如何在Python中将PDF转换为灰度

Question

是否可以使用Python库将PDF文件转换为其灰度等效文件？我尝试了ghostscript模块：

import locale
from io import BytesIO
import ghostscript as gs

ENCO = locale.getpreferredencoding()
STDOUT = BytesIO()
STDERR = BytesIO()

with open('adob_in.pdf', 'r') as infile:
    ARGS = f"""DUMMY -sOutputFile=adob_out.pdf -sDEVICE=pdfwrite
     -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray
     -dNOPAUSE -dBATCH {infile.name}"""

    ARGSB = [arg.encode(ENCO) for arg in ARGS.split()]

    gs.Ghostscript(*ARGSB, stdout=STDOUT, stderr=STDERR)

print(STDOUT.getvalue().decode(ENCO))
print(STDERR.getvalue().decode(ENCO))

标准输出和错误流为：

GPL Ghostscript 9.52 (2020-03-19)
Copyright (C) 2020 Artifex Software, Inc.  All rights reserved.
This software is supplied under the GNU AGPLv3 and comes with NO WARRANTY:
see the file COPYING for details.
Processing pages 1 through 1.
Page 1

不幸的是，灰度PDF已损坏。实际上，使用Ghostscript进行调试会显示以下错误：

GPL Ghostscript 9.52 (2020-03-19)
Copyright (C) 2020 Artifex Software, Inc.  All rights reserved.
This software is supplied under the GNU AGPLv3 and comes with NO WARRANTY:
see the file COPYING for details.
   **** Error: Cannot find a 'startxref' anywhere in the file.
               Output may be incorrect.
   **** Error:  An error occurred while reading an XREF table.
   **** The file has been damaged.  This may have been caused
   **** by a problem while converting or transfering the file.
   **** Ghostscript will attempt to recover the data.
   **** However, the output may be incorrect.
   **** Error:  Trailer dictionary not found.
                Output may be incorrect.
   No pages will be processed (FirstPage > LastPage).

   **** This file had errors that were repaired or ignored.
   **** Please notify the author of the software that produced this
   **** file that it does not conform to Adobe's published PDF
   **** specification.

   **** The rendered output from this file may be incorrect.
GS>

请注意，字符串ARGS包含有效的ghostscript代码（在Linux命令行中使用GPL Ghostscript 9.52进行测试），并且ARGSB只是字符串的对应二进制表示形式：

print(ARGSB)
[b'DUMMY', b'-sOutputFile=adob_out.pdf', b'-sDEVICE=pdfwrite', b'-sColorConversionStrategy=Gray', b'-dProcessColorModel=/DeviceGray', b'-dNOPAUSE', b'-dBATCH', b'adob_in.pdf']

这项任务如何才能妥善完成？我的示例输入和输出文件可以在here中找到。事先非常感谢

Answer 1

我不知道如何通过ghostscript实现，但是下面使用pdf2image和img2pdf的代码可以达到这个目的：

from os.path import join
from tempfile import TemporaryDirectory
from pdf2image import convert_from_path # https://pypi.org/project/pdf2image/
from img2pdf import convert # https://pypi.org/project/img2pdf/

with TemporaryDirectory() as temp_dir: # Saves images temporarily in disk rather than RAM to speed up parsing
    # Converting pages to images
    print("Parsing pages to grayscale images. This may take a while")
    images = convert_from_path(
        "your_pdf_path.pdf",
        output_folder=temp_dir,
        grayscale=True,
        fmt="jpeg",
        thread_count=4
    )

    image_list = list()
    for page_number in range(1, len(images) + 1):
        path = join(temp_dir, "page_" + str(page_number) + ".jpeg")
        image_list.append(path)
        images[page_number-1].save(path, "JPEG") # (page_number - 1) because index starts from 0

    with open("Gray_PDF.pdf", "bw") as gray_pdf:
        gray_pdf.write(convert(image_list))

    print("The new page is saved as Gray_PDF.pdf in the current directory.")

带有灰度图像的PDF文件将保存为同一目录中的Gray_PDF.PDF

说明： 以下代码：

with TemporaryDirectory() as temp_dir: # Saves images temporarily in disk rather than RAM. This speeds up parsing
    # Converting pages to images
    print("Parsing pages to grayscale images. This may take a while")
    images = convert_from_path(
        "your_pdf_path.pdf",
        output_folder=temp_dir,
        grayscale=True,
        fmt="jpeg",
        thread_count=4
    )

执行以下任务：

1. 将PDF页面转换为灰度图像
2. 将其临时存储在目录中
3. 创建PIL图像对象的列表images

现在输入以下代码：

    image_list = list()
    for page_number in range(1, len(images) + 1):
        path = join(temp_dir, "page_" + str(page_number) + ".jpeg")
        image_list.append(path)
        images[page_number-1].save(path, "JPEG") # (page_number - 1) because index starts from 0

在同一目录中将图像再次保存为page_1.jpeg、page_2.jpeg等。它还列出了这些新图像的路径

最后，输入以下代码：

    with open("Gray_PDF.pdf", "bw") as gray_pdf:
        gray_pdf.write(convert(image_list))

从先前创建的灰度图像创建名为Gray\u PDF的PDF，并将其保存在工作目录中

附加提示：如果您想使用OpenCV执行更多图像处理操作，则此方法为您提供了很大的灵活性，因为所有页面现在都是图像形式。只需确保所有操作都在第一个with语句中，即：

with TemporaryDirectory() as temp_dir: # Saves images temporarily in disk rather than RAM. This speeds up parsing