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Gert Pellin - GPEandGitHub
58289b2822
A camera JPEG (e.g. Nikon D3200) records its resolution only in EXIF IFD0, so it opened at 72 ppi instead of 300, and saving wrote JFIF 72 next to the source's EXIF 300. Reading: a JPEG takes the first usable of APP13 Photoshop ResolutionInfo (0x03ED), XMP tiff:X/YResolution, EXIF IFD0 X/YResolution + ResolutionUnit (2 inch, also when absent; 3 cm), and only then the JFIF density. That is the order Photoshop resolves them in; JFIF comes last because cameras write EXIF only while many tools write a default JFIF 72. Other formats (WebP, HEIF, PNG without pHYs, ...) fall back to XMP then EXIF when they have no resolution of their own. All parsing is bounds-checked; malformed data reads as no resolution. Writing: JPEG, PNG, WebP, TIFF (XMP), PSD and Save for Web JPEG rewrite the EXIF/XMP resolution to the image's: RATIONAL values in place, ResolutionUnit 2, and an entry that can't be rewritten safely is removed from IFD0 without moving any other data.