mirror of
https://github.com/python-pillow/Pillow.git
synced 2025-11-04 09:57:43 +03:00
419 lines
16 KiB
ReStructuredText
419 lines
16 KiB
ReStructuredText
.. _image-plugins:
|
||
|
||
Writing Your Own Image Plugin
|
||
=============================
|
||
|
||
The Pillow uses a plug-in model which allows you to add your own
|
||
decoders to the library, without any changes to the library
|
||
itself. Such plug-ins usually have names like
|
||
:file:`XxxImagePlugin.py`, where ``Xxx`` is a unique format name
|
||
(usually an abbreviation).
|
||
|
||
.. warning:: Pillow >= 2.1.0 no longer automatically imports any file
|
||
in the Python path with a name ending in
|
||
:file:`ImagePlugin.py`. You will need to import your
|
||
image plugin manually.
|
||
|
||
Pillow decodes files in 2 stages:
|
||
|
||
1. It loops over the available image plugins in the loaded order, and
|
||
calls the plugin's ``accept`` function with the first 16 bytes of
|
||
the file. If the ``accept`` function returns true, the plugin's
|
||
``_open`` method is called to set up the image metadata and image
|
||
tiles. The ``_open`` method is not for decoding the actual image
|
||
data.
|
||
2. When the image data is requested, the ``ImageFile.load`` method is
|
||
called, which sets up a decoder for each tile and feeds the data to
|
||
it.
|
||
|
||
An image plug-in should contain a format handler derived from the
|
||
:py:class:`PIL.ImageFile.ImageFile` base class. This class should
|
||
provide an :py:meth:`_open` method, which reads the file header and
|
||
sets up at least the :py:attr:`~PIL.Image.Image.mode` and
|
||
:py:attr:`~PIL.Image.Image.size` attributes. To be able to load the
|
||
file, the method must also create a list of :py:attr:`tile`
|
||
descriptors, which contain a decoder name, extents of the tile, and
|
||
any decoder-specific data. The format handler class must be explicitly
|
||
registered, via a call to the :py:mod:`~PIL.Image` module.
|
||
|
||
.. note:: For performance reasons, it is important that the
|
||
:py:meth:`_open` method quickly rejects files that do not have the
|
||
appropriate contents.
|
||
|
||
Example
|
||
-------
|
||
|
||
The following plug-in supports a simple format, which has a 128-byte header
|
||
consisting of the words “SPAM” followed by the width, height, and pixel size in
|
||
bits. The header fields are separated by spaces. The image data follows
|
||
directly after the header, and can be either bi-level, greyscale, or 24-bit
|
||
true color.
|
||
|
||
**SpamImagePlugin.py**::
|
||
|
||
from PIL import Image, ImageFile
|
||
import string
|
||
|
||
class SpamImageFile(ImageFile.ImageFile):
|
||
|
||
format = "SPAM"
|
||
format_description = "Spam raster image"
|
||
|
||
def _open(self):
|
||
|
||
# check header
|
||
header = self.fp.read(128)
|
||
if header[:4] != "SPAM":
|
||
raise SyntaxError, "not a SPAM file"
|
||
|
||
header = string.split(header)
|
||
|
||
# size in pixels (width, height)
|
||
self._size = int(header[1]), int(header[2])
|
||
|
||
# mode setting
|
||
bits = int(header[3])
|
||
if bits == 1:
|
||
self.mode = "1"
|
||
elif bits == 8:
|
||
self.mode = "L"
|
||
elif bits == 24:
|
||
self.mode = "RGB"
|
||
else:
|
||
raise SyntaxError, "unknown number of bits"
|
||
|
||
# data descriptor
|
||
self.tile = [
|
||
("raw", (0, 0) + self.size, 128, (self.mode, 0, 1))
|
||
]
|
||
|
||
Image.register_open(SpamImageFile.format, SpamImageFile)
|
||
|
||
Image.register_extension(SpamImageFile.format, ".spam")
|
||
Image.register_extension(SpamImageFile.format, ".spa") # dos version
|
||
|
||
The format handler must always set the
|
||
:py:attr:`~PIL.Image.Image.size` and :py:attr:`~PIL.Image.Image.mode`
|
||
attributes. If these are not set, the file cannot be opened. To
|
||
simplify the plugin, the calling code considers exceptions like
|
||
:py:exc:`SyntaxError`, :py:exc:`KeyError`, :py:exc:`IndexError`,
|
||
:py:exc:`EOFError` and :py:exc:`struct.error` as a failure to identify
|
||
the file.
