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D. Writing Your Own File Decoder

The Python Imaging Library 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 have names like XxxImagePlugin.py, where Xxx is a unique format name (usually an abbreviation).

A decoder plug-in should contain a decoder class, based on the ImageFile base class defined in the module with the same name. This class should provide an _open method, which reads the file header and sets up at least the mode and size attributes. To be able to load the file, the method must also create a list of tile descriptors. The class must be explicitly registered, via a call to the Image module.

For performance reasons, it is important that the _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 colour.

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("SPAM", SpamImageFile)
        
Image.register_extension("SPAM", ".spam")
Image.register_extension("SPAM", ".spa") # dos version

The format handler must always set the size and mode attributes. If these are not set, the file cannot be opened. To simplify the decoder, the calling code considers exceptions like SyntaxError, KeyError, and IndexError, as a failure to identify the file.

Note that the decoder must be explicitly registered using the register_open function in the Image module. Although not required, it is also a good idea to register any extensions used by this format.

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.

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 tile attribute contains a list of tile descriptors, not just a single descriptor.

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 Image.fromstring function, use the following syntax:

    image = Image.fromstring(
        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:

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.

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:

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 fromstring function, use the following syntax:

    image = fromstring(
        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.