如何将彩色pdf转换为黑白？

我想将一个带有一些彩色文本和图像的pdf转换为另一个只有黑白的pdf，以减小其尺寸。此外，我想将文本保留为文本，而不变换图片中的页面元素。我尝试了以下命令：

convert -density 150 -threshold 50% input.pdf output.pdf

在另一个问题中找到了一个link，但它确实做了我不想要的：输出中的文本被转换为较差的图像，并且不再可以选择。我尝试了Ghostscript：

gs      -sOutputFile=output.pdf \
        -q -dNOPAUSE -dBATCH -dSAFER \
        -sDEVICE=pdfwrite \
        -dCompatibilityLevel=1.3 \
        -dPDFSETTINGS=/screen \
        -dEmbedAllFonts=true \
        -dSubsetFonts=true \
        -sColorConversionStrategy=/Mono \
        -sColorConversionStrategyForImages=/Mono \
        -sProcessColorModel=/DeviceGray \
        $1

但是它给了我以下错误信息：

./script.sh: 19: ./script.sh: output.pdf: not found

还有其他创建文件的方法吗？

gs示例

gs您在上面运行的命令带有尾部$1，通常用于将命令行参数传递到脚本中。因此，我不确定您实际尝试了什么，但是我猜测您尝试将该命令放入脚本中script.sh：

#!/bin/bash

gs      -sOutputFile=output.pdf \
        -q -dNOPAUSE -dBATCH -dSAFER \
        -sDEVICE=pdfwrite \
        -dCompatibilityLevel=1.3 \
        -dPDFSETTINGS=/screen \
        -dEmbedAllFonts=true \
        -dSubsetFonts=true \
        -sColorConversionStrategy=/Mono \
        -sColorConversionStrategyForImages=/Mono \
        -sProcessColorModel=/DeviceGray \
        $1

并像这样运行它：

$ ./script.sh: 19: ./script.sh: output.pdf: not found

不确定如何设置此脚本，但是它需要可执行。

$ chmod +x script.sh

但是，该脚本显然不正确。当我尝试它时，我得到了这个错误：

无法恢复的错误：.putdeviceprops中的rangecheck

替代

我会使用SU问题中的该脚本代替该脚本。

#!/bin/bash

gs \
 -sOutputFile=output.pdf \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dBATCH \
 $1

然后像这样运行它：

$ ./script.bash LeaseContract.pdf 
GPL Ghostscript 8.71 (2010-02-10)
Copyright (C) 2010 Artifex Software, Inc.  All rights reserved.
This software comes with NO WARRANTY: see the file PUBLIC for details.
Processing pages 1 through 2.
Page 1
Page 2

