sips (scriptable image processing system)
The command line "sips" program can query or modify images.
I particularly like that you can be very specific:
$ sips -g dpiHeight -g dpiWidth *jpg
The syntax for changing formats is a bit odd, but there's a lot of power
$ sips -s format jpeg picture.png --out picture.jpg
$ ls -l picture*
-rw-r--r-- 1 tony staff 240011 Sep 8 13:16 picture.jpg
-rw-r--r-- 1 tony staff 23092 Aug 5 2004 picture.png
$ file picture*
picture.jpg: JPEG image data, JFIF standard 1.01
picture.png: PNG image data, 1085 x 1410, 2-bit colormap, non-interlaced
Interestingly, the man page for sips doesn't tell you about a lot of
the syntax it recognizes. Probably most importantly it neglects to mention
that "sips -h" and "sips -H" give much more information (and note that -h and -H
give different help - you need both). You can flip images, rotate, change width and
height, and much, much more.
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© 2009-11-07 Tony Lawrence