Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
When identifying a TIFF image that was created by the Macs Screenshot tool grab, magick 6.8.0-10 return 0 (zero). Magick 6.7 returned correctly 72 dpi.
I think your file was created badly and is missing the Resolution. Even exiftool is missing the resolution when run on this file. Have you tried any other screen snap tools?
It does the same for me using Grab -- no resolution field -- when I take a selection screen snap. However, if I open the tiff result in Preview and save it again to tiff, then it does include 72 dpi for the resolution and IM sees it.
So it is clearly a problem with Grab.
PS Use Preview to take your screen snaps (see screen capture under the File menu). It has similar functionality and you can save to tiff and get the resolution directly.
I understand that there could be an error with images created by Grab. But what I wanted to point out is, that magick altered its behaviour here. When you run the image trough an older version, you receive the "correct" 72dpi:
A resolution of 0 means the resolution is not defined. This is a result of a bug fix where a user needed to be able to distinguish between an image whose resolution is defined @ 72DPI and those whose resolution is not defined.
