Merging Physics Objects

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egg651
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Merging Physics Objects

Post by egg651 » Sun Apr 22, 2012 4:32 pm

This weekend's Ludum Dare inspired me to get back to making games (tried once before ages ago in XNA but eventually fizzled out after being too ambitious), and love2d seems perfect for what I'm looking to achieve. My idea is for a game about planets which move around by ejecting mass and gain mass by smashing in to other planets/rocks - And this is where we get to my problem. What's the best way to implement planet growth? (Taking in to account I'm using love.physics for all my objects, and that currently they are all circular)

Source code can be found here, and the .love file is attatched.

Thanks in advance!

P.S: Apologies for any ugly code, hopefully I can still use the 'lol I'm a newbie idiot' excuse.
P.P.S: As a random side question: Why does it restart itself multiple times on launch?
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Inny
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Re: Merging Physics Objects

Post by Inny » Sun Apr 22, 2012 5:24 pm

It's "restarting" itself multiple times because you keep calling love.graphic.setMode. That function is for changing the window size or fullscreen dimensions, so only call it once, or supply a conf.lua file that sets the window size you want. See: https://love2d.org/wiki/conf.lua

As for merging two object, at the time of collision you'll basically want to calculate the area of the two objects, add the area of the smaller to the larger, and then recalculate the radius of the larger object from its new area. That formula is R=sqrt(A/pi)

iemfi
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Re: Merging Physics Objects

Post by iemfi » Sun Apr 22, 2012 5:32 pm

It looks like it's reloading because you use setMode 4 times! Do you know how to handle collisions? Wiki has a good tutorial on that. https://love2d.org/wiki/Tutorial:Physic ... nCallbacks

After you have the 2 objects colliding, you just need a function which merges them. You could simply use getMass() to find the smaller one and add the 2 masses together then use setMass() on the larger body and and setRadius() on the larger shape and destroy() on the smaller body. The hard part would be making it look good and making the smashing together look correct. For that you can play around with the restitution and maybe add your own forces when a collision is detected.

egg651
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Re: Merging Physics Objects

Post by egg651 » Sun Apr 22, 2012 8:00 pm

iemfi wrote:It looks like it's reloading because you use setMode 4 times! Do you know how to handle collisions? Wiki has a good tutorial on that. https://love2d.org/wiki/Tutorial:Physic ... nCallbacks
Inny wrote:It's "restarting" itself multiple times because you keep calling love.graphic.setMode.
Ah! It seems that I accidentally copied it when duplicating the code to create the walls. :oops:

As for the main problem - I've set up the world callbacks, and written a small function to merge two balls. The new problem is that if I destroy the smaller body, the program crashes as it tries to update the body that no longer exists, since it is still in dynamicobjects. Is there an easy way to remove it from that array or will I have to search through it looking for the right ball?

Ninja edit: Updated code here.

iemfi
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Re: Merging Physics Objects

Post by iemfi » Sun Apr 22, 2012 8:17 pm

I think you have to search through it, this is what I use.

Code: Select all

function deleteObject(tab,obj)
    for i,v in ipairs(tab) do
        if v==obj then
            table.remove(tab,i)
            return
        end
    end
end
ninja edit: The other way would be to keep track of the position of each ball in the table but I wouldn't bother with that unless you had a gazillion balls.

egg651
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Re: Merging Physics Objects

Post by egg651 » Sun Apr 22, 2012 9:14 pm

Yeah, I figured as much - Was just wondering if there was some magical lua/love2d feature I was unaware of.

Anyway, I've implemented a basic linear search as suggested to remove the items from the table and have been left with one last bug - The objects radii increase as they should, but the circle drawn to the screen remains the same size.

[Updated code]

Edit: Added .love file

Edit 2: Solved the problem by changing the following line in love.draw:

Code: Select all

love.graphics.circle("fill", ball.body:getX(), ball.body:getY(), ball.shape:getRadius())
To this:

Code: Select all

love.graphics.circle("fill", ball.body:getX(), ball.body:getY(), ball.fixture:getShape():getRadius())
I don't know why it works, but it does.
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tentus
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Re: Merging Physics Objects

Post by tentus » Mon Apr 23, 2012 2:39 am

egg651 wrote: Edit 2: Solved the problem by changing the following line in love.draw:

Code: Select all

love.graphics.circle("fill", ball.body:getX(), ball.body:getY(), ball.shape:getRadius())
To this:

Code: Select all

love.graphics.circle("fill", ball.body:getX(), ball.body:getY(), ball.fixture:getShape():getRadius())
Wow, that is... not lovely. The new Box2d API makes me sad. :(
Kurosuke needs beta testers

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slime
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Re: Merging Physics Objects

Post by slime » Mon Apr 23, 2012 1:34 pm

In box2d 2.2, shapes created by the user are prototypes. When you create a fixture and pass a shape in as an argument, box2d internally clones the shape and attaches the cloned shape to the fixture. I don't think that's unlovely, it's just.. different. It means that if your game uses a lot of the same shape with the same dimensions for different objects, you only need to create one and box2d will do the rest for you when you make fixtures. :P

egg651
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Re: Merging Physics Objects

Post by egg651 » Mon Apr 23, 2012 4:14 pm

Ah, I see. Using that knowledge, I've changed the ball creation code to this:

Code: Select all

function GenerateBalls(count)
	for i=0,count - 1 do
		ball = {}
		ball.body = love.physics.newBody(world, math.random(650), math.random(650), "dynamic")
		ball.fixture = love.physics.newFixture(ball.body, love.physics.newCircleShape(math.random(5, 60)) , planetdensity)
		ball.shape = ball.fixture:getShape()
		ball.fixture:setRestitution(planetrestitution)
		ball.r = math.random(255)
		ball.g = math.random(255)
		ball.b = math.random(255)
		ball.body:setMass(ball.shape:getRadius() * ball.fixture:getDensity())
		ball.body:setLinearDamping(0.75)
		ball.fixture:setUserData("ball")
		table.insert(dynamicobjects, ball)
	end
end
Which in turn allows me to return to the more sensible looking draw code. Thanks for all your help guys, looks like my problems are solved (for now). :awesome:

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