Simple Collision Detection

ruxpin · November 2011

I was looking for a way to check if two sprites had collided without getting into Box2d stuff. I came up with this solution after a little bit of searching/trial and error. It checks if the sprites' rectangles are overlapping. Maybe something like this can be implemented in the engine to speed up the checks?

Put this code in your init.lua file so all Sprites can use it.

--Input (sprite2): another sprite to check for collision detection
--Return: return true if the rectangles overlap otherwise return false
function Sprite:collision(sprite2)
 
	-- self bottom < other sprite top
	if self:getY() + self:getHeight() < sprite2:getY() then
		return false
	end
	-- self top > other sprite bottom
	if self:getY() > sprite2:getY() + sprite2:getHeight() then
		return false
	end
	-- self left > other sprite right
	if self:getX() > sprite2:getX() + sprite2:getWidth() then
		return false
	end
	-- self right < other sprite left
	if self:getX() + self:getWidth() < sprite2:getX() then
		return false
	end
 
	return true
end

atilim · November 2011

Thank you for the contribution

Right now this way is the best way. We'll consider embedding collision detection or geometry engine into Gideros API.

soy_ismael · May 2012

Please, could you give an example of its use? I'm starting

Thank!

alexzheng · May 2012

take care of the anchor point for a bitmap.

MikeHart · May 2012

if (yoursprite1:collision(yoursprite2) = true) then
    -- a collision has happened, do something
end

Mells · May 2012

take care of the anchor point for a bitmap.

Talking about the anchor point, do people usually work with it set at top left for Sprites and center for Bitmaps (mix)? Or do you prefer to work with all anchors set at the same point and would it be top left or center?
(Even though it's not possible for Sprites right now but I believe @atilim said that support is coming soon.)

chipster123 · May 2012

if (yoursprite1:collision(yoursprite2) = true) then
    -- a collision has happened, do something
end

Shouldn't it be "== true" ?

evs · May 2012

Actually you can simplify it to

 
if yoursprite1:collision(yoursprite2) then
    -- a collision has happened, do something
end

cheers

evs

phongtt · May 2012

I prefer "collideWith" to "collision"

Garyk1968 · May 2012

Where do you get all the method details from?

I asked on another thread about certain methods for TileMap which I couldn't find in the reference manual and was told because TileMap descends from Sprite it inherits the methods. Thats fine but I dont see a class diagram anywhere so not sure how I discover all these goodies!

Cheers

Gary

ar2rsawseen · May 2012

If you look at documentation:
http://www.giderosmobile.com/documentation/reference_manual.html

You can notice things like:
Sprite > EventDispatcher meaning Sprite inherits all EventDispatcher methods
TileMap > Sprite meaning TileMap inherits all Sprite methods and EventDispatcher methods

Garyk1968 · May 2012

Thanks ar2!

kontinyu · July 2012

Put this code in your init.lua file so all Sprites can use it.

--Input (sprite2): another sprite to check for collision detection
--Return: return true if the rectangles overlap otherwise return false
function Sprite:collision(sprite2)
 
	-- self bottom < other sprite top
	if self:getY() + self:getHeight() < sprite2:getY() then
		return false
	end
	-- self top > other sprite bottom
	if self:getY() > sprite2:getY() + sprite2:getHeight() then
		return false
	end
	-- self left > other sprite right
	if self:getX() > sprite2:getX() + sprite2:getWidth() then
		return false
	end
	-- self right < other sprite left
	if self:getX() + self:getWidth() < sprite2:getX() then
		return false
	end
 
	return true
end

Well i've been using this method and it's working fine
BUT since i'm using it on a lots of objects, it's slowing my game down.
Is there any better way? Or API implementation is anywhere close on the roadmap?

ar2rsawseen · July 2012

Well, now let's see if any lua optimization really increases the performance or not.

Will something like this work better?

--Input (sprite2): another sprite to check for collision detection
--Return: return true if the rectangles overlap otherwise return false
function Sprite:collision(sprite2)
 	local myX = self:getX()
 	local myY = self:getY()
 	local myWidth = self:getWidth()
 	local myHeight = self:getHeight()
 	local otherX = sprite2:getX()
 	local otherY = sprite2:getY()
 	local otherWidth = sprite2:getWidth()
 	local otherHeight = sprite2:getHeight()
	-- self bottom < other sprite top
	if myY + myHeight < otherY then
		return false
	end
	-- self top > other sprite bottom
	if myY > otherY + otherHeight then
		return false
	end
	-- self left > other sprite right
	if myX > otherX + otherWidth then
		return false
	end
	-- self right < other sprite left
	if myX  + myWidth < otherX then
		return false
	end
 
	return true
end

Maybe combining in one "OR" statement will also work faster?

kontinyu · July 2012

oh i did the variable thingy. it didn't help.
but i don't know how to implement any "OR"s ?

Scouser · July 2012

I think @ar2rsawseen meant something like this when he mentioned using OR statements

--Input (sprite2): another sprite to check for collision detection
--Return: return true if the rectangles overlap otherwise return false
function Sprite:collision(sprite2)
 	local myX = self:getX()
 	local myY = self:getY()
 	local myWidth = self:getWidth()
 	local myHeight = self:getHeight()
 	local otherX = sprite2:getX()
 	local otherY = sprite2:getY()
 	local otherWidth = sprite2:getWidth()
 	local otherHeight = sprite2:getHeight()
	-- self bottom < other sprite top or self top > other sprite bottom
	if myY + myHeight < otherY or myY > otherY + otherHeight then
		return false
	end
	-- self left > other sprite right or self right < other sprite left
	if myX > otherX + otherWidth or myX  + myWidth < otherX then
		return false
	end
 
	return true
end

ar2rsawseen · July 2012

Yeah, but I guess that also won't do any good.
Sprite:getBounds maybe?\
I wonder what is the slowest statement in this function

victor_wujitouch · July 2012

Thanks for the code. I found (as alexzheng mentioned) that you have to be take the anchor point on the Bitmap into consideration. Then I found the post below that uses Sprite:getBounds(). Using this function returns the correct bounds relative to the stage, solving the anchor point problem.

