I'm an occasional (→non-expert) user of the ImageMagick's marvelous convert program. Today I wanted to convert a nice PDF file to a high-resolution (300dpi, say) PNG file for some editing.
But I was having trouble with it; sayingconvert foo.pdf bar.png
resulted in a 72dpi image. What to do? I wondered. Perhaps the help would be
useful?
collin@p3:~> convert --help|grep -i resol
-units type the units of image resolution
-resample geometry change the resolution of an image
collin@p3:~> convert --help|grep -i size
-page geometry size and location of an image canvas (setting)
-pointsize value font point size
-size geometry width and height of image
-adaptive-resize geometry
adaptively resize image using 'mesh' interpolation
-extent geometry set the image size
-geometry geometry preferred size or location of the image
-repage geometry size and location of an image canvas
-resize geometry resize the image
collin@p3:~> convert --help|grep -i dots
collin@p3:~> convert --help|grep -i dpi
collin@p3:~>
None of those looked promising. I didn't want to change the resolution
of the image, or resize the image; I just wanted the pre-existing information
to be used! Well, when all else fails, Read The Fine Manual.
convert(1) convert(1)
NAME
convert - convert between image formats as well as resize an image,
blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much
more.
SYNOPSIS
convert [input-options] input-file [output-options] output-file
OVERVIEW
The convert program is a member of the ImageMagick(1) suite of tools.
Use it to convert between image formats as well as resize an image,
blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much
more.
For more information about the convert command, point your browser to
file:///usr/share/doc/packages/ImageMagick/www/convert.html or
http://www.imagemagick.org/script/convert.php.
Ah-HA! The manual is actually online. So I went to the link and looked for "resolution" --
it pointed me at resample, which I knew I didn't want, but I clicked on it
anyway, which got me to http://www.imagemagick.org/script/command-line-options.php#resample, where I did a
search for "dpi" (dots per inch). That led me to this entry:
-density widthxheight
The default resolution is 72 dots per inch, which is equivalent to one point per pixel (Macintosh and Postscript standard).
collin@p3:/tmp> convert foo.pdf bar.png collin@p3:/tmp> convert foo.pdf -resample 300x300 bar3-resampled.png collin@p3:/tmp> convert -density 300x300 foo.pdf bar3.pngThe first command is the naive command to convert foo.pdf to bar.png; the top image is a screenshot, magnified, of bar.png. You can see that the image is pretty ugly.
The second command is the result of resampling the interpreted-as-72dpi image. The information was lost when foo.pdf was read (i.e., it was read at 72dpi); we then tried to expand the image to 300dpi. The edges are smoothed, so it doesn't look quite as ugly as the first image—if you don't look too close that is. But still, a lot of information was lost.
The last command retains more information from the original, and in the bottom image you can see how much nicer it looks.
