What's the right way to move a BODY ?

RogerT

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

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

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

twisttap

@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

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

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

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.