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 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 ImagePlugin.py
. You will need to import your image plugin manually.
Pillow decodes files in 2 stages:
- 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
acceptfunction returns true, the plugin's
_openmethod is called to set up the image metadata and image tiles. The
_open`` method is not for decoding the actual image data. - 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.
A decoder plug-in should contain a decoder class, based on the :pyPIL.ImageFile.ImageFile
base class. This class should provide an :py_open
method, which reads the file header and sets up at least the :py~PIL.Image.Image.mode
and :py~PIL.Image.Image.size
attributes. To be able to load the file, the method must also create a list of :pytile
descriptors. The class must be explicitly registered, via a call to the :py~PIL.Image
module.
For performance reasons, it is important that the :py_open
method quickly rejects files that do not have the appropriate contents.
The raw
decoder is useful for uncompressed image formats, but many formats require more control of the decoding context, either with a decoder written in C
or by linking in an external library to do the decoding. (Examples of this include PNG, Tiff, and Jpeg support)
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~PIL.Image.Image.size
and :py~PIL.Image.Image.mode
attributes. If these are not set, the file cannot be opened. To simplify the decoder, the calling code considers exceptions like :pySyntaxError
, :pyKeyError
, :pyIndexError
, :pyEOFError
and :pystruct.error
as a failure to identify the file.
Note that the decoder must be explicitly registered using :pyPIL.Image.register_open
. Although not required, it is also a good idea to register any extensions used by this format.
To be able to read the file as well as just identifying it, the :pytile
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 :pytile
attribute contains a list of tile descriptors, not just a single descriptor.
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 :pyPIL.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 :pyUnpack.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 (blue, green, red, pad). |
RGB;L |
24-bit true colour, line interleaved (first all red pixels, the 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 decode.c
source file, and the standard plug-in implementations provided with the library.
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. |
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 frombytes function, use the following syntax:
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.
There are 3 stages in a file decoder's lifetime:
- Setup: Pillow looks for a function named
[decodername]_decoder
on the internal core image object. That function is called with theargs
tuple from thetile
setup in the_open
method. - Decoding: The decoder's decode function is repeatedly called with chunks of image data.
- 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.
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.
- handles_eof
UNDONE, set if your code handles EOF errors.
- 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 incodec_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.3.3.0
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.
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.