|
||
|
||
Note that the image plugin must be explicitly registered using
|
||
:py:func:`PIL.Image.register_open`. Although not required, it is also a good
|
||
idea to register any extensions used by this format.
|
||
|
||
The :py:attr:`tile` attribute
|
||
-----------------------------
|
||
|
||
To be able to read the file as well as just identifying it, the :py:attr:`tile`
|
||
attribute must also be set. This attribute consists of a list of tile
|
||
descriptors, where each descriptor specifies how data should be loaded to a
|
||
given region in the image. In most cases, only a single descriptor is used,
|
||
covering the full image.
|
||
|
||
The tile descriptor is a 4-tuple with the following contents::
|
||
|
||
(decoder, region, offset, parameters)
|
||
|
||
The fields are used as follows:
|
||
|
||
**decoder**
|
||
Specifies which decoder to use. The ``raw`` decoder used here supports
|
||
uncompressed data, in a variety of pixel formats. For more information on
|
||
this decoder, see the description below.
|
||
|
||
**region**
|
||
A 4-tuple specifying where to store data in the image.
|
||
|
||
**offset**
|
||
Byte offset from the beginning of the file to image data.
|
||
|
||
**parameters**
|
||
Parameters to the decoder. The contents of this field depends on the
|
||
decoder specified by the first field in the tile descriptor tuple. If the
|
||
decoder doesn’t need any parameters, use None for this field.
|
||
|
||
Note that the :py:attr:`tile` attribute contains a list of tile descriptors,
|
||
not just a single descriptor.
|
||
|
||
Decoders
|
||
========
|
||
|
||
The raw decoder
|
||
---------------
|
||
|
||
The ``raw`` decoder is used to read uncompressed data from an image file. It
|
||
can be used with most uncompressed file formats, such as PPM, BMP, uncompressed
|
||
TIFF, and many others. To use the raw decoder with the
|
||
:py:func:`PIL.Image.frombytes` function, use the following syntax::
|
||
|
||
image = Image.frombytes(
|
||
mode, size, data, "raw",
|
||
raw mode, stride, orientation
|
||
)
|
||
|
||
When used in a tile descriptor, the parameter field should look like::
|
||
|
||
(raw mode, stride, orientation)
|
||
|
||
The fields are used as follows:
|
||
|
||
**raw mode**
|
||
The pixel layout used in the file, and is used to properly convert data to
|
||
PIL’s internal layout. For a summary of the available formats, see the
|
||
table below.
|
||
|
||
**stride**
|
||
The distance in bytes between two consecutive lines in the image. If 0, the
|
||
image is assumed to be packed (no padding between lines). If omitted, the
|
||
stride defaults to 0.
|
||
|
||
**orientation**
|
||
Whether the first line in the image is the top line on the screen (1), or
|
||
the bottom line (-1). If omitted, the orientation defaults to 1.
|
||
|
||
The **raw mode** field is used to determine how the data should be unpacked to
|
||
match PIL’s internal pixel layout. PIL supports a large set of raw modes; for a
|
||
complete list, see the table in the :py:mod:`Unpack.c` module. The following
|
||
table describes some commonly used **raw modes**:
|
||
|
||
+-----------+-----------------------------------------------------------------+
|
||
| mode | description |
|
||
+===========+=================================================================+
|
||
| ``1`` | 1-bit bilevel, stored with the leftmost pixel in the most |
|
||
| | significant bit. 0 means black, 1 means white. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``1;I`` | 1-bit inverted bilevel, stored with the leftmost pixel in the |
|
||
| | most significant bit. 0 means white, 1 means black. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``1;R`` | 1-bit reversed bilevel, stored with the leftmost pixel in the |
|
||
| | least significant bit. 0 means black, 1 means white. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``L`` | 8-bit greyscale. 0 means black, 255 means white. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``L;I`` | 8-bit inverted greyscale. 0 means white, 255 means black. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``P`` | 8-bit palette-mapped image. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``RGB`` | 24-bit true colour, stored as (red, green, blue). |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``BGR`` | 24-bit true colour, stored as (blue, green, red). |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``RGBX`` | 24-bit true colour, stored as (red, green, blue, pad). The pad |
|
||
| | pixels may vary. |
|
||
+-----------+-----------------------------------------------------------------+
|
||
| ``RGB;L`` | 24-bit true colour, line interleaved (first all red pixels, then|
|
||
| | all green pixels, finally all blue pixels). |
|
||
+-----------+-----------------------------------------------------------------+
|
||
|
||
Note that for the most common cases, the raw mode is simply the same as the mode.
