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Circle collisions with physics - Gideros Forum

Circle collisions with physics

totebototebo Member
edited August 2017 in General questions
I'm planning to use Box2D to create a game with only circle collisions. Hundreds of circles all changing sizes all the time.

Box 2D is not ideal because you can't resize fixtures. You have to remove them and recreate them. This is slow. Pooling is unlikely to work, because there will be hundreds of different sizes.

Is there an alternative physics engine that allows changing size of a circle collision object?
My Gideros games: www.totebo.com

Comments

  • Here is a quick (and dirty, oh so dirty) test, which went slightly better than I thought. The limit seems to be around 100 circles. Any more than that and it starts lagging on slower devices.

    Open for suggestions on how to make this more efficient!
    -- Localize math functions
    local random = math.random
     
    -- Start up Box2D
    require "box2d"
    local world = b2.World.new( 0, 10, true )
    local sprite_debug_draw = b2.DebugDraw.new()
    world:setDebugDraw( sprite_debug_draw )
    stage:addChild( sprite_debug_draw )
     
     
    -- Create walls to contain the circles
    local body_floor = world:createBody{ type = b2.STATIC_BODY }
    local poly = b2.PolygonShape.new()
    poly:setAsBox(400, 10)
    local fixture_definition1 =	{
    								shape = poly,
    								density = 1.0, 
    								friction = 0.1,
    								restitution = 0.9
    							}
    local fixturez = body_floor:createFixture( fixture_definition1 )
    body_floor:setPosition( 0,400 )
    local body_left_wall = world:createBody{ type = b2.STATIC_BODY }
    local poly = b2.PolygonShape.new()
    poly:setAsBox(10, 400)
    local fixture_definition1 =	{
    								shape = poly,
    								density = 1.0, 
    								friction = 0.1,
    								restitution = 0.9
    							}
    local fixturez = body_left_wall:createFixture( fixture_definition1 )
    body_left_wall:setPosition( 0,0 )
    local body_right_wall = world:createBody{ type = b2.STATIC_BODY }
    local poly = b2.PolygonShape.new()
    poly:setAsBox(10, 400)
    local fixture_definition1 =	{
    								shape = poly,
    								density = 1.0, 
    								friction = 0.1,
    								restitution = 0.9
    							}
    local fixturez = body_right_wall:createFixture( fixture_definition1 )
    body_right_wall:setPosition( 300,0 )
     
    local bodies = {}
    local fixtures = {}
     
    -- Destroy and recreate a fixture
    local resizeFixture =
    	function( body, fixture )
     
    		if fixture then
    			body:destroyFixture( fixture )
    			fixtures[body] = nil
    		end
     
    		if body.radius > 40 then
    			body.size_diff = -1
    		elseif body.radius < 1 then
    			body.size_diff = 1
    		end
     
    		body.radius = body.radius + body.size_diff
     
    		local circle = b2.CircleShape.new( 0, 0, body.radius )	
     
    		local fixture_definition =	{
    										shape = circle,
    										density = 1.0, 
    										friction = 0.1,
    										restitution = 0.2
    									}
     
    		local fixture = body:createFixture( fixture_definition )
    		fixtures[body] = fixture
     
    	end
     
     
    -- Create 100 bodies, each with one circle fixture
    for i=1, 100 do
     
    	local body = world:createBody{ type = b2.DYNAMIC_BODY }
    	body.radius = random(40)
    	body.size_diff = 1
    	body:setPosition( random(300),random(300) )
    	resizeFixture( body )
     
    end
     
    -- Resize all fixtures
    local resizeFixtures =
    	function()
    		for body,fixture in pairs(fixtures) do
    			resizeFixture( body,fixture )
    		end
    	end
     
    -- Gameloop
    local enterFrame =
    	function()
     
    		resizeFixtures()
    		world:step(1/60, 8, 3)
     
    	end
    stage:addEventListener(Event.ENTER_FRAME, enterFrame )
    My Gideros games: www.totebo.com
  • I suppose it depends on how these circles will move and stuff. Will all circles always be moving? If not then you can make some sleep so they aren't needing to do collision checking. The real problem however is when you get too many circles onscreen at one time, that's when the CPU grinds to a halt :D
  • HubertRonaldHubertRonald Member
    edited August 2017
    Hi @totebo if you're thinking make a game like agar.io
    Here there is a sample on javascript

    https://codepen.io/adrian_po_11235/pen/NqXyvq

    Maybe you can take some ideas from there

    In the case Box2D I had a similar problem than you (because I needed 700 circles per level ) so like @antix said you, you can make some sleep.. if these isn't on your screen for example like a google maps (if you're thinking multiplayer) only wake up circles those than you need show on your screen
  • Cheers guys! Yeah, like Agario, but with physics. I also think the key will be to disable stuff outside of view, good call.
    My Gideros games: www.totebo.com
  • @HubertRonald, nice link, that's pretty cool. Basically that's acting just like Bump.lua :)

    With my own prototype that used circles I used bump.lua. Each frame I would use queryRect() to get the circles in view and process those. Each time a circle left the screen I would start a counter and after 2 seconds it would go to sleep and not be processed if it did not come back onto the screen.

    This worked pretty good and I was able to have many circles (thousands). Unfortunately I could never figure out how to stop circles clumping together.

    And again there was the an issue when too many circles came onscreen :(
  • Thousands! That's advanced.

    Box2D is pretty good at stopping overlapping objects, I suppose that's what a physics engine is for. As long as you don't create objects (fixtures) on top of each other you're pretty safe.

