Solution for JPEGs in GraphicConverter
Don't do this:
1) Open image in GraphicConverter
2) Menu → Picture → Resolution…
3) "Scale Picture" is OFF (=no pixel interpolations, just a metadata operation) + Change Unified PPI to your desired PPI.
4) But then when you save the picture with ⌘-S it recompresses the image. We won't to avoid that.
Hence do this:
1) Open image parent folder in GraphicConverter and select the image.
2) Menu → General → JPEGs → Change Resolution Value lossless…
3) Change Resolution Losslessly to: Desired PPI.
4) Click OK.
5) All fine: The resolution metadata gets updated. The modification date gets updated. The file is overwritten! So make a backup before in case you are worried. Note: Changing back the resolution value without a backup is easy too: Just run again with the resolution value it had before.
Note "Losslessly" is not a correct adverb in English AFAIK. "Lossless" is already an adverb.
Solution for JPEGs in ExifTool
Code: Select all
exiftool /path/to/image.jpg -xresolution=600 -yresolution=600 -v2
• The -v2 argument is not necessary. Just gives you verbose info what exactly was updated (or not in case of failure).
• No need for a file backup. ExifTool creates a backup with an "_original" suffix (e.g. "image.jpg_original") and only then overwrites.
Processed file outcomes GraphicConverter vs. ExifTool
• Sometimes the same outcome.
• Sometimes slightly different outcome (e.g. 88 bytes difference).
• 3 permutations possible: Source contains no EXIF + Source contains EXIF and you choose Modify only values in JPEG APP0 Marker vs. Modify JPEG APP0 Marker and Exif if available. I guess that explains the sometimes slightly different outcome.
• Does GraphicConverter use ExifTool for that operation? Or its own methods?
Is there also a lossless Resolution metadata change for other lossless file formats, e.g. webp ?
• If a WEBP is selected you can still open: Menu → General → JPEGs → Change Resolution Value lossless…
• And run the dialog. No error is thrown. But the file remains untouched.