我在这里找到了可以执行此操作的脚本。它要求gs您似乎也要拥有pdftk。您没有提到您的发行版，但是在基于Debian的系统上，您应该能够使用

sudo apt-get install pdftk

您可以在此处找到RPM 。

安装后pdftk，将脚本另存为graypdf.sh并按如下方式运行：

./greypdf.sh input.pdf

它将创建一个名为的文件input-gray.pdf。我在此处包括整个脚本，以避免链接腐烂：

# convert pdf to grayscale, preserving metadata
# "AFAIK graphicx has no feature for manipulating colorspaces. " http://groups.google.com/group/latexusersgroup/browse_thread/thread/5ebbc3ff9978af05
# "> Is there an easy (or just standard) way with pdflatex to do a > conversion from color to grayscale when a PDF file is generated? No." ... "If you want to convert a multipage document then you better have pdftops from the xpdf suite installed because Ghostscript's pdf to ps doesn't produce nice Postscript." http://osdir.com/ml/tex.pdftex/2008-05/msg00006.html
# "Converting a color EPS to grayscale" - http://en.wikibooks.org/wiki/LaTeX/Importing_Graphics
# "\usepackage[monochrome]{color} .. I don't know of a neat automatic conversion to monochrome (there might be such a thing) although there was something in Tugboat a while back about mapping colors on the fly. I would probably make monochrome versions of the pictures, and name them consistently. Then conditionally load each one" http://newsgroups.derkeiler.com/Archive/Comp/comp.text.tex/2005-08/msg01864.html
# "Here comes optional.sty. By adding \usepackage{optional} ... \opt{color}{\includegraphics[width=0.4\textwidth]{intro/benzoCompounds_color}} \opt{grayscale}{\includegraphics[width=0.4\textwidth]{intro/benzoCompounds}} " - http://chem-bla-ics.blogspot.com/2008/01/my-phd-thesis-in-color-and-grayscale.html
# with gs:
# http://handyfloss.net/2008.09/making-a-pdf-grayscale-with-ghostscript/
# note - this strips metadata! so:
# http://etutorials.org/Linux+systems/pdf+hacks/Chapter+5.+Manipulating+PDF+Files/Hack+64+Get+and+Set+PDF+Metadata/
COLORFILENAME=$1
OVERWRITE=$2
FNAME=${COLORFILENAME%.pdf}
# NOTE: pdftk does not work with logical page numbers / pagination;
# gs kills it as well;
# so check for existence of 'pdfmarks' file in calling dir;
# if there, use it to correct gs logical pagination
# for example, see
# http://askubuntu.com/questions/32048/renumber-pages-of-a-pdf/65894#65894
PDFMARKS=
if [ -e pdfmarks ] ; then
PDFMARKS="pdfmarks"
echo "$PDFMARKS exists, using..."
# convert to gray pdf - this strips metadata!
gs -sOutputFile=$FNAME-gs-gray.pdf -sDEVICE=pdfwrite \
-sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH "$COLORFILENAME" "$PDFMARKS"
else # not really needed ?!
gs -sOutputFile=$FNAME-gs-gray.pdf -sDEVICE=pdfwrite \
-sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH "$COLORFILENAME"
fi
# dump metadata from original color pdf
## pdftk $COLORFILENAME dump_data output $FNAME.data.txt
# also: pdfinfo -meta $COLORFILENAME
# grep to avoid BookmarkTitle/Level/PageNumber:
pdftk $COLORFILENAME dump_data output | grep 'Info\|Pdf' > $FNAME.data.txt
# "pdftk can take a plain-text file of these same key/value pairs and update a PDF's Info dictionary to match. Currently, it does not update the PDF's XMP stream."
pdftk $FNAME-gs-gray.pdf update_info $FNAME.data.txt output $FNAME-gray.pdf
# (http://wiki.creativecommons.org/XMP_Implementations : Exempi ... allows reading/writing XMP metadata for various file formats, including PDF ... )
# clean up
rm $FNAME-gs-gray.pdf
rm $FNAME.data.txt
if [ "$OVERWRITE" == "y" ] ; then
echo "Overwriting $COLORFILENAME..."
mv $FNAME-gray.pdf $COLORFILENAME
fi
# BUT NOTE:
# Mixing TEX & PostScript : The GEX Model - http://www.tug.org/TUGboat/Articles/tb21-3/tb68kost.pdf
# VTEX is a (commercial) extended version of TEX, sold by MicroPress, Inc. Free versions of VTEX have recently been made available, that work under OS/2 and Linux. This paper describes GEX, a fast fully-integrated PostScript interpreter which functions as part of the VTEX code-generator. Unless specified otherwise, this article describes the functionality in the free- ware version of the VTEX compiler, as available on CTAN sites in systems/vtex.
# GEX is a graphics counterpart to TEX. .. Since GEX may exercise subtle influence on TEX (load fonts, or change TEX registers), GEX is op- tional in VTEX implementations: the default oper- ation of the program is with GEX off; it is enabled by a command-line switch.
# \includegraphics[width=1.3in, colorspace=grayscale 256]{macaw.jpg}
# http://mail.tug.org/texlive/Contents/live/texmf-dist/doc/generic/FAQ-en/html/FAQ-TeXsystems.html
# A free version of the commercial VTeX extended TeX system is available for use under Linux, which among other things specialises in direct production of PDF from (La)TeX input. Sadly, it���s no longer supported, and the ready-built images are made for use with a rather ancient Linux kernel.
# NOTE: another way to capture metadata; if converting via ghostscript:
# http://compgroups.net/comp.text.pdf/How-to-specify-metadata-using-Ghostscript
# first:
# grep -a 'Keywo' orig.pdf
# /Author(xxx)/Title(ttt)/Subject()/Creator(LaTeX)/Producer(pdfTeX-1.40.12)/Keywords(kkkk)
# then - copy this data in a file prologue.ini:
#/pdfmark where {pop} {userdict /pdfmark /cleartomark load put} ifelse
#[/Author(xxx)
#/Title(ttt)
#/Subject()
#/Creator(LaTeX with hyperref package + gs w/ prologue)
#/Producer(pdfTeX-1.40.12)
#/Keywords(kkkk)
#/DOCINFO pdfmark
#
# finally, call gs on the orig file,
# asking to process pdfmarks in prologue.ini:
# gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 \
# -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -dDOPDFMARKS \
# -sOutputFile=out.pdf in.pdf prologue.ini
# then the metadata will be in output too (which is stripped otherwise;
# note bookmarks are preserved, however). 

