python - 获取图像大小而不将图像加载到内存中

Question

我了解您可以通过以下方式使用 PIL 获取图像大小

from PIL import Image
im = Image.open(image_filename)
width, height = im.size

但是，我想获得图像的宽度和高度，而不必将图像加载到内存中。那可能吗？我只对图像大小进行统计，并不关心图像内容。我只是想让我的处理速度更快。

Answer 1

如果你不关心图像内容，PIL 可能是矫枉过正。

我建议解析python魔术模块的输出:

>>> t = magic.from_file('teste.png')
>>> t
'PNG image data, 782 x 602, 8-bit/color RGBA, non-interlaced'
>>> re.search('(\d+) x (\d+)', t).groups()
('782', '602')

这是一个 libmagic 的包装器，它读取尽可能少的字节以识别文件类型签名。

脚本的相关版本:

https://raw.githubusercontent.com/scardine/image_size/master/get_image_size.py

[更新]

Hmmm, unfortunately, when applied to jpegs, the above gives "'JPEG image data, EXIF standard 2.21'". No image size! – Alex Flint

似乎 jpeg 具有抗魔力。 :-)

我明白为什么:为了获得 JPEG 文件的图像尺寸，您可能需要读取比 libmagic 喜欢读取的更多的字节数。

我卷起袖子来了this very untested snippet (get it from GitHub)不需要第三方模块。

#-------------------------------------------------------------------------------
# Name:        get_image_size
# Purpose:     extract image dimensions given a file path using just
#              core modules
#
# Author:      Paulo Scardine (based on code from Emmanuel VAÏSSE)
#
# Created:     26/09/2013
# Copyright:   (c) Paulo Scardine 2013
# Licence:     MIT
#-------------------------------------------------------------------------------
#!/usr/bin/env python
import os
import struct

class UnknownImageFormat(Exception):
    pass

def get_image_size(file_path):
    """
    Return (width, height) for a given img file content - no external
    dependencies except the os and struct modules from core
    """
    size = os.path.getsize(file_path)

    with open(file_path) as input:
        height = -1
        width = -1
        data = input.read(25)

        if (size >= 10) and data[:6] in ('GIF87a', 'GIF89a'):
            # GIFs
            w, h = struct.unpack("<HH", data[6:10])
            width = int(w)
            height = int(h)
        elif ((size >= 24) and data.startswith('\211PNG\r\n\032\n')
              and (data[12:16] == 'IHDR')):
            # PNGs
            w, h = struct.unpack(">LL", data[16:24])
            width = int(w)
            height = int(h)
        elif (size >= 16) and data.startswith('\211PNG\r\n\032\n'):
            # older PNGs?
            w, h = struct.unpack(">LL", data[8:16])
            width = int(w)
            height = int(h)
        elif (size >= 2) and data.startswith('\377\330'):
            # JPEG
            msg = " raised while trying to decode as JPEG."
            input.seek(0)
            input.read(2)
            b = input.read(1)
            try:
                while (b and ord(b) != 0xDA):
                    while (ord(b) != 0xFF): b = input.read(1)
                    while (ord(b) == 0xFF): b = input.read(1)
                    if (ord(b) >= 0xC0 and ord(b) <= 0xC3):
                        input.read(3)
                        h, w = struct.unpack(">HH", input.read(4))
                        break
                    else:
                        input.read(int(struct.unpack(">H", input.read(2))[0])-2)
                    b = input.read(1)
                width = int(w)
                height = int(h)
            except struct.error:
                raise UnknownImageFormat("StructError" + msg)
            except ValueError:
                raise UnknownImageFormat("ValueError" + msg)
            except Exception as e:
                raise UnknownImageFormat(e.__class__.__name__ + msg)
        else:
            raise UnknownImageFormat(
                "Sorry, don't know how to get information from this file."
            )

    return width, height

[2019 年更新]

查看一个 Rust 实现:https://github.com/scardine/imsz