You can use the man command tiffutil with the option -cathidpicheck. The command lets you manipulate TIFF files using the specified options. The -cathidpdicheck option writes a single output file containing the files supplied as arguments to the option. This option also checks to make sure that the standard- and high-resolution files you supply are sized correctly. That is, the dimensions of the high-resolution image must be twice that of the standard-resolution image. Running tiffutil explicitly changes the dpi. Using tiffutil also compresses the resulting output file, so there is no need for you to perform additional compression.
Running the following command creates a single file from the two input files:
tiffutil -cathidpicheck infile1 infile2 -out outfile
For example, if the input files are:
-
myimage.pngwith width = 32 pixels, height = 32 pixels -
myimage@2x.pngwith width = 64 pixels, height = 64 pixels
running this command:
tiffutil -cathidpicheck myimage.png myimage@2x.png -out myimage.tiff
will produce a single TIFF file that contains the two input images.
See the tiffutil man pages documentation in Terminal for more information, including options for extracting an image from a multirepresentation TIFF file.