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PPI (Pixels Per Inch) measures screen resolution density — how many pixels fit in one linear inch of a display. Higher PPI means sharper text and images. Apple branded 326+ PPI as 'Retina' — the point where individual pixels become indistinguishable at normal viewing distance.

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1. 1PPI = √(width² + height²) / diagonal screen size (inches)
2. 2Diagonal pixels = √(horizontal_res² + vertical_res²)
3. 3Retina threshold ≈ 300 PPI at ~12 inch viewing distance
4. 4Effective viewing PPI depends on viewing distance

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