I'm using Ghostcript to convert some PDF files to JPG. All is working when converting the program consider the resolution of 600dpi when converting and output jpeg quality is good.
Here is my code :
gs -sDEVICE=jpegcmyk -dTextAlphaBits=4 -r600 -dSAFER -dBATCH -dNOPAUSE -o my_output_file.jpg my_input_file.pdf
But when I open the file in Photoshop, the properties contains 72dpi instead of 600dpi I expected :
When I try with RGB profile for output, it is ok, I have got 600dpi.
So what I want is CMYK + 600dpi in image properties.
As can be seen from your screenshots, both images are of the same dimensions, 6803 by 709 pixels.
And that is all that matters.
Also, the size of the CMYK version is bigger by about 33% compared to the RGB version -- as is to be expected for an image with 4 color channels instead of 3.
Ghostscript used the -r600 CLI parameter to correctly expand the number of pixels when converting the PDF file.
Ghostscript does not add any EXIF metadata to its output when converting a PDF to raster.
The DPI or PPI information would be an internal metadata hint to tell any compliant viewers how big to render the image on screen. It would not change anything substantial in the image information itself.
Why Photoshop does think it should use 72 dpi for one, but 600 dpi for the other, you may ask Adobe about.
I bet Photoshop also renders the 72dpi file about 7 times larger on screen than the other. Is that the case?
P.S.: See also "What DPI do web images need to be?"
来源：https://stackoverflow.com/questions/48166403/ghostscript-convert-pdf-to-jpg-cmyk-profile-resolution-error