Reduce dpi but don't enlarge it in IM

[jwilheim]

I am trying to figure out how to configure IM to change the dpi of an image only if it is larger than a desired value.
For example, if given any file with a dpi larger than 200, I want to change it to 200 dpi.
But if given a file of less than 200 dpi, I want to keep it at the original dpi.
Any help would be greatly appreciated.

[fmw42]

I believe that you will likely need to write a script to loop over each image and then test the resolution and then decide if you want to change it. See %[xresolution], %[yresolution] and %x and %y for x and y resolutions in http://www.imagemagick.org/script/escape.php You can also get density and units from the -verbose info:

xres=`convert image -format "%[xresolution]" info:`
if [ $xres > 200 ]; then
convert image -density 200 resultimage
fi

Note this is unix. For windows see http://www.imagemagick.org/Usage/windows/

You can also use "%x" in place of "%x" "%[xresolution]". This will give you both the resolution (density) and the units, but you will have to separate the two first. Units may be important as png units are pixelspercentimeter. So you may need both to convert to one common units such as pixelsperinch.