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The hypotenuse of a right-angle triangle (diagonal) is longer than its sides and would contain fewer pixels than an inch measured horizontally or vertically.

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  • PPI isn't calculated using a diagonal inch.
    – Billy Kerr
    Aug 17, 2022 at 8:09

2 Answers 2

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Its usually measured in the direction of the pixel matrix because its describing the image, not output. So horizontal and vertical of the image. Yes its possible to have two different values.

Pitch like measurements in general choose closest item to closest item because thats easier to distinguish.

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Just to complement.

As the values are orthogonal, you have an X PPI value and a Y PPI value.

Normally the pixels are square, so you only mention one value. If you have a non-square pixel, you have now a pixel ratio.

This was common on some old video formats, for example when the video formats transitioned from analog standard definition to HD or a 16:9 aspect ratio, or when the video was converted to DVD. The pixel ratio is an instruction for the device to stretch the image back to the desired proportion.

Generally, printers have also different X vs Y DPI. You can see that by reading the specs of the device. There is no "Dot ratio" here. The dot is considered round, but the different X vs Y resolution is only given by the proximity or overlapping of the dots.

Scanners can also have different PPI ratios. The final result does not make evident a "rectangular" pixel, because normally the result of a scanner is interpolated, giving as result a square pixel.

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