|
||
|
||
The Python Imaging Library supports many other decoders, including JPEG, PNG,
|
||
and PackBits. For details, see the :file:`decode.c` source file, and the
|
||
standard plug-in implementations provided with the library.
|
||
|
||
Decoding floating point data
|
||
----------------------------
|
||
|
||
PIL provides some special mechanisms to allow you to load a wide variety of
|
||
formats into a mode ``F`` (floating point) image memory.
|
||
|
||
You can use the ``raw`` decoder to read images where data is packed in any
|
||
standard machine data type, using one of the following raw modes:
|
||
|
||
============ =======================================
|
||
mode description
|
||
============ =======================================
|
||
``F`` 32-bit native floating point.
|
||
``F;8`` 8-bit unsigned integer.
|
||
``F;8S`` 8-bit signed integer.
|
||
``F;16`` 16-bit little endian unsigned integer.
|
||
``F;16S`` 16-bit little endian signed integer.
|
||
``F;16B`` 16-bit big endian unsigned integer.
|
||
``F;16BS`` 16-bit big endian signed integer.
|
||
``F;16N`` 16-bit native unsigned integer.
|
||
``F;16NS`` 16-bit native signed integer.
|
||
``F;32`` 32-bit little endian unsigned integer.
|
||
``F;32S`` 32-bit little endian signed integer.
|
||
``F;32B`` 32-bit big endian unsigned integer.
|
||
``F;32BS`` 32-bit big endian signed integer.
|
||
``F;32N`` 32-bit native unsigned integer.
|
||
``F;32NS`` 32-bit native signed integer.
|
||
``F;32F`` 32-bit little endian floating point.
|
||
``F;32BF`` 32-bit big endian floating point.
|
||
``F;32NF`` 32-bit native floating point.
|
||
``F;64F`` 64-bit little endian floating point.
|
||
``F;64BF`` 64-bit big endian floating point.
|
||
``F;64NF`` 64-bit native floating point.
|
||
============ =======================================
|
||
|
||
The bit decoder
|
||
---------------
|
||
|
||
If the raw decoder cannot handle your format, PIL also provides a special “bit”
|
||
decoder that can be used to read various packed formats into a floating point
|
||
image memory.
|
||
|
||
To use the bit decoder with the :py:func:`PIL.Image.frombytes` function, use
|
||
the following syntax::
|
||
|
||
image = Image.frombytes(
|
||
mode, size, data, "bit",
|
||
bits, pad, fill, sign, orientation
|
||
)
|
||
|
||
When used in a tile descriptor, the parameter field should look like::
|
||
|
||
(bits, pad, fill, sign, orientation)
|
||
|
||
The fields are used as follows:
|
||
|
||
**bits**
|
||
Number of bits per pixel (2-32). No default.
|
||
|
||
**pad**
|
||
Padding between lines, in bits. This is either 0 if there is no padding, or
|
||
8 if lines are padded to full bytes. If omitted, the pad value defaults to
|
||
8.
|
||
|
||
**fill**
|
||
Controls how data are added to, and stored from, the decoder bit buffer.
|
||
|
||
**fill=0**
|
||
Add bytes to the LSB end of the decoder buffer; store pixels from the MSB
|
||
end.
|
||
|
||
**fill=1**
|
||
Add bytes to the MSB end of the decoder buffer; store pixels from the MSB
|
||
end.
|
||
|
||
**fill=2**
|
||
Add bytes to the LSB end of the decoder buffer; store pixels from the LSB
|
||
end.
|
||
|
||
**fill=3**
|
||
Add bytes to the MSB end of the decoder buffer; store pixels from the LSB
|
||
end.
|
||
|
||
If omitted, the fill order defaults to 0.
|
||
|
||
**sign**
|
||
If non-zero, bit fields are sign extended. If zero or omitted, bit fields
|
||
are unsigned.
|
||
|
||
**orientation**
|
||
Whether the first line in the image is the top line on the screen (1), or
|
||
the bottom line (-1). If omitted, the orientation defaults to 1.
