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# Writing your own image plugin

Pillow uses a plugin model which allows you to add your own decoders and encoders to the library, without any changes to the library itself. Such plugins 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 two 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 plugin should contain a format handler derived from the `PIL.ImageFile.ImageFile` base class. This class should provide an `_open` method, which reads the file header and set at least the internal `_size` and `_mode` attributes so that `mode` and `size` are populated. To be able to load the file, the method must also create a list of `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 `Image` module.

Note

For performance reasons, it is important that the `_open` method quickly rejects files that do not have the appropriate contents.

## Example

The following plugin 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, grayscale, or 24-bit true color.

SpamImagePlugin.py:

    from PIL import Image, ImageFile

    def _accept(prefix: bytes) -> bool:
        return prefix.startswith(b"SPAM")

    class SpamImageFile(ImageFile.ImageFile):

        format = "SPAM"
        format_description = "Spam raster image"

        def _open(self) -> None:

            header = self.fp.read(128).split()

            # 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:
                msg = "unknown number of bits"
                raise SyntaxError(msg)

            # data descriptor
            self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 128, (self.mode, 0, 1))]

    Image.register_open(SpamImageFile.format, SpamImageFile, _accept)

    Image.register_extensions(
        SpamImageFile.format,
        [
            ".spam",
            ".spa",  # DOS version
        ],
    )

The format handler must always set the internal `_size` and `_mode` attributes so that `size` and `mode` are populated. If these are not set, the file cannot be opened. To simplify the plugin, the calling code considers exceptions like `SyntaxError`, `KeyError`, `IndexError`, `EOFError` and `struct.error` as a failure to identify the file.

Note that the image plugin must be explicitly registered using `PIL.Image.register_open()`. Although not required, it is also a good idea to register any extensions used by this format.

Once the plugin has been imported, it can be used:

    from PIL import Image
    import SpamImagePlugin

    with Image.open("hopper.spam") as im:
        pass

## The `tile` attribute

To be able to read the file as well as just identifying it, the `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. `PsdImagePlugin.PsdImageFile` uses multiple tiles to combine channels within a single layer, given that the channels are stored separately, one after the other.

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.

  A list of C decoders can be seen under codecs section of the function array in `_imaging.c`. Python decoders are registered within the relevant plugins.

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 `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 `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 `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 grayscale. 0 means black, 255 means white.
`L;I` | 8-bit inverted grayscale. 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 `decode.c` source file, and the standard plugin 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 `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.

# Writing your own file codec in C

There are 3 stages in a file codec’s lifetime:

1. Setup: Pillow looks for a function in the decoder or encoder registry, falling back to a function named `[codecname]_decoder` or `[codecname]_encoder` on the internal core image object. That function is called with the `args` tuple from the `tile`.

2. Transforming: The codec’s `decode` or `encode` function is repeatedly called with chunks of image data.

3. Cleanup: If the codec has registered a cleanup function, it will be called at the end of the transformation process, even if there was an exception raised.

## Setup

The current conventions are that the codec setup function is named `PyImaging_[codecname]DecoderNew` or `PyImaging_[codecname]EncoderNew` and defined in `decode.c` or `encode.c`. The Python binding for it is named `[codecname]_decoder` or `[codecname]_encoder` and is set up from within the `_imaging.c` file in the codecs section of the function array.

The setup function needs to call `PyImaging_DecoderNew` or `PyImaging_EncoderNew` and at the very least, set the `decode` or `encode` function pointer. The fields of interest in this object are:

decode/encode

  Function pointer to the decode or encode function, which has access to `im`, `state`, and the buffer of data to be transformed.

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 codec to store any format specific state or options.

pulls_fd/pushes_fd

  If the decoder has `pulls_fd` or the encoder has `pushes_fd` set to 1, `state->fd` will be a pointer to the Python file like object. The codec may use the functions in `codec_fd.c` to read or write directly with the file like object rather than have the data pushed through a buffer.

  Added in version 3.3.0.

## Transforming

The decode or encode function is called with the target (core) image, the codec state structure, and a buffer of data to be transformed.

It is the codec’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 codec will include the previous unconsumed tail. The codec function will be called multiple times as the data processed.

Alternatively, if `pulls_fd` or `pushes_fd` is set, then the decode or encode function is called once, with an empty buffer. It is the codec’s responsibility to transform the entire tile in that one call. Using this will provide a codec with more freedom, but that freedom may mean increased memory usage if the entire tile is held in memory at once by the codec.

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 codec returns a negative value, or if there is an error. This function should free any allocated memory and release any resources from external libraries.

# Writing your own file codec in Python

Python file decoders and encoders should derive from `PIL.ImageFile.PyDecoder` and `PIL.ImageFile.PyEncoder` respectively, and should at least override the decode or encode method. They should be registered using `PIL.Image.register_decoder()` and `PIL.Image.register_encoder()`. As in the C implementation of the file codecs, there are three stages in the lifetime of a Python-based file codec:

1. Setup: Pillow looks for the codec in the decoder or encoder registry, then instantiates the class.

2. Transforming: The instance’s `decode` method is repeatedly called with a buffer of data to be interpreted, or the `encode` method is repeatedly called with the size of data to be output.

   Alternatively, if the decoder’s `_pulls_fd` property (or the encoder’s `_pushes_fd` property) is set to `True`, then `decode` and `encode` will only be called once. In the decoder, `self.fd` can be used to access the file-like object. Using this will provide a codec with more freedom, but that freedom may mean increased memory usage if entire file is held in memory at once by the codec.

   In `decode`, once the data has been interpreted, `set_as_raw` can be used to populate the image.

3. Cleanup: The instance’s `cleanup` method is called once the transformation is complete. This can be used to clean up any resources used by the codec.

   If you set `_pulls_fd` or `_pushes_fd` to `True` however, then you probably chose to perform any cleanup tasks at the end of `decode` or `encode`.

For an example `PIL.ImageFile.PyDecoder`, see DdsImagePlugin. For a plugin that uses both `PIL.ImageFile.PyDecoder` and `PIL.ImageFile.PyEncoder`, see BlpImagePlugin