DTF Gang Sheet Issue in Photoshop

[gofortheheart]

I'm trying to make a DTF Gang sheet in Photoshop for t-shirts. The gang sheet is 22" x 120". I create a new PS file with those dimensions at 300 dpi. When I export the file as a png it ends up much larger. (55" x 300+"). I then have to open the png file and manually reduce the size back to 22" x 120".  Can anyone explain why this happens?

[kglad]

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[davescm]

Exporting, without including all metadata, strips out the ppi value which is normally held in the file alongside the image data. This is not usually an issue as export was designed for screen use where ppi is irrelevant.  

If an image is opened with no ppi metadata then a value of 72ppi is used. Hence the reported physical size is different, although no actual pixels are changed. 

The answer is to export including all metadata or use Save a copy. That way 300 ppi value will be retained in the file.

Dave

[Stephen Marsh]

Further to Dave's reply, PNG was originally designed for monitor use, it was never intended as a print format. The PNG specification uses Pixels Per Metre, not Pixels Per Inch, for the resolution metadata.

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, it performs a conversion from PPM to PPI:

118.11 (CM) x 2.54 (CM/INCH) = 299.9994 ppi... rounded up this is 300 (PPI).

So technically there is no such thing as a 300ppi PNG.