|
||
|
||
.. _file-decoders:
|
||
|
||
Writing Your Own File Decoder in C
|
||
==================================
|
||
|
||
There are 3 stages in a file decoder's lifetime:
|
||
|
||
1. Setup: Pillow looks for a function in the decoder registry, falling
|
||
back to a function named ``[decodername]_decoder`` on the internal
|
||
core image object. That function is called with the ``args`` tuple
|
||
from the ``tile`` setup in the ``_open`` method.
|
||
|
||
2. Decoding: The decoder's decode function is repeatedly called with
|
||
chunks of image data.
|
||
|
||
3. Cleanup: If the decoder has registered a cleanup function, it will
|
||
be called at the end of the decoding process, even if there was an
|
||
exception raised.
|
||
|
||
|
||
Setup
|
||
-----
|
||
|
||
The current conventions are that the decoder setup function is named
|
||
``PyImaging_[Decodername]DecoderNew`` and defined in ``decode.c``. The
|
||
python binding for it is named ``[decodername]_decoder`` and is setup
|
||
from within the ``_imaging.c`` file in the codecs section of the
|
||
function array.
|
||
|
||
The setup function needs to call ``PyImaging_DecoderNew`` and at the
|
||
very least, set the ``decode`` function pointer. The fields of
|
||
interest in this object are:
|
||
|
||
**decode**
|
||
Function pointer to the decode function, which has access to
|
||
``im``, ``state``, and the buffer of data to be added to the image.
|
||
|
||
**cleanup**
|
||
Function pointer to the cleanup function, has access to ``state``.
|
||
|
||
**im**
|
||
The target image, will be set by Pillow.
|
||
|
||
**state**
|
||
An ImagingCodecStateInstance, will be set by Pillow. The **context**
|
||
member is an opaque struct that can be used by the decoder to store
|
||
any format specific state or options.
|
||
|
||
**pulls_fd**
|
||
**EXPERIMENTAL** -- **WARNING**, interface may change. If set to 1,
|
||
``state->fd`` will be a pointer to the Python file like object. The
|
||
decoder may use the functions in ``codec_fd.c`` to read directly
|
||
from the file like object rather than have the data pushed through a
|
||
buffer. Note that this implementation may be refactored until this
|
||
warning is removed.
|
||
|
||
.. versionadded:: 3.3.0
|
||
|
||
|
||
Decoding
|
||
--------
|
||
|
||
The decode function is called with the target (core) image, the
|
||
decoder state structure, and a buffer of data to be decoded.
|
||
|
||
**Experimental** -- If ``pulls_fd`` is set, then the decode function
|
||
is called once, with an empty buffer. It is the decoder's
|
||
responsibility to decode the entire tile in that one call. The rest of
|
||
this section only applies if ``pulls_fd`` is not set.
|
||
|
||
It is the decoder's responsibility to pull as much data as possible
|
||
out of the buffer and return the number of bytes consumed. The next
|
||
call to the decoder will include the previous unconsumed tail. The
|
||
decoder function will be called multiple times as the data is read
|
||
from the file like object.
|
||
|
||
If an error occurs, set ``state->errcode`` and return -1.
|
||
|
||
Return -1 on success, without setting the errcode.
|
||
|
||
Cleanup
|
||
-------
|
||
|
||
The cleanup function is called after the decoder returns a negative
|
||
value, or if there is a read error from the file. This function should
|
||
free any allocated memory and release any resources from external
|
||
libraries.
|
||
|
||
.. _file-decoders-py:
|
||
|
||
Writing Your Own File Decoder in Python
|
||
=======================================
|
||
|
||
Python file decoders should derive from
|
||
:py:class:`PIL.ImageFile.PyDecoder` and should at least override the
|
||
decode method. File decoders should be registered using
|
||
:py:meth:`PIL.Image.register_decoder`. As in the C implementation of
|
||
the file decoders, there are three stages in the lifetime of a
|
||
Python-based file decoder:
|
||
|
||
1. Setup: Pillow looks for the decoder in the registry, then
|
||
instantiates the class.
|
||
|
||
2. Decoding: The decoder instance's ``decode`` method is repeatedly
|
||
called with a buffer of data to be interpreted.
|
||
|
||
3. Cleanup: The decoder instance's ``cleanup`` method is called.
|
||
|
||
|