This function should be added back to the class reference documentation.

http://www.giderosmobile.com/forum/discussion/1279/how-do-i-get-the-bounds-of-a-sprite

--[[
	Input (sprite2): another sprite to check for collision detection
	Return: return true if the rectangles overlap otherwise return false
]]
function Sprite:collidesWith(sprite2)
 
	local x,y,w,h = self:getBounds(stage)
	local x2,y2,w2,h2 = sprite2:getBounds(stage)
 
	-- self bottom < other sprite top
	if y + h < y2 then
		return false
	end
	-- self top > other sprite bottom
	if y > y2 + h2 then
		return false
	end
	-- self left > other sprite right
	if x > x2 + w2 then
		return false
	end
	-- self right < other sprite left
	if x + w < x2 then
		return false
	end
 
	print('self bounds:',x,y,w,h,' sprite2 bounds:',x,y,w,h)
 
	return true
end

kontinyu · July 2012

""Sprite:getBounds maybe?""

why there is no such thing in api?

kontinyu · July 2012

Thanks for the code. I found (as alexzheng mentioned) that you have to be take the anchor point on the Bitmap into consideration. Then I found the post below that uses Sprite:getBounds(). Using this function returns the correct bounds relative to the stage, solving the anchor point problem.

This function should be added back to the class reference documentation.

--[[
	Input (sprite2): another sprite to check for collision detection
	Return: return true if the rectangles overlap otherwise return false
--]]
function Sprite:collidesWith(sprite2)
 
	local x,y,w,h = self:getBounds(stage)
	local x2,y2,w2,h2 = sprite2:getBounds(stage)
 
	-- self bottom < other sprite top
	if y + h < y2 then
		return false
	end
	-- self top > other sprite bottom
	if y > y2 + h2 then
		return false
	end
	-- self left > other sprite right
	if x > x2 + w2 then
		return false
	end
	-- self right < other sprite left
	if x + w < x2 then
		return false
	end
 
	print('self bounds:',x,y,w,h,' sprite2 bounds:',x,y,w,h)
 
	return true
end

wow, that works clearly faster than the previous.
everyone should use this code over the old one

and ofc this should be added to documentation.

GregBUG · July 2012

... and you can speed-up things even more (about 7%-10% faster) with this implementation (not mine; only converted):

function Sprite:collidesWith(sprite2)
	local x,y,w,h = self:getBounds(stage)
	local x2,y2,w2,h2 = sprite2:getBounds(stage)
 
	return not ((y+h < y2) or (y > y2+h2) or (x > x2+w2) or (x+w < x2))
end

ar2rsawseen · July 2012

@GregBUG this should go straight into FAQ

Tom2012 · September 2012

Thanks GregBUG for posting the code.

One question though - if I wanted to shrink the hitbox so that there was a padding of say 5 pixels around the outside, how would I do that?

Thanks

GregBUG · September 2012

@Tom2012

try this...

taken from my upcoming collision lib (this is original prototype lua code... it's quite fast)

my next lib will be C++ native and for this example/collision check is about 30% faster...

anyway...

local cHandle = { x = 0, y = 0 }
 
-- set Anchor Point
function setCollisionAnchorPoint(x, y)
    cHandle.x = x
    cHandle.y = y
end
 
-- get Anchor Point
function getCollisionAnchorPoint()
    return cHandle.x, cHandle.y
end
 
function boxToBox(spriteAX, spriteAY, widthA, heightA, spriteBX, spriteBY, widthB, heightB)
    -- Check for vertical GAP
    local Ay1, By1 =  spriteAY - (heightA * cHandle.y ), spriteBY - (heightB * cHandle.y ) 
    if Ay1 + heightA < By1 then
        return false
    end
    if Ay1 > By1 + heightB then
        return false
    end
 
  -- Check for horizontal GAP
    local Ax1, Bx1  = spriteAX - (widthA * cHandle.x ), spriteBX - (widthB * cHandle.x ) 
    if Ax1 > Bx1 + widthB then
        return false
    end
    if Ax1 + widthA < Bx1 then
        return false
    end
    -- Boxes Collide...
    return true
end

you can set width and height and also set collision handle so...

let me know...

OZApps · October 2012

The Bitmaps do not have a getBounds method, do they?

ar2rsawseen · October 2012

@OZApps Bitmap is extended from Sprite, and Sprite has getBounds method, so yeah, Bitmap also has it

GregBUG · October 2012

i have not used getBounds because in c plugin version i must to call
form c code a "Gideros lua" function (getBounds) and get back result to c code
....but is it's very slow...

but yes in this lua version you can use getBounds...

Tom2012 · October 2012

Hey GregBUG - nice script. Do you have an example on how to use the function?

Thanks

Tom

Cyberience · January 2013

Is there a way to do this for odd shapes? Like a Star. or TRiangle?

hgvyas123 · January 2013

Is there a way to do this for odd shapes? Like a Star. or TRiangle?

i don't think so unless you go with the physics

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