How to find pixels per inch (PPI) using ImageMagick

Question

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


        

Answer 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

Answer 2

1 inch =~ 2.54 cm. So it is trivial to convert between the two.

Answer 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.

Answer 4

You can simply add the option

-units PixelsPerInch