Collision, need help.

[rokit boy]

Ok so I'm making collision in my game, here it is:

Code: Select all

function leftcollide(x1,y1,w1,h1,x2,y2,w2,h2)
	local i
	for i = 0, h1 - 1 do
		if y1 + i > y2 and y1 + i < y2 + h2 and x1 > x2 and x1 < x2 + w2 then
			return true
		end
	end
	return false
end

function rightcollide(x1,y1,w1,h1,x2,y2,w2,h2)
	local i
	for i = 0, h1 - 1 do
		if y1 + i > y2 and y1 + i < y2 + h2 and x1 + w1 > x2 and x1< x2 then
			return true
		end
	end
	return false
end

function upcollide(x1,y1,w1,h1,x2,y2,w2,h2)
	local i
	for i = 0, h1 - 1 do
		if x1 + i > x2 and y1 > y2 and y1 < y2 + h2 then
			return true
		end
	end
	return false
end

function downcollide(x1,y1,w1,h1,x2,y2,w2,h2)
	local i
	for i = 0, h1 - 1 do
		if x1 + i > x2 and y1 < y2 and y2 < y1 + h2 then
			return true
		end
	end
	return false
end

right and down work like a charm, but when I do left it also returns down and up, and up also returns left and right, can anyone help me?

[Roland_Yonaba]

Are you trying to make a bounding-box collision checks (AABB versus AABB) ?
Isn't this enough.

[rokit boy]

No, because i need to check which side is touched, this is the extra bit:

Code: Select all

if CheckCollision(self.x-16,self.y-16,32,32,ax,ay,32,32) and rightcollide(self.x-16,self.y-16,32,32,ax,ay,32,32) == true and self.x < ax then
						rightcollision = true
						leftcollision = false
						--print("right")
					end
					if CheckCollision(self.x-16,self.y-16,32,32,ax,ay,32,32) and upcollide(self.x-16,self.y-16,32,32,ax,ay,32,32) == true and self.y > ay then
						upcollision = true
						downcollision = false
						--print("up")
					end
					if CheckCollision(self.x-16,self.y-16,32,32,ax,ay,32,32) and downcollide(self.x-16,self.y-16,32,32,ax,ay,32,32) == true and self.y < ay then
						downcollision = true
						upcollision = false
						--print("down")
					end
					if CheckCollision(self.x-16,self.y-16,32,32,ax,ay,32,32) and leftcollide(self.x-16,self.y-16,32,32,ax,ay,32,32) == true and self.x > ax then
						leftcollision = true
						rightcollision = false
						--print("left")
					end

[kikito]

You may want to use a displacement vector then:

viewtopic.php?f=4&t=9341&p=57738

On the demo attached there, when the two boxes collide, a vector (the white line that appears in the screen) is generated. This vector can be used to check whether the collision was up,down,left or right, in a single operation.

[Roland_Yonaba]

Well, just a silly idea...
How's that working for you ?

Code: Select all

-- Bounding-box collision check
function CheckCollision(ax1,ay1,aw,ah, bx1,by1,bw,bh)
  local ax2,ay2,bx2,by2 = ax1 + aw, ay1 + ah, bx1 + bw, by1 + bh
  return ax1 < bx2 and ax2 > bx1 and ay1 < by2 and ay2 > by1
end

-- Gets sides where collision occurs, from the perspective of object placed at ax1,ay1
local function  GetSides(ax1,ay1,aw,ah, bx1,by1,bw,bh)
  if ChekCollision(ax1,ay1,aw,ah, bx1,by1,bw,bh) then
    local onx = ax1 < bx1 and 'right' or (ax1 > bx1 and 'left' or 'superposed')
    local ony = ay1 < by1 and 'down' or (ay1 > by1 and 'up' or 'superposed')
    return onx,ony
  end
end

Kikito's proposal is more robust, though.

[Qcode]

Not sure if this is any help for you, but I started learning collisions off of this tutorial: http://www.opentk.com/node/869. It's written in C# (I think) but I managed to translate it to Lua. The function ends up like this.

Code: Select all

function calculatemtd(v1, v2)
	local v1minx = v1.x --Min = top left
	local v1miny = v1.y
	local v2minx = v2.x
	local v2miny = v2.y
	local v1maxx = v1.x + v1.width -- Max = bottom right
	local v1maxy = v1.y + v1.height
	local v2maxx = v2.x + v2.width
	local v2maxy = v2.y + v2.height

	--K, we got our first set of values.

	local mtd = {} --Make our mtd table.

	local left = v2minx - v1maxx
	local right = v2maxx - v1minx
	local up = v2miny - v1maxy
	local down = v2maxy - v1miny
	if left > 0 or right < 0 or up > 0 or down < 0 then --Not colliding. This is the easy part.
		return false
	end

	if math.abs(left) < right then --Determine the collision on both axises? Axis'? I give up. On the x and y axis
		mtd.x = left
	else
		mtd.x = right
	end

	if math.abs(up) < down then
		mtd.y = up
	else
		mtd.y = down
	end
	if math.abs(mtd.x) < math.abs(mtd.y) then
		mtd.y = 0
	else
		mtd.x = 0
	end
	return mtd
end

(Of course you can specify the x, y, width, and height of each object in the parameters, but I prefer to just use 2 tables.)
Then you can do your colliding like this.
(For aabb it's just a bounding box check)

Code: Select all

function collide(v1, v2)
local collision = aabb(v1, v2)
local horizontal, vertical
local mtd
if collision then
	mtd = calculatemtd(v1, v2)
end

if mtd then
	if mtd.x ~= 0 then
		horizontal = true
	end
	if mtd.y ~= 0 then
		vertical = true
	end
end
if horizontal then
        v1.x = v1.x + mtd.x
        --At this point you should set your vertical speed to zero as well. For me it's xSpeed
        v1.xSpeed = 0
end
if vertical then
        v1.y = v1.y + mtd.y
        v1.ySpeed = 0
end

[Le Codex]

Code: Select all

-- Gets sides where collision occurs, from the perspective of object placed at ax1,ay1
local function  GetSides(ax1,ay1,aw,ah, bx1,by1,bw,bh)
  if ChekCollision(ax1,ay1,aw,ah, bx1,by1,bw,bh) then
    local onx = ax1 < bx1 and 'right' or (ax1 > bx1 and 'left' or 'superposed')
    local ony = ay1 < by1 and 'down' or (ay1 > by1 and 'up' or 'superposed')
    return onx,ony
  end
end

I don't quite understand what you're doing here. Why are there strings in if-statements ? What value do they represent? Are they always true ?

[Nixola]

Dude. That's a 4+ years necropost. Check the date of the last post before replying.

[zorg]

This is like the third time too... If you have a question, open a new thread.