Wow Igawa, this is a surprisingly good idea.
Offmap artillery should be the strategic weapon it represents by forcing an opponent out of an area, not a unit killer. Now dodging artillery will be about hoping a shell doesn't randomly land on your head or simply moving out of the zone just like before, but staying will be more viable...
I like it as an idea for offmap, but I would strongly recommend to keep the smoke - otherwise it's just an unexpected pwn button instead of an pwn win button.
This Idea is much better than the I win buttons we have now.
In terms of implementation I suggest that the warning be displayed on the minmap as a red blinking area or perhaps similar to the Sector Arty Icon that PE get when the deploy it but everyone can see.
In terms of calling it in.
Would it be possible to incorporate the tactical map, so when you want to call in arty you switch to the tactical map draw a square/shape on the map (or click on a sector) then the arty would occur only in that area. With the highlighted area then being shown on the mini map so people can react to the incoming fire. Some points:
1. The bigger the area the less concentrated the arty would be but there would be less area to be evacuated receiving side.
2. There must be no LOS restrictions.
3. There must be an initial period with no offmaps, so you can't arty the spawn area.
4. There must be a significant lag between the warning and when the first shell strikes. So people have to plan ahead when using it and can't use it as an I win button, or instant kill vet unit button)
5. Offcourse this might be impossible to program.
The next question would be how would you diversify the off maps between doctrines and faction? But I guess you can cross that bridge when you get to it.
Poll
