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The recognition speed depends on how much the color and shape attract the eyes. So the solid icons are easier to recognize.
What is a solid icon?
Solid icons grab the users' attention with a simple design and careful coloring. As you can see in the image below, the solid icons stand out more than an outline icon, and as a result, they don't need to contain as many subtle cues or shapes around the edge of the icon.
Why use outline icons?
These icons have subtle characteristic cues that appear on the outer edges of the shape. Because of this, an outline style makes those cues easier to notice. The comment bubble tail is easy to miss as a solid, but more apparent as outlines.
