Game Dev - Having trouble with AABB collision detection during jumps - lua

Question

My collision detection works by getting intersection between rectangles and reversing the effect. This is happening during each frame. It works great except in the case where the player is sitting on top of a corner and jumps. every once in a while the vertical intersection is greater than the horizontal intersection which makes my player slide down the side of the platform. Any suggestions?
-- detect initial collision
if mathy.hasCollided(player, platform) then
local playerBoundaries = player:boundaries()
-- list of intersections between platform and player
local bottomBoundary = mathy.bottomBoundary( playerBoundaries, platform )
local topBoundary = mathy.topBoundary( playerBoundaries, platform )
local rightBoundary = mathy.rightBoundary( playerBoundaries, platform )
local leftBoundary = mathy.leftBoundary( playerBoundaries, platform )
local smallestDist = ""
local smallestBoundary
local boundaries = {
bottom = bottomBoundary,
top = topBoundary,
right = rightBoundary,
left = leftBoundary
}
-- get the smallest intersection (thats the side we're probably closest to)
for direction, boundary in pairs(boundaries) do
if not smallestBoundary then
smallestBoundary = boundary
smallestDist = direction
end
if smallestBoundary > boundary then
smallestBoundary = boundary
smallestDist = direction
end
end
-- reverse effects depending on collision location
if smallestDist == "bottom" then
player.desiredPos:add(diffX, -bottomBoundary)
player.velocity.y = 0
player.onGround = true
elseif smallestDist == "top" then
player.velocity.y = 250
player.desiredPos:add(0, topBoundary)
elseif smallestDist == "right" then
player.desiredPos:add(-rightBoundary, 0)
elseif smallestDist == "left" then
player.desiredPos:add(leftBoundary, 0)
end
end

Answer 1

Its hard to tell from the short clip, but I think the issue is a result of checking for the smallest intersection instead of checking for the intersection in the direction of the object's velocity. You could try something like this in place of the smallestBoundary loop:
local boundary = nil
if (player.velocity.y > 0) then
boundary = topBoundary
elseif (player.velocity.y < 0) then
boundary = bottomBoundary
elseif (player.velocity.x > 0) then
boundary = rightBoundary
elseif (player.velocity.x < 0) then
boundary = leftBoundary
end
Of course, this isn't as robust as it can be. You might also try to combine the two approaches and do something like this in place of the smallestBoundary loop:
local yMod = math.abs(player.velocity.y)
local xMod = math.abs(player.velocity.x)
local topMod = player.velocity.y > 0 and yMod or 1
local bottomMod = player.velocity.y < 0 and yMod or 1
local rightMod = player.velocity.x > 0 and xMod or 1
local leftMod = player.velocity.x < 0 and xMod or 1
local boundaries = {
bottom = (bottomMod / MAX_VELOCITY) * bottomBoundary,
top = (topMod / MAX_VELOCITY) * topBoundary,
right = (rightMod / MAX_VELOCITY) * rightBoundary,
left = (leftMod / MAX_VELOCITY) * leftBoundary
}
for direction, boundary in pairs(boundaries) do
if not smallestBoundary then
smallestBoundary = boundary
smallestDist = direction
end
if smallestBoundary > boundary then
smallestBoundary = boundary
smallestDist = direction
end
end
This will do exactly what you're doing now, but will adjust the size of the boundary (in context of the comparison only) by the velocity of the player in that direction. So if you're moving in x with -5 and in y with -10, downward collisions in the y plane will have twice the weight of leftward collisions in the x plane. Of course, there's always the other option of adjusting the player in every plane of collision:
if bottomBoundary > 0 then
player.desiredPos:add(diffX, -bottomBoundary)
player.velocity.y = 0
player.onGround = true
end
if topBoundary > 0 then
player.velocity.y = 250
player.desiredPos:add(0, topBoundary)
end
if rightBoundary > 0 then
player.desiredPos:add(-rightBoundary, 0)
end
if leftBoundary > 0 then
player.desiredPos:add(leftBoundary, 0)
end
This last method would make the most sense, except you don't seem to handle collisions in all directions uniformly, for whatever reason, so it might not fit your architecture.
Keep in mind that I'm not familiar with the framework you're using, so this code might not work out of the box. Also, this post assumes that +y is up, -y is down, +x is right, and -x is left.