Open PNG file that contains pHYs chuck.
"Image infomation" -> "Resolution" -> Change DPI
Save as, check "Use PNGOUIT Plugin"(Keep chunks include pHYs)
Saved image has not changed pHYs chunk.
uncheck "Use PNGOUIT Plugin", saved image has changed pHYs chunk.
Open PNG file that not contains pHYs chuck, saved image has new correct pHYs chunk.
"Image infomation" -> "Resolution" -> Change DPI
Save as, check "Use PNGOUIT Plugin"(Keep chunks include pHYs)
Saved image has not changed pHYs chunk.
uncheck "Use PNGOUIT Plugin", saved image has changed pHYs chunk.
Open PNG file that not contains pHYs chuck, saved image has new correct pHYs chunk.