    Too many circles on screen would be a problem, unless their behaviour were predictable and simple, in which case you could use PARTICLES!
    My Gideros games: www.totebo.com
  • Particles would be good but I don;t think they are very good at resizing on the fly, which is what you wanted right?
  • piepie Member
    non physic Particles can be resized individually, but they won't handle collisions http://docs.giderosmobile.com/reference/gideros/Particles/setParticleSize
  • totebototebo Member
    edited August 2017
    You could technically use particles like sprites and control them individually. So may work with my game, but I have a feeling I would hit another bottleneck somewhere else, it doesn't feel like the "right thing to do".

    But I love a bit of a hack, so may just try that!
    My Gideros games: www.totebo.com
  • Here is something I figured out...
    function collision(x1,y1,r1,x2,y2,r2)
    	local x,y,r=x2-x1,y2-y1,r1+r2
    	if ((x*x)+(y*y))<(r*r) then
    		return true
    	else
    		return false
    	end
    end</pre>
  • Just modded my old code to now use particles:
    application:setBackgroundColor(0)
     
     
    frameRate=application:getFps()
     
    local bit = require("bit")
     
    local sub=string.sub
    local random,floor,sin,cos,pi,abs,rad,pi,sqrt=math.random,math.floor,math.sin,math.cos,math.pi,math.abs,math.rad,math.pi,math.sqrt
    local bnot,band,bor,bxor,lshift,rshift,rol,ror=bit.bnot,bit.band,bit.bor,bit.bxor,bit.lshift,bit.rshift,bit.rol,bit.ror
    math.randomseed(os.time())
     
    width=application:getContentWidth()
    height=application:getContentHeight()
    orientation=application:getOrientation()
     
    function collision(x1,y1,r1,x2,y2,r2)
    	local x,y,r=x2-x1,y2-y1,r1+r2
    	if ((x*x)+(y*y))<(r*r) then
    		return true
    	else
    		return false
    	end
    end
     
    particles=Particles.new()
    stage:addChild(particles)
     
    function circle(r,c1,c2,a)
    	p=particles:addParticles(0,0,-r,0,0)
    	if p then
    		particles:setParticleColor(p,c2,a)
    	end
    	return p
    end
     
    local frameCounter=0
    circles={}
     
    repeat
     
    	local x,y,r=random(0,width),random(0,height),random(2,20)
    	local collide=false
    	for loop=1,#circles do
    		local c=circles[loop]
    		if collision(x,y,r,c.x,c.y,c.r) then
    			collide=true
    			break
    		end
    	end
    	if not collide then
    		local c1,c2,c3=random(0x40,0xff),random(0x40,0xff)*0x100,random(0x40,0xff)*0x10000
    		local p=circle(100,0xffffff,c1+c2+c3,1)
    		if p then
    			circles[#circles+1]={}
    			local c=circles[#circles]
    			c.x=x
    			c.y=y
    			c.r=r
    			c.wr=r
    			c.particle=p
    			particles:setParticleSize(p,r*2)
    			particles:setParticlePosition(p,x,y)
    		end
    	end
     
    until #circles==100
     
    function gameLoop(event)
    	local skip=event.deltaTime*frameRate
    	if skip>1.5 then skip=2 else skip=1 end
     
    	local usage=(application:getTextureMemoryUsage()/1024)
    	if usage~=oldUsage then
    		oldUsage=usage
    		print("Texture usage: "..usage.."MB")
    	end
        local fps = 1/event.deltaTime  
    	if fps<40 then print("FPS: "..floor(fps))
     
    	end
     
    	frameCounter=frameCounter+1
     
    	for loop=1,#circles do
    		c=circles[loop]
    		local x,y,r=c.x,c.y+1,c.wr
    		if y>height then
    			y=height
    		end
    		local pos=false
    		if r>0 then
    			local collide=false
     
    			for loop2=1,#circles do
    				if loop~=loop2 then
    					local c2=circles[loop2]
    					if collision(x,y,r,c2.x,c2.y,c2.wr) then
    						local a,a2=pi*r*r,pi*c2.wr*c2.wr	
    						local d=20--abs(a-a2)/50
    						if aheight then
    					y=height
    				end
    				c.y=y
    				pos=true
    			end
    		end
    		local r=c.r
    		if r~=c.wr then
    			local d=(c.wr-r)/4
    			r=r+d
    			if r
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  • That's pretty darn sweet. One question I had about particles was their lifespan. If you don't specify a TTL, do they live forever?

    Likes: SinisterSoft

    My Gideros games: www.totebo.com
    +1 -1 (+1 / -0 ) Share on Facebook
  • yes, I even use them for the 3D stars and particle effects in RetroStar!
  • Makes sense. Performance wise there is just no comparison.
    My Gideros games: www.totebo.com
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  • @SinisterSoft could this be turned into a super-fast Gideros version of Bump, but with circles instead of axis-aligned rectangles? Your code is way to efficient for me to follow, but it feels like it's begging to be turned into a reusable class.
    My Gideros games: www.totebo.com
  • SinisterSoftSinisterSoft Maintainer
    edited August 2017
    It's not really that efficient. It still has a sqrt to find real distances. The easiest way to optimise things I've found is to look how people used to do this on integer based machines - like the amiga, etc in assembly language. There is usually some math trick to do things much faster than you would think.

    The collision is pretty simple & fast though - that could be done inline with the main code to speed things up.

    The x*x (square) is used quite a lot in my code. I'm sure we could add an new operator to Lua to do this really common operation - it would shave time off a lot of code all over the place as gideros wouldn't need to look up the variable twice - just once. Maybe this ^^var would work?

    Maybe ^% could be used to give a very fast LUT based sqrt ?

    so that would been to find a distance would be:

    distance=^%(^^dx+^^dy)

    Likes: antix

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