It's just that easy.
I do have a png input file (file.png), which is a 720 PPI one.
If i try these simple commands:
convert file.png file2.png
convert file.png -units PixelsPerInch -density 720 file2.png
By opening file2.png in Photoshop, it reports the resolution of the file is 719,988 pixels/inch.
Why does this happen?
The question is now posted in very simple terms, but the main reason is i'm working on a small tool to create interlaced files for lenticular printing, and to do so i'd need a very precise control against the resolution only, without resampling the image.
On the test i'm making, for example, i'd need to final file to be a 754.8 PPI. But step after step, the PPI i can see in photoshop is everytime more different than the value i set via the density option.
IM version i'm using is 6.6.3-Q16 on win7-64 (version: Portable Win32 static at 16 bits-per-pixel).