将 PDF 转换为 PNG - 开卷题库

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最佳答案

You can use one commandline with two commands (gs, convert) connected through a pipe, if the first command can write its output to stdout, and if the second one can read its input from stdin.

Luckily, gs can write to stdout (... -o %stdout ...).
Luckily, convert can read from stdin (convert -background transparent - output.png).

Problem solved:

GS used for alpha channel handling a special image,
convert used for creating transparent background,
pipe used to avoid writing out a temp file on disk.

Complete solution:

gs -sDEVICE=pngalpha       \
-o %stdout              \
-r144 cover.pdf         \
|                       \
convert                    \
-background transparent \
-                       \
cover.png

Update

If you want to have a separate PNG per PDF page, you can use the %d syntax:

gs -sDEVICE=pngalpha -o file-%03d.png -r144 cover.pdf

This will create PNG files named page-000.png, page-001.png, ... (Note that the %d-counting is zero-based -- file-000.png corresponds to page 1 of the PDF, 001 to page 2...

Or, if you want to keep your transparent background, for a 100-page PDF, do

for i in {1..100}; do        \
\
gs -sDEVICE=pngalpha       \
-dFirstPage="${i}"      \
-dLastPage="${i}"       \
-o %stdout              \
-r144 input.pdf         \
|                       \
convert                    \
-background transparent \
-                       \
page-${i}.png ;        \
\
done

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I'll add my solution, even thought his thread is old. Maybe this will help someone anyway.

First, I need to generate the PDF. I use XeLaTeX for that:

xelatex test.tex

Now, ImageMagick and GraphicMagic both parse parameters from left to right, so the leftmost parameter, will be executed first. I ended up using this sequence for optimal processing:

gm convert -trim -transparent white -background transparent -density 1200x1200 -resize 25% test.pdf test.png

It gives nice graphics on transparent background, trimmed to what is actually on the page. The -density and -resize parameters, give a better granularity, and increase overall resolution.

I suggest checking if the density can be decreased for you. It'll cut down converting time.

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Out of all the available alternatives I found Inkscape to produce the most accurate results when converting PDFs to PNG. Especially when the source file had transparent layers, Inkscape succeeded where Imagemagick and other tools failed.

This is the command I use:

inkscape "$pdf" -z --export-dpi=600 --export-area-drawing --export-png="$pngfile"

And here it is implemented in a script:

#!/bin/bash


while [ $# -gt 0 ]; do


pdf=$1
echo "Converting "$pdf" ..."
pngfile=`echo "$pdf" | sed 's/\.\w*$/.png/'`
inkscape "$pdf" -z --export-dpi=600 --export-area-drawing --export-png="$pngfile"
echo "Converted to "$pngfile""
shift


done


echo "All jobs done. Exiting."

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My solution is much simpler and more direct. At least it works that way on my PC (with the following specs):

me@home: my.folder$ uname -a
Linux home 3.2.0-54-generic-pae #82-Ubuntu SMP Tue Sep 10 20:29:22 UTC 2013 i686 i686 i386 GNU/Linux

with

me@home: my.folder$ convert --version
Version: ImageMagick 6.6.9-7 2012-08-17 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2011 ImageMagick Studio LLC
Features: OpenMP

So, here's what I run on my file.pdf:

me@home: my.folder$ convert -density 300 -quality 100 file.pdf file.png

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Couldn't get the accepted answer to work. Then found out that actually the solution is much simpler anyway as Ghostscript not just natively supports PNG but even multiple different "encodings":

png256
png16
pnggray
pngmono
...

The shell command that works for me is:

gs -dNOPAUSE -q -sDEVICE=pnggray -r500 -dBATCH -dFirstPage=2 -dLastPage=2 -sOutputFile=test.png test.pdf

It will save page 2 of test.pdf to test.png using the pnggray encoding and 500 DPI.

