问题
I can get PPI for a JPEG image using the following command:
$ identify -format "%w x %h %x x %y" mypic.jpg
1600 x 1200 72 PixelsPerInch x 72 PixelsPerInch
However, when I run the same command on PNG format I get Pixels Per Centimeter:
$ identify -format "%w x %h %x x %y" mypic.png
1600 x 1200 28.35 PixelsPerCentimeter x 28.35 PixelsPerCentimeter
Is there a way to change the command to get Pixels Per Inch (PPI) for PNG format as well or perhaps calculate the PPI based on the pixels per centimeter?
回答1:
The resolution and units used are stored in the file, so if the resolution is stored in PixelsPerCentimeter, that's how identify will display it. There isn't a way to do the conversion automatically through identify. But it's just cm to inch conversion math:
PixelsPerInch = PixelsPerCentimeter * 2.54
回答2:
You can simply add the option
-units PixelsPerInch
回答3:
You can use the fx operator and some smart formatting in the output of identify like this:
identify -format "%[fx:int(resolution.x*2.54)]" image.png
299
Of course, the true joy of this is that it is platform-independent, so you don't have to shell out to dc on OS X and Linux, or do whatever muppetry is required on Windows to do the maths.
回答4:
1 inch =~ 2.54 cm. So it is trivial to convert between the two.
来源：https://stackoverflow.com/questions/14632254/how-to-find-pixels-per-inch-ppi-using-imagemagick