Spring Simulation

[TechnoCat]

I'm trying to make a simulation of a spring.
It is almost working, except that the spring skews about 2 pixels downward every oscillation. Anyone want to point out what is wrong?

http://love.pastebin.com/f62228309

Code: Select all

function load()

	ball={}
	ball.y=50
	ball.v=0
	gravity=30
	spring={}
	spring.y=200
	spring.top=200
	spring.k=200

end

function keypressed(key) 
	if key == love.key_tab then
		love.system.restart() 
	end
end

function update(dt)
	
	--local dt=dt*5
	ball.v=ball.v+gravity*dt
	ball.y=ball.y+ball.v*dt
	if ball.y>=spring.k then
		spring.y=ball.y
		ball.v=ball.v-(spring.k-spring.top)*dt
		spring.top=spring.top-ball.v*dt
		if spring.top>200 then
			spring.top=200
		end
	end

end

function draw()

	love.graphics.setColor(0,255,0)
		love.graphics.rectangle(love.draw_fill,85,spring.y,30,spring.top)
	love.graphics.setColor(255,0,0)
		love.graphics.circle(love.draw_fill,100,ball.y,10)

end

[Raider]

It moves down by one pixel; I think this is just an aliasing artifact.

[TechnoCat]

It moves down by one pixel; I think this is just an aliasing artifact.

I don't agree.

[Raider]

hrm.. aye, dunno for sure.

It only happens on the upwards stroke..

But it does *look* like aliasing at first.
Still looking then.

[TechnoCat]

I figured it out

Code: Select all

if ball.y>=spring.k then
	spring.y=ball.y
	ball.v=ball.v-(spring.k-spring.top)*dt
	spring.top=spring.top-ball.v*dt
	if spring.top>200 then
		spring.top=200
	end
end

needs to be:

Code: Select all

if ball.y>=spring.k then
	spring.y=ball.y
	spring.top=spring.top-ball.v*dt
	ball.v=ball.v-(spring.k-spring.top)*dt
end

ball.v needs to be updated after i use it to update spring.top. Otherwise the ball.v I used to calculate the new spring.top will be the next iterations velocity and not the current one.
I also didn't need to check if it was 200 or not anymore because it should function correctly now.

http://love.pastebin.com/f2dadd810 --I added some other things in, added a reference line.

EDIT: Now it shrinks!