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For a PDF that ImageMagick was giving inaccurate colors I found that GraphicsMagick did a better job:

$ gm convert -quality 100 -thumbnail x300 -flatten journal.pdf\[0\] cover.jpg

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To convert pdf to image files use following commands:

For PNG gs -sDEVICE=png16m -dTextAlphaBits=4 -r300 -o a.png a.pdf

For JPG gs -sDEVICE=jpeg -dTextAlphaBits=4 -r300 -o a.jpg a.pdf

If you have multiple pages add to name %03d gs -o a%03d.jpg a.pdf

What each option means:

sDEVICE={jpeg,pngalpha,png16m...} - filetype
-o - output file (%stdout to stdout)
-dTextAlphaBits=4 - font antialiasing.
-r300 - 300 dpi

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One can also use the command line utilities included in poppler-utils package:

sudo apt-get install poppler-utils
pdftoppm --help
pdftocairo --help

Example:

pdftocairo -png mypage.pdf mypage.png

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As this page also lists alternative tools I'll mention xpdf which has command line tools ready compiled for Linux/Windows/Mac. Supports transparency. Is free for commercial use - opposed to Ghostscript which has truly outrageous pricing.

In a test on a huge PDF file it was 7.5% faster than Ghostscript.

(It also has PDF to text and HTML converters)

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You can use ImageMagick without separating the first page of the PDF with other tools. Just do

convert -density 288 cover.pdf[0] -resize 25% cover.png

Here I increase the nominal density by 400% (72*4=288) and then resize by 1/4 (25%). This gives a much better quality for the resulting png.

However, if the PDF is CMYK, PNG does not support that. It would need to be converted to sRGB, especially if it has transparency, since Ghostscript cannot handle CMYK with alpha.

convert -density 288 -colorspace sRGB -resize 25% cover.pdf[0] cover.png

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Try to extract a single page.

$page = 4

gs -sDEVICE=pngalpha -dFirstPage="$page" -dLastPage="$page" -o thumb.png -r144 input.pdf

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I studied the answers presented and then compared them to my scanner which can scan to PDF or can scan to image (typically JPEG). I found the following:

Typically scanners work at 300 DPI, sometimes higher at 600 DPI or lower at 150 DPI
The scanned images are typically full 24 bit RGB color (8 bits per band) and are typically compressed as JPEG
PNGs, however, are typically used for digital drawings and are typically 8-bit with a palette
PDFs are typically multi-page, whereas PNG and JPEG are typically a single page

When converting from PDF to PNG we have to ask ourselves whether we want to use the default 300 DPI or whether we want to resample the image at a higher resolution 600 DPI or lower resolution 150 DPI. We also have to ask ourselves if the PDF contains photos and therefore we need 24-bit images, i.e. png16m. Alternatively, if the PDF just contains digital documents, predominantly black and white text but may contain a limited number of colors, then, an 8-bit image format is sufficient, i.e. png256. We also need to ask ourselves whether we want to have access to multiple pages or are content with one page.

For the rest of the answer, I will assume 300 DPI and digital documents that do not contain photos, i.e. 8-bit format (or 256-color format).

For a single page extraction, I determine the parameters need to be PDF, PNG, and PAGENO:

#!/bin/bash
# Usage: pdf2png.sh input.pdf output.png pageno
PDF=$1
PNG=$2
PAGENO=$3
FORMAT=png256 # png16m png16 pngmono
DPI=300 # 600 150
cmd=(gs)
cmd+=(-dNOPAUSE)
cmd+=(-q)
cmd+=(-sDEVICE=${FORMAT})
cmd+=(-r${DPI})
cmd+=(-dBATCH)
cmd+=(-dFirstPage=${PAGENO})
cmd+=(-dLastPage=${PAGENO})
cmd+=(-sOutputFile=${PNG})
cmd+=(${PDF})
${cmd[@]}

For a multiple-page extraction, I determine the parameters need to be PDF and DIR:

#!/bin/bash
# Usage: pdf2pngs.sh input.pdf outputdir
PDF=$1
DIR=$2
FORMAT=png256 # png16m png16 pngmono
DPI=300 # 600 150
mkdir -p ${DIR}
cmd=(gs)
cmd+=(-dNOPAUSE)
cmd+=(-q)
cmd+=(-sDEVICE=${FORMAT})
cmd+=(-r${DPI})
cmd+=(-dBATCH)
cmd+=(-sOutputFile=${DIR}/'p%d.png')
cmd+=(${PDF})
${cmd[@]}

To join the pages back together to PNG, we can make use of ImageMagick convert as follows:

#!/bin/bash
# pngs2pdf.sh dir output.pdf
DIR=$1
PDF=$2
convert ${DIR}/*.png ${PDF}