collision detection help?

Axis Aligned Collision Detection

If you are only placing buildings on a grid that is aligned with the global axes, then the collision detection is quite simple. All you need to check is if there is a collsion on each axis. If there is an overlap on all of the axes, then you can say that the two bounding boxes collide.

function checkForCollision(model1, model2)
    local collided = false
    local x, y, z = false, false, false
    local size1, size2 = model1.PrimaryPart.Size, model2.PrimaryPart.Size
    local pos1, pos2 = model1.PrimaryPart.Position, model2.PrimaryPart.Position
    if (pos1.X < pos2.X + size2.X and pos1.X + size1.X > pos2.X) or (pos1.X > pos2.X + size2.X and pos1.X + size1.X < pos2.X) then
        x = true
        -- X Axis Collision
    end
    if (pos1.Y < pos2.Y + size2.Y and pos1.Y + size1.Y > pos2.Y) or (pos1.Y > pos2.Y + size2.Y and pos1.Y + size1.Y < pos2.Y) then
        y = true
        -- Y Axis Collision
    end
    if (pos1.Z < pos2.Z + size2.Z and pos1.Z + size1.Z > pos2.Z) or (pos1.Z < pos2.Z + size2.Z and pos1.Z + size1.Z > pos2.Z) then
        z = true
        -- Z Axis Collision
    end
    if x and y and z then
        collided = true
    end
    return collided
end

In this example I used a PrimaryPart in each of the models that had completely encased the entire model, such that all parts were physically inside of the PrimaryPart and the outermost portions were lined up with the faces of the PrimaryPart. You could change it to use the method of model GetExtentsSize in order to determine the size, and find the position else how, but this seemed like to easiest way to do it for me.

This can be simplified down to use only one if statement, but I figure it's easier to visualize what all the different conditions are actually representing this way.

Collisions and Arbitrary Rotations

Detecting collisions starts to become a quite a bit more difficult when your bounding boxes are rotated at some arbitrary angle in each axis. It isn't as simple as a check which axis have overlaps, as both parts could have their own separate local axes.

The easiest way to do this, is probably by using the property of BasePart GetTocuchingParts. You'll just have to look through the parts and see if anything that should collide with it is touching it.

However, you should note that this method only captures parts that have CanCollide set to true.