There is no total (or improved) cover anywhere on that map. Improved cover is intended to cover cases where you can't draw any corner to corner lines at all but can still see a little bit of the target.
There is regular (+4 AC) cover from all squares except the top two rows of the map; from those two rows there is no cover for ranged attacks and only the square you have marked orange has cover for melee attacks. The partial cover thing is sort of up to you; I tend to ignore it unless it's really obvious that it shouldn't be regular cover.
Blue dot (target) can make ranged attacks to every square on the map with no cover.
I get your point, wordelo. It seems like the blue dot should get total cover from the farthest purples - but only if you assume blue is not fighting back.
If they're firing at each other, in combat against each other somehow, then blue is exposing himself during the round and therefore doesn't have total cover.
Now, if all blue is doing is hiding behind that wall without ever sticking his head out to see what the purple dot is doing, and all purple is dong is standing there waiting to play target-practice with blue, then your point might be valid that the blue might have total cover from the purple dots. Still not the official rules, but arguably worth a house rule.
there are no rules about "sticking your head out", nor any rules about whether or not you're fighting back (unless you can find it and point it out). Either you are in a square or you are not in a square.
See, I don't see how you can say the bit above (which is exactly true) and then say the bit below (which is exactly a made up house rule).
If you are the green guy the blue target has no cover
if you are the yellow guy the blue target has partial cover
if you are the orange guy the blue target has cover
if you are the red guy the blue target has improved cover
if you are the purple guy the blue target has total cover and cannot be attacked.
No, the reality of the rule is that the blue target never has total cover from any of the dots. Not even the purple ones.
Total Cover
If you don't have line of effect to your target (that is, you cannot draw any line from your square to your target's square without crossing a solid barrier), he is considered to have total cover from you. You can't make an attack against a target that has total cover.
From every purple dot I can draw at least one line (actually two) to the rightmost corners of the blue dot's square. Therefore, the blue target has cover and never total cover because I cannot truly say "you cannot draw any line from your square to your target's square without crossing a solid barrier" - the most I can say is that out of four lines, I cannot draw two of them, but I certainly can draw the other two. No total cover.
Again, if the blue target has his back to the wall and keeps his head down, I could totally justify your house rule, even against many dots that are not purple. But if he's fighting and able, sometimes, to see those dots, then they can see him, and the rules don't allow total cover in that case.
Furthermore, all the yellow, orange, and red dots simply have cover. Yes, at your discretion, they can have partial cover, normal cover, or improved cover. Clearly it's your discretion to assign these kinds of cover according to your diagram. But again, that is one GM's discretion, others might be differently discretionary. None of which is a rule, just GM's discretion.
Sure, your idea has merit as a house rule. But it's not a rule.
There is no total (or improved) cover anywhere on that map. Improved cover is intended to cover cases where you can't draw any corner to corner lines at all but can still see a little bit of the target.
There is regular (+4 AC) cover from all squares except the top two rows of the map; from those two rows there is no cover for ranged attacks and only the square you have marked orange has cover for melee attacks. The partial cover thing is sort of up to you; I tend to ignore it unless it's really obvious that it shouldn't be regular cover.
Blue dot (target) can make ranged attacks to every square on the map with no cover.
This.
If the Blue dot took a five foot step to the left, it would have total cover from the Purple and Red dots, and half of the Orange dots depending on your interpretation.
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
Improved Cover is, by the book, reserved for special circumstances like shooting through an arrow slit or attacking someone underwater. But as the GM it is entirely at your discretion to expand the definition for a better table experience.