What's the right way to move a BODY ?

RogerT · February 2013

Hi guys,
Say I have a world with no gravity and have a body, that I want to move in a direction it is facing.

So far I used

local ang = body:getAngle()
local x, y = body:getPosition()

x = x - math.sin(ang)*speed
y = y + math.cos(ang)*speed

body:setPosition(x,y)

on every frame. Yet it appears that setPosition is not the best solution when it bumps into other objects, it tries to go through them and look rather unnatural.

Please suggest, what method should I use to move this body? some applyForce or something?

Thanks

Scouser · February 2013

Have you tried something like:

local ang = body:getAngle()
 
local vx = -math.sin(ang)*speed
local vy = math.cos(ang)*speed
 
body:setLinearVelocity(vx, vy)

I'm not 100% sure but I think this is probably what you require

RogerT · February 2013

Yes, looks like setLinearVelocity is what I need. ) THanks

twisttap · February 2013

@RogerT, @Scouser In my opinion both setPosition and setLinearVelocity are not good ways to move a body because in real world and with real physics, it is impossible to move a body in a way like those two functions do.
I think using impulses or applyForce works best and seems more real.
Below is something I use with TNTpad

 
function main:leftJoy(e)
local actor = assets.Actors["Player"].body   -- Get actor body from asset pool
local xVel, yVel = actor:getLinearVelocity()  -- Get Current Velocity
local force = 0
local desiredSpeed = 0
	if e.data.power > 0.7 then
 
			local direction = 1
			if (math.cos(e.data.angle) < 0) then
				desiredSpeed = self:speedMax(actor, -1, -10)			
			else
				desiredSpeed = self:speedMax(actor, 1, 10)
			end
 
 
	elseif e.data.power == 0 then
		desiredSpeed = 0
	else
		desiredSpeed = 0
	end
 
local velchange = desiredSpeed - xVel
force = actor:getMass() * velchange / (1/60)
actor:applyForce(force, 0 , actor:getWorldCenter() )
 
end
 
-- This is a function to increase bodys speed to the maxValue in the specified steps.
function main:speedMax(_body, _step, _maxValue)
	local velx, vely = _body:getLinearVelocity()
	if (math.abs(velx) < math.abs(_maxValue)) then
		velx = velx + _step	
	end
	return velx
end

Cheers

techdojo · February 2013

Damm all these sensible answers, I was going to make some glib comment about waiting till it's really dark and then getting two trustworthy friends, a large piece of canvas, a pickup truck and some good shovels, but it's too late now

)

ar2rsawseen · February 2013

agree with @twisttap I use setLinearVelocity only when I have a moving body and I need to destroy for some reason (for example to change a shape or size) so I get velocity (angular and linear) and then set it to a new body so it would continue to move as if nothing happened

@techdojo don't worry, if I had to move a body, I'd do it your way!

plamen · February 2013

There are some important things to consider. 1. What kind of body is it? Dynamic, kinematic , static. They have different properties and some of them are not simulated other are partial and some are fully simulated. If you set position of dynamic body with body:setPosition() you override all the simulations made by box2d. According to box2d docs the best way to move dynamic bodies is to use applyLinearVelocity or applyLinearImpulse both give natural looking motion. Kinematic bodies are moved by setting their velocity. Dynamic by the forces and static only by their coordinates.

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