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With io7, Apple made a switch in the tabbars to icons that for the most part are line based. In other places, such as their app icons they are using mostly solid icons. That made me wonder if there is any evidence on if one icon style is more readable than the other.

flat/hollow icons Solid icons

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up vote 10 down vote accepted

Evidence says there's not a significant difference between the speed of recognition for 'line-based' vs. 'filled' icons.

When it comes to icon readability (interpreting 'readability' as inverse to 'time it takes to understand') other factors are more important than style differences.

Here's an excellent, annotated article from covering how to optimize icons for faster recognition, which covers an actual experiment done on lined-vs-filled icons. It's also got great references that include published papers in visual theory, semiotics and linguistics.

Relevant quote:

There was no noticeable difference in speed of recognition for the two styles. So we could suggest that, if an icon is well designed and represents a particular recognizable shape clearly, its style doesn’t have a significant effect on the speed of recognition. The more notable difference, however, was in how individual signs relate to the concepts they represent–the relation of signifier to signified in particular signs.

The icons that performed faster in both sets were iconic signs–‘calculator,’ ‘camera,’ ‘mail.’ On the other hand, the icons that seemed to have required a short pause were symbolic: ‘download,’ ‘back,’ ‘copy.’ Even the ‘delete’ icon caused a slight delay in participants’ response.

Hope that helps!

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