我也有一些要转换为黑白的扫描彩色pdf和灰度pdf。我尝试使用此处列出gs的代码，并且仍然存在pdf文本，因此图像质量很好。但是，该gs代码仅转换为灰度（如问题所要求），并且文件大小仍然很大。convert直接使用时，效果很差。

我希望黑白PDF具有良好的图像质量和较小的文件大小。我本来可以尝试使用terdon的解决方案，但pdftk使用yum（在撰写本文时）无法进入centOS 7。

我的解决方案用于gs从pdf中提取灰度bmp文件，convert将这些bmp 阈值化为bw并将其保存为tiff文件，然后使用img2pdf压缩tiff图像并将它们全部合并为一个pdf。

我尝试直接从pdf转到tiff，但是质量不一样，所以我将每个页面保存到bmp。对于一页pdf文件，convert从bmp到pdf都做得很好。例：

gs -sDEVICE=bmpgray -dNOPAUSE -dBATCH -r300x300 \
   -sOutputFile=./pdf_image.bmp ./input.pdf

convert ./pdf_image.bmp -threshold 40% -compress zip ./bw_out.pdf

对于多页，gs可以将多个pdf文件合并为一个，但是img2pdf文件大小比gs小。tiff文件必须解压缩为img2pdf的输入。请记住，对于大量页面，中间的bmp和tiff文件通常会很大。pdftk或者joinpdf，如果他们可以合并来自的压缩pdf文件，效果会更好convert。

我想有一个更优雅的解决方案。但是，我的方法产生的结果具有非常好的图像质量和更小的文件大小。要使文本恢复为bw pdf，请再次运行OCR。

我的shell脚本使用gs，convert和img2pdf。根据需要更改开头列出的参数（页面数，扫描dpi，阈值％等），然后运行chmod +x ./pdf2bw.sh。这是完整的脚本（pdf2bw.sh）：

#!/bin/bash

num_pages=12
dpi_res=300
input_pdf_name=color_or_grayscale.pdf
bw_threshold=40%
output_pdf_name=out_bw.pdf
#-------------------------------------------------------------------------
gs -sDEVICE=bmpgray -dNOPAUSE -dBATCH -q -r$dpi_res \
   -sOutputFile=./%d.bmp ./$input_pdf_name
#-------------------------------------------------------------------------
for file_num in `seq 1 $num_pages`
do
  convert ./$file_num.bmp -threshold $bw_threshold \
          ./$file_num.tif
done
#-------------------------------------------------------------------------
input_files=""

for file_num in `seq 1 $num_pages`
do
  input_files+="./$file_num.tif "
done

img2pdf -o ./$output_pdf_name --dpi $dpi_res $input_files
#-------------------------------------------------------------------------
# clean up bmp and tif files used in conversion

for file_num in `seq 1 $num_pages`
do
  rm ./$file_num.bmp
  rm ./$file_num.tif
done

RHEL6和RHEL5都是Ghostscript的8.70基准，不能使用上面给出的命令形式。假设脚本或函数将PDF文件作为第一个参数“ $ 1”，则以下内容应更易于移植：

gs \
    -sOutputFile="grey_$1" \
    -sDEVICE=pdfwrite \
    -sColorConversionStrategy=Mono \
    -sColorConversionStrategyForImages=/Mono \
    -dProcessColorModel=/DeviceGray \
    -dCompatibilityLevel=1.3 \
    -dNOPAUSE -dBATCH \
    "$1"

输出文件将以“ grey_”作为前缀的位置。

RHEL6和5可以使用CompatibilityLevel = 1.4更快，但是我的目标是可移植性。

我得到可靠的结果，清理了扫描的pdf文件，与此脚本形成了很好的对比。

#!/bin/bash
# 
# $ sudo apt install poppler-utils img2pdf pdftk imagemagick
#
# Output is still greyscale, but lots of scanner light tone fuzz removed.
#

pdfimages $1 pages

ls ./pages*.ppm | xargs -L1 -I {} convert {}  -quality 100 -density 400 \
  -fill white -fuzz 80% -auto-level -depth 4 +opaque "#000000" {}.jpg

ls -1 ./pages*jpg | xargs -L1 -I {} img2pdf {} -o {}.pdf

pdftk pages*.pdf cat output ${1/.pdf/}_bw.pdf

rm pages*