Regardless of saving an image in resolution 120 ppcm, it switches next time when opening to ppi

[defaultfz35vasww8kk]

We have to print large scale wall prints. As there is a number of collaborators it was in the end decided to save pictures in the exact size as the wall, Because the measures are in mm, the size is in millimeters and we switched to ppcm instead of ppi, and the closest resolution to 300ppi would be 120ppcm, because we wanted to work with round numbers.

When we change it in photoshop and save PNG it all looks OK, however when we reopen PNG resolution is approx 305ppi as this is the equivalent to 120ppcm, but the size is no longer 1200mm, but 1199,21mm.

Because of the number of collaborators during the installation, and language barriers we dropped out all printing margins we would usually have when preparing for printing.

Is there a reason why Photoshop cannot keep the resolution and size? If everything is changed to decimal units there should be no such issue.

Thank you all in advance!

[Stephen Marsh]

PNG is not a great format for professional print IMHO (it can be useful in consumer/office apps, particularly for transparency). I would suggest PDF.

The official PNG specification states that the PNG resolution unit is in metres (not inches or mm etc). This can be verified using metadata tools such as ExifTool.

So, a 300 ppi save as PNG from Photoshop would embed a value of 11811 ppm (pixels per metre). When Photoshop opens a PNG with this resolution metadata, I believe that it performs an “on the fly translation” from PPM to PPI:

118.11 x 2.54 = 299.9994 ppi... rounded up this is 300 ppi.

In your example a 120 cm x 120 cm @ 120 ppcm image has the following metadata entries:

[PNG] ImageWidth: 14400
[PNG] ImageHeight: 14400
[PNG-pHYs] PixelsPerUnitX: 12000
[PNG-pHYs] PixelsPerUnitY: 12000
[PNG-pHYs] PixelUnits: meters

As you know, this translates into Photoshop as 119.92 cm x 119.92 cm @ 120.079 ppcm, so you would need to then "normalise" the resolution to 120 ppcm to get the correct print size of 120 cm.

P.S. Do you really need 120 ppcm (304.8 ppi) for such work?

[mglush]

Stephen gae a great reason not to use png as you file format. I would suggest changing it to a true printing format like pdf and see if you still have the same issue. When printed, png does not give you the color gamut you would want to print your image, where as pdf will.

Michelle