问题
After upgrading ImageMagick text quality got degraded when convert pdf to jpeg:
Old image
New Image
Conversion command: convert foo.pdf foo.jpeg
Old ImageMagick version:
[root@home]# convert -version
Version: ImageMagick 6.2.8 05/07/12 Q16 file:/usr/share/ImageMagick-6.2.8/doc/index.html
Copyright: Copyright (C) 1999-2006 ImageMagick Studio LLC
generated files size:
-rw-r--r-- 1 root root 139K Apr 2 16:11 foo-0.jpeg
-rw-r--r-- 1 root root 130K Apr 2 16:11 foo-1.jpeg
-rw-r--r-- 1 root root 334K Mar 24 14:27 foo.pdf
After upgrading ImageMagick
[root@home]# convert -version
Version: ImageMagick 6.7.8-10 2012-08-17 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2012 ImageMagick Studio LLC
Features: OpenMP
generated files size:
-rw-r--r-- 1 root root 60K Apr 2 16:11 foo-0.jpeg
-rw-r--r-- 1 root root 55K Apr 2 16:11 foo-1.jpeg
-rw-r--r-- 1 root root 334K Mar 24 14:27 foo.pdf
I've tried using antialias flag:
convert -antialias foo.pdf foo.jpeg
Which did nothing, I've tried setting an higher quality:
convert -quality 100 foo.pdf foo.jpeg
and super sampling:
convert -density 288 -background white -alpha off foo.pdf -resize 25% foo.jpeg
both gave bigger files and better results, but ran more time and had lower quality that the old ImageMagick version.
any advises?
Link to the file
回答1:
I see the same problem with your sample file. It looks like ImageMagick's delegates for the PDF conversion may have changed with the new install.
If you try convert -verbose foo.pdf foo.jpeg, do you see -sDEVICE=pngalpha in the command that gets sent to gs? The pnmraw device has been used in the past, and switching back to that seems to fix the problem for me.
In ImageMagick's delegates.xml file (which may be in /etc/ImageMagick, but could be somewhere else depending on your setup), look for the decode="ps:alpha" delegate line and change -sDEVICE=pngalpha in the command to -sDEVICE=pnmraw. (You can probably just search for pngalpha in the file.)
回答2:
it seem that problem at DPI. when convert pdf, imagemagick using Ghostscript. you can skip using imagemagick.
$ gs -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dGridFitTT=2 -dUseCropBox -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -r200x200 -sDEVICE=jpeg -dJPEGQ=100 -sOutputFile=foo-%05d.jpg foo.pdf
set -r option higher value. Ghostscript have default value is 100DPI.
or using convert option -density. this option set pdf converted DPI.
$ convert -density 200x200 foo.pdf foo.jpg
回答3:
PDF files are vector files and have no specific size. Their size is controlled by defining the density and units before reading in the PDF file. You can get better quality for the same desired output file size by supersampling. That means rasterize the PDF to a large size and then resize to your desired actual size. For example in ImageMagick:
convert -units pixelsperinch -density 288 image.pdf -resize 25% output.jpg
The nominal density if left off is 72 dpi. So 72*4=288. Then resize by 1/5=25% gets back to the same default size, but should look much better. Change the density or resize to deal with quality and final size as desired.
来源：https://stackoverflow.com/questions/15769623/imagemagick-convert-pdf-to-jpeg-has-poor-text-quality-after-upgrading-imagemagic