If you haven’t been immersed in iOS interface design, you might look at Apple’s icons and think that they’re just a rounded square or a ‘roundrect’. If you’ve been designing icons, you know that they’re something different and may have heard the word squircle used (mathematical intermediate of a square and a circle). And if you’re an Industrial Designer, you recognize this as a core signature of their hardware products.
A ‘secret’ of Apple’s physical products is that they avoid tangency (where a radius meets a line at a single point) and craft their surfaces with what’s called curvature continuity.
On the right you see what curvature continuity looks like. The curvature comb transition is a curve itself, starting from zero curvature. There’s no sudden break in curvature and, as a result, the highlight is smoother. This difference in curvature is harder to spot in an icon, but the important thing is that now the icons and the hardware are part of the same design language.
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