Two creatures can see each other if they have line of sight to each other.
To determine line of sight, draw an imaginary line between the two creatures. If any such line is clear (not blocked), then there's line of sight between the creatures. The line is clear if it doesn't intersect or even touch squares that block line of sight.
If a character can't see a target (for instance, if he is blinded or the target is invisible), he can't have line of sight to it even an unblocked line could be drawn between them.
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