This is not a question, but an attempt to improve my understanding of how DPI is stored in various file formats, and discuss why it works.

I work a lot with images (JPG, PNG and Tiff) and detect their resolution.

For example, I can run this on an image in Imagick:

$imagick = new Imagick($filename);
$data = $imagick->identifyimage();
var_dump($data);

It will give me the resolution and units (PixelsPerCentimeter or PixelsPerInch)

In Windows (11), I can only see the resolution in the properties of the JPG but not the PNG, whether you set the PSD to PixelsPerCentimeter or PixelsPerInch, it shows up as ( 300 dpi).

In Imagick (version: ImageMagick 6.9.11-60 Q16 x86_64) - it is able to see the difference between a JPG saved with PixelsPerCentimeter and a JPG saved with PixelsPerInch. But for PNG, no matter how I create/save it in Photoshop, it gives PixelsPerCentimeter.

Photoshop (24.1.0) can also detect PixelsPerCentimeter on JPGs - but the PNGs all "say" they are PixelsPerInch, even if I save them as PixelsPerCentimeter.

Now I totally understand 118.11 PixelsPerCentimeter = 300 PixelsPerInch but I'm really interested in why Windows/Photoshop and Imagick treat them like this?

I did consider that creating the file in Photoshop initially might have side effects, so I created a file using Imagick (PNG, 300dpi, PixelsPerInch), but Imagick specified it as 118 PixelsPerCentimeter.

So I guess my question is: Do these differences depend on how the file is read or the actual metadata stored in the file type?