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You're right that in a world with varying pixel densities, it's not enough to just design an icon for a given pixel size. But physical size isn't the right design target either. An icon designed for a given physical size on a display close to the user's eyes won't work if the display is much further away.

So when you go from thinking about pixel size to thinking about physical size, you're going in the right direction, but you're not going far enough. The relevant metric is more like "the angle subtended at the user's eyes", kinda like how the "CSS pixel" unit works.

Now, given that it's almost impossible for software on the device to know how far away the user is, trying to design up front for that metric (angle subtended by the icon) is also almost impossible.

And even if you did know both the pixel density and the distance to the display, that still doesn't take into account the fact that users will vary in their visual acuity. The right design choice for a 20-year-old user holding the display close to their face is going to be dramatically different from the right choice for a 70-year-old holding the display at arms length.

You can't really design well for all those possibilities up front. The right answer has to involve allowing the user to adjust the "zoom level" to their liking. And that level of flexibility means that automated approaches (possibly something like freetype's autohinting, extended to work in color) are probably going to be a better approach than manual pixel tweaking.

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