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Re:AVIF – AV1 based image format by sjwright on Thursday April 12, 2018 @09:52PM (#16826566)
All true, but irrelevant for images. All the number does is indicate the intended physical dimensions of the image.

For example, an image with pixel dimensions of 1800 by 1200 and a “DPI” metadata value of 300 will print with a physical dimension of 6.00 by 4.00 inches.

Re:AVIF – AV1 based image format by tatersolid on Sunday April 15, 2018 @08:50AM (#16842434)
You misunderstand: the file format itself needs to have at least horizontal and vertical DPI values to be generally useful.

Many film scanners for instance are say 2400 dpi vertically and then some other DPI value horizontal depending on the film or picture format. Anamorphic lenses are used to scale a wide-aspect image onto a 4:3 sensor.

Some scanners and printers even have different DPI for different color components.