Box2D - FRIM (Frame Rate Independent Movement) - actionscript-3

Question

I've been trying to implement a FRIM system for my game for the last couple of days.I did some research and came upon this article - it seemed simple enough to implement so I got started.
Everything seems to work fine except I get some temporal aliasing (moving bodies seem to jump ahead a bit) - this happens when more Box2D steps are processed....I think.
private const FIXED_TIMESTEP:Number = 1 / 60;
private const velocityIterations:int = 8;
private const positionIterations:int = 3;
private var fixedTimestepAccumulator:Number = 0;
private var fixedTimestepAccumulatorRatio:Number = 0;
public function Step(dt:Number):void
{
//dt - time between frames - I'm passing the e.passedTime - from the enter frame event; using Starling
fixedTimestepAccumulator += dt;
var nSteps:uint = Math.floor(fixedTimestepAccumulator / FIXED_TIMESTEP);
if (nSteps > 0)
{
fixedTimestepAccumulator = fixedTimestepAccumulator - nSteps * FIXED_TIMESTEP;
}
fixedTimestepAccumulatorRatio = fixedTimestepAccumulator / FIXED_TIMESTEP;
var nStepsClamped:int = Math.min(nSteps, MAX_STEPS);
for (var i:int = 0; i < nStepsClamped; ++i)
{
resetSