Constraints I'm noticing:
- How quick / easy is it to spot on the app screen?
  - realfavicongenerator says "make sure your favicon will always be prominent, whatever the background"
- How easy is it to recognize it from afar?
  - This is very similar to a test you can do for camouflage
  - It's also similar to the test I applied to advertisements, where they should aim to grab your attention from afar rather than just being a wall of text.
- How easy is it to distinguish the icon from other popular icons?
- How well does the icon give an impression of what the app is for?
  - This may be more important for new apps.
- How much of the physical space allotted to the icon is used up by the icon?

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