How to do tile based collision

Question

I am trying to make a simple platform game, and obviously I need tile collision. The problem with the code I have so far is that it moves the character first, then checks to see if it is colliding with something, but sometimes it thinks its colliding at the wrong times depending on if I check the x-axis for collisions first or the y-axis first. Am I going about this the wrong way? Here's some code.

function checkCollision(val, axis, oldPos)
    if axis == "x" and char.tX then
        local tileX = math.ceil(val/absoluteTileSize)
        local tileY = math.floor(oldPos/absoluteTileSize)

        local tl, tr, bl ,br = getTouchingTiles(tileX, tileY)
        local isOnFlatSurface = math.abs(oldPos/absoluteTileSize-tileY) <= .00001--might not be a good i

        if isOnFlatSurface then
            if tr.canCollide then
                char.tX = nil
                char.x = tileX * absoluteTileSize - absoluteTileSize
            end
        else
            if br.canCollide then
                char.tX = nil
                char.x = tileX * absoluteTileSize - absoluteTileSize
            end
        end
    elseif axis == "y" then
        local tileX = math.ceil(oldPos/absoluteTileSize)
        local tileY = math.floor(val/absoluteTileSize)

        local tl, tr, bl ,br = getTouchingTiles(tileX, tileY)

        if bl.canCollide or br.canCollide then
            char.tY = nil
            char.y = tileY * absoluteTileSize --// - absoluteTileSize
            --/////////////idk why i don't need to subtract that but it works
        elseif not char.tY then--start falling if walk off something
            char.tY = love.timer.getTime()
            char.yi = char.y
            char.vyi = 0
        end
    end
end

score 0 · Answer 1 · answered Jun 02 '14 at 02:00

local tileX = math.ceil(val/absoluteTileSize)
local tileY = math.floor(oldPos/absoluteTileSize)

It seems strange that you would use math.ceil for the x values and math.floor for the y. This may be why you are getting some strange occurrences. I would recommend this little debugging trick that may help you:

-- Since you are using LÖVE, this is what you would use:
love.graphics.setColor( 255, 0, 0, 255 )
love.graphics.rectangle( 'line', ( tileX - 1 ) * absoluteTileSize, ( tileY - 1 ) * absoluteTileSize, absoluteTileSize, absoluteTileSize )
-- assuming absoluteTileSize represents the width/height of the tiles?

This would go at the end of your drawing function and would draw a red box in the "tile" your player is currently inside.

How to do tile based collision

1 Answers1