The diagram in the example is as follows:
x x a o a
x o a a a
x o a a a
a a a a a
a a a o o
This is what I think it ought to be by my reckoning of the rules:
x a a o a
x o a a a
a o a a a
a a a a a
a a a o o
I replaced 2 of the x's with a's. The upper x=a because I can trace an imaginary line from the chosen origin square's corner to the target square in question without touching an obstacle.
The diagram in the book is correct. The 2 squares you mention are outside of the burst not because of cover, but because they are out of range. To go around the corner of a wall, you can't go diagonally. PHB p. 273 says you count range from the origin by taking one step to
all adjacent squares (allowing you to cut corners only for the first step), and then count further steps normally (without diagonally cutting corners). So those 2 squares are at a range of 3 from the origin square. The statue doesn't fill its square like a wall, so you can count through its space.
The lower x=a because the imaginary line runs along the edge of the wall, so it isn't considered blocked for determining whether the target square in question is affected by the burst. Now, I don't think that it makes a whole lot of sense that a square that's directly on the other side of a wall from the origin square is affected by a burst, so I think some errata needs to be written...
Actually, if the only line you can draw to the target square passes along a wall, you don't have line of effect. The rules say the line is blocked if it passes through
or touches a blocking obstacle. (It seems that that the 3.5E concept of a spread no longer exists in 4E -- no more fireballs going around corners.)
In this same vein, I would also like to propose some errata. All the examples seem to suggest that you determine line of sight/effect by tracing an imaginary line from a chosen corner of the origin square to any one of the corners of the target square.
The rules say (page 273),
"...pick a corner of your space and trace an imaginary line from that corner to any part of the target’s space..."
The ought to say,
"...pick a corner of your space and trace an imaginary line from that corner to any corner of the target’s space..."
This change makes sense to me because there are an infinite number of lines from a corner of one square to any part of another square. This change is further reinforced by the following rule on page 281, "If three or four of those lines are blocked but you have line of effect, the target has superior cover." If you were to follow both rules as written, then any kind of obstruction, no matter how small, would provide superior cover!
I think the rules are OK as written. You seem to be mixing up line-of-sight/line-of-effect with cover, but the rules are slightly different for those two concepts. PHB p. 273 is about line-of-sight/line-of-effect. If you can draw a line to any point of a square, even if all the corner points are blocked, you can target that square with an effect.
PHB p. 281 is about cover. Superior cover only checks to see if lines to the target square's
corners are blocked. So no, a small obstruction doesn't automatically provide superior cover, because we're not checking every single line of sight to see if 4 are blocked. We only check the 4 lines to the corners.
Moreover the following rule,
"If three or four of those lines are blocked but you have line of effect, the target has superior cover."
Ought to say,
"If three of those lines are blocked, the target has superior cover. If all four lines are blocked, you don't have line of effect."
Since there are only 4 imaginary lines that can be drawn from the chosen origin square corner to any of the target square's corners, then you wouldn't have line of effect if all 4 lines are blocked.
In summary, if one or two lines are blocked the target has cover. If three lines are blocked the target has superior cover. If all four lines are blocked, then you don't have line of effect. This falls in line with the line of effect rules (page 273), "If every imaginary line you trace to a target passes through or touches a solid obstacle, you don’t have line of effect to the target."
You can have all 4 target space corners blocked but still have line of effect. For example, an archer behind an arrow slit would have all of her space's corners blocked, but one would still have line of effect to her space. She has superior cover, but one can still shoot her or throw a spell at her. The arrow slit is the classic example of the difference between cover and line of effect. It's harder to show this happening on a map, but you could have:
Code:
. . . . . . . . . . .
. . . . . . . . . . w
. X . . . . . . . . .
m . . X . . . . . . .
. . . . . . . . . . .
If I've drawn this right, the walls 'X' keep the wizard 'w' from drawing a line to any corner of the monster's 'm' space, but he can still draw a line to the middle of the right edge of the 'm' space. So 'w' has line-of-effect to 'm', but 'm' has superior cover from 'w'.
-- k.fan