._fp
 91            pass
 92
 93        def _get_length(self):
 94            # Return the number of images. Can be np.inf
 95            return self._length
 96
 97        def _get_data(self, index):
 98            # Return the data and meta data for the given index
 99            if index >= self._length:
100                raise IndexError("Image index %i > %i" % (index, self._length))
101            # Read all bytes
102            if self._data is None:
103                self._data = self._fp.read()
104            # Put in a numpy array
105            im = np.frombuffer(self._data, "uint8")
106            im.shape = len(im), 1
107            # Return array and dummy meta data
108            return im, {}
109
110        def _get_meta_data(self, index):
111            # Get the meta data for the given index. If index is None, it
112            # should return the global meta data.
113            return {}  # This format does not support meta data
114
115    # -- writer
116
117    class Writer(Format.Writer):
118        def _open(self, flags=0):
119            # Specify kwargs here. Optionally, the user-specified kwargs
120            # can also be accessed via the request.kwargs object.
121            #
122            # The request object provides two ways to write the data.
123            # Use just one:
124            #  - Use request.get_file() for a file object (preferred)
125            #  - Use request.get_local_filename() for a file on the system
126            self._fp = self.request.get_file()
127
128        def _close(self):
129            # Close the reader.
130            # Note that the request object will close self._fp
131            pass
132
133        def _append_data(self, im, meta):
134            # Process the given data and meta data.
135            raise RuntimeError("The dummy format cannot write image data.")
136
137        def set_meta_data(self, meta):
138            # Process the given meta data (global for all images)
139            # It is not mandatory to support this.
140            raise RuntimeError("The dummy format cannot write meta data.")
141
142
143# Register. You register an *instance* of a Format class. Here specify:
144format = DummyFormat(
145    "dummy",  # short name
146    "An example format that does nothing.",  # one line descr.
147    ".foobar .nonexistentext",  # list of extensions
148    "iI",  # modes, characters in iIvV
149)
150formats.add_format(format)