教程¶

使用Image类¶

The most important class in the Python Imaging Library is the Image class, defined in the module with the same name. You can create instances of this class in several ways; either by loading images from files, processing other images, or creating images from scratch.

从文件加载图像，可以使用:吡啶:FUNC:在〜PIL.Image.open`功能:PY:MOD:〜PIL.Image`模块:

>>> from PIL import Image
>>> im = Image.open("hopper.ppm")

If successful, this function returns an Image object. You can now use instance attributes to examine the file contents:

>>> print(im.format, im.size, im.mode)
PPM (512, 512) RGB

The format attribute identifies the source of an image. If the image was not read from a file, it is set to None. The size attribute is a 2-tuple containing width and height (in pixels). The mode attribute defines the number and names of the bands in the image, and also the pixel type and depth. Common modes are “L” (luminance) for greyscale images, “RGB” for true color images, and “CMYK” for pre-press images.

如果该文件无法打开，一:PY:EXC:`IOError`异常.

Once you have an instance of the Image class, you can use the methods defined by this class to process and manipulate the image. For example, let’s display the image we just loaded:

>>> im.show()

注解

The standard version of show() is not very efficient, since it saves the image to a temporary file and calls a utility to display the image. If you don’t have an appropriate utility installed, it won’t even work. When it does work though, it is very handy for debugging and tests.

以下各节提供的这个库中提供的不同功能的概述.

读取和写入图像¶

The Python Imaging Library supports a wide variety of image file formats. To read files from disk, use the open() function in the Image module. You don’t have to know the file format to open a file. The library automatically determines the format based on the contents of the file.

To save a file, use the save() method of the Image class. When saving files, the name becomes important. Unless you specify the format, the library uses the filename extension to discover which file storage format to use.

将文件转换为JPEG¶

import os, sys
from PIL import Image

for infile in sys.argv[1:]:
    f, e = os.path.splitext(infile)
    outfile = f + ".jpg"
    if infile != outfile:
        try:
            Image.open(infile).save(outfile)
        except IOError:
            print("cannot convert", infile)

A second argument can be supplied to the save() method which explicitly specifies a file format. If you use a non-standard extension, you must always specify the format this way:

创建JPEG缩略图¶

import os, sys
from PIL import Image

size = (128, 128)

for infile in sys.argv[1:]:
    outfile = os.path.splitext(infile)[0] + ".thumbnail"
    if infile != outfile:
        try:
            with Image.open(infile) as im:
                im.thumbnail(size)
                im.save(outfile, "JPEG")
        except IOError:
            print("cannot create thumbnail for", infile)

It is important to note that the library doesn’t decode or load the raster data unless it really has to. When you open a file, the file header is read to determine the file format and extract things like mode, size, and other properties required to decode the file, but the rest of the file is not processed until later.

This means that opening an image file is a fast operation, which is independent of the file size and compression type. Here’s a simple script to quickly identify a set of image files:

识别图像文件¶

import sys
from PIL import Image

for infile in sys.argv[1:]:
    try:
        with Image.open(infile) as im:
            print(infile, im.format, "%dx%d" % im.size, im.mode)
    except IOError:
        pass

剪切，粘贴，和合并图像¶

The Image class contains methods allowing you to manipulate regions within an image. To extract a sub-rectangle from an image, use the crop() method.

从图像复制子矩形¶

box = (100, 100, 400, 400)
region = im.crop(box)

The region is defined by a 4-tuple, where coordinates are (left, upper, right, lower). The Python Imaging Library uses a coordinate system with (0, 0) in the upper left corner. Also note that coordinates refer to positions between the pixels, so the region in the above example is exactly 300x300 pixels.

该区域现在可以以一定的方式进行处理和粘贴回.

处理子矩形，并将其粘贴回¶

region = region.transpose(Image.ROTATE_180)
im.paste(region, box)

When pasting regions back, the size of the region must match the given region exactly. In addition, the region cannot extend outside the image. However, the modes of the original image and the region do not need to match. If they don’t, the region is automatically converted before being pasted (see the section on 颜色变换 below for details).

这里有一个额外的例子:

滚动图像¶

def roll(image, delta):
    """Roll an image sideways."""
    xsize, ysize = image.size

    delta = delta % xsize
    if delta == 0: return image

    part1 = image.crop((0, 0, delta, ysize))
    part2 = image.crop((delta, 0, xsize, ysize))
    image.paste(part1, (xsize-delta, 0, xsize, ysize))
    image.paste(part2, (0, 0, xsize-delta, ysize))

    return image

For more advanced tricks, the paste method can also take a transparency mask as an optional argument. In this mask, the value 255 indicates that the pasted image is opaque in that position (that is, the pasted image should be used as is). The value 0 means that the pasted image is completely transparent. Values in-between indicate different levels of transparency. For example, pasting an RGBA image and also using it as the mask would paste the opaque portion of the image but not its transparent background.

The Python Imaging Library also allows you to work with the individual bands of an multi-band image, such as an RGB image. The split method creates a set of new images, each containing one band from the original multi-band image. The merge function takes a mode and a tuple of images, and combines them into a new image. The following sample swaps the three bands of an RGB image:

拆分和合并波段¶

r, g, b = im.split()
im = Image.merge("RGB", (b, g, r))

Note that for a single-band image, split() returns the image itself. To work with individual color bands, you may want to convert the image to “RGB” first.

