I'm making a map editor for further RTS game (2D), something like the first StarCraft Editor. Terrain tiles will be a rhombuses. So I'll probably just make an 2D array with intigers, that corresponds proper tiles. Then, after creating the terrain, you'd probably like to add some units and structures to map (especially if you're making a scenario). I'm not sure how do I check if the unit can be placed somewhere. I decided to make units almost independent from terrain tiles like in StarCraft (1), so the units are not placed in any grid. I'll give a simple image:
As you can see, those green tiles represent a passable area (grass, let's say), and a blue one impassable (for example water). That stickman in a box is our unit. You can place it only in green area. That means the box can't go outside the green rhombuses.
I have some ideas how to achieve this, but I'm not sure which one would be the most optimal.
1) Since I work in Unity, I could add a collider to every tile. That will be very simple to do and will probably have terrible optimalisation in maps 256x256.
2) Math. I can calculate if any of those boxes crosses tile's borders. If so, I'll have to check if all those tiles are passable. It won't be that easy to programme (well, for me at least) and... yeah, here comes my dillema. Am I right that it would have much better optimalisation? Is it worth the effort?
What about saving this map? I can easily save the array with tiles, but how can I do it with units? Just save their positions or is there a better way to do it?
EDIT: I've tested the first idea (I've written script to create 256x256 map - that will be probably the biggest, and to move a camera). The map generates quite long (comparing to tiles without collider), but after that there isn't much difference in performance (it doesn't mean it won't cause problems when I add more features than moving a camera ;)). I can also attach colliders only to impassable tiles, not all. This should improve the performance even more. Still not sure if that's a good idea.
EDIT 2: I've just realised how bad my idea was. Let me visualise this: So if you consider not the whole rectangle, but only a circle, it's not only easier to implement, but also more reasonable. You can now convert rhombus' vertices' position from isometric to cartesian like Bálint said:
CartX = ( 2 * IsoY + IsoX) / 2
CartY = ( 2 * IsoY - IsoX) / 2
You can also do it with circle's center point. Radius is equal to half of square (rhombus) side length OR sqrt(2) * square side length (when square is inscribed in circle).
So now I have a square grid and a circle. How can I check if all squares that this circle lays on are passable? What's the most optimal way to do it?