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 theRequest
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 beinf
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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 | # -*- coding: utf-8 -*-
# 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.extension in 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.extension in 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)
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