几何变换¶

The PIL.Image.Image class contains methods to resize() and rotate() an image. The former takes a tuple giving the new size, the latter the angle in degrees counter-clockwise.

简单的几何变换¶

out = im.resize((128, 128))
out = im.rotate(45) # degrees counter-clockwise

To rotate the image in 90 degree steps, you can either use the rotate() method or the transpose() method. The latter can also be used to flip an image around its horizontal or vertical axis.

移调图像¶

out = im.transpose(Image.FLIP_LEFT_RIGHT)
out = im.transpose(Image.FLIP_TOP_BOTTOM)
out = im.transpose(Image.ROTATE_90)
out = im.transpose(Image.ROTATE_180)
out = im.transpose(Image.ROTATE_270)

transpose(ROTATE) operations can also be performed identically with rotate() operations, provided the expand flag is true, to provide for the same changes to the image’s size.

PY:图像变换的更一般的形式可通过进行甲基:`〜PIL.Image.Image.transform`方法.

颜色变换¶

The Python Imaging Library allows you to convert images between different pixel representations using the convert() method.

模式之间转换¶

from PIL import Image
with Image.open("hopper.ppm") as im:
    im = im.convert("L")

The library supports transformations between each supported mode and the “L” and “RGB” modes. To convert between other modes, you may have to use an intermediate image (typically an “RGB” image).

图像增强¶

Python图像库提供了许多可用于增强图像的方法和模块.

Filters¶

The ImageFilter module contains a number of pre-defined enhancement filters that can be used with the filter() method.

应用滤镜¶

from PIL import ImageFilter
out = im.filter(ImageFilter.DETAIL)

Point Operations¶

The point() method can be used to translate the pixel values of an image (e.g. image contrast manipulation). In most cases, a function object expecting one argument can be passed to this method. Each pixel is processed according to that function:

运用点变换¶

# multiply each pixel by 1.2
out = im.point(lambda i: i * 1.2)

Using the above technique, you can quickly apply any simple expression to an image. You can also combine the point() and paste() methods to selectively modify an image:

处理单个带¶

# split the image into individual bands
source = im.split()

R, G, B = 0, 1, 2

# select regions where red is less than 100
mask = source[R].point(lambda i: i < 100 and 255)

# process the green band
out = source[G].point(lambda i: i * 0.7)

# paste the processed band back, but only where red was < 100
source[G].paste(out, None, mask)

# build a new multiband image
im = Image.merge(im.mode, source)

注意:用于创建蒙语法:

imout = im.point(lambda i: expression and 255)

Python only evaluates the portion of a logical expression as is necessary to determine the outcome, and returns the last value examined as the result of the expression. So if the expression above is false (0), Python does not look at the second operand, and thus returns 0. Otherwise, it returns 255.

增强¶

For more advanced image enhancement, you can use the classes in the ImageEnhance module. Once created from an image, an enhancement object can be used to quickly try out different settings.

您可以调整这样的对比度，亮度，色彩平衡和清晰度.

增强图像¶

from PIL import ImageEnhance

enh = ImageEnhance.Contrast(im)
enh.enhance(1.3).show("30% more contrast")

图像序列¶

The Python Imaging Library contains some basic support for image sequences (also called animation formats). Supported sequence formats include FLI/FLC, GIF, and a few experimental formats. TIFF files can also contain more than one frame.

When you open a sequence file, PIL automatically loads the first frame in the sequence. You can use the seek and tell methods to move between different frames:

读出序列¶

from PIL import Image

with Image.open("animation.gif") as im:
    im.seek(1) # skip to the second frame

    try:
        while 1:
            im.seek(im.tell()+1)
            # do something to im
    except EOFError:
        pass # end of sequence

如本例中看到的那样，你会得到一个:PY:EXC:`EOFError`例外序时结束.

下面的类可以让你在序列使用for语句循环:

使用ImageSequence迭代器类¶

from PIL import ImageSequence
for frame in ImageSequence.Iterator(im):
    # ...do something to frame...

Postscript打印¶

Python图像库包括功能在PostScript打印机上打印图像，文字和图形. 这里有一个简单的例子:

绘图后记¶

from PIL import Image
from PIL import PSDraw

with Image.open("hopper.ppm") as im:
    title = "hopper"
    box = (1*72, 2*72, 7*72, 10*72) # in points

    ps = PSDraw.PSDraw() # default is sys.stdout
    ps.begin_document(title)

    # draw the image (75 dpi)
    ps.image(box, im, 75)
    ps.rectangle(box)

    # draw title
    ps.setfont("HelveticaNarrow-Bold", 36)
    ps.text((3*72, 4*72), title)

    ps.end_document()

控制解码器¶

Some decoders allow you to manipulate the image while reading it from a file. This can often be used to speed up decoding when creating thumbnails (when speed is usually more important than quality) and printing to a monochrome laser printer (when only a greyscale version of the image is needed).

The draft() method manipulates an opened but not yet loaded image so it as closely as possible matches the given mode and size. This is done by reconfiguring the image decoder.

阅读草稿模式¶

这仅适用于JPEG和MPO文件.

from PIL import Image

with Image.open(file) as im:
    print("original =", im.mode, im.size)

    im.draft("L", (100, 100))
    print("draft =", im.mode, im.size)

这将打印像:

original = RGB (512, 512)
draft = L (128, 128)

Note that the resulting image may not exactly match the requested mode and size. To make sure that the image is not larger than the given size, use the thumbnail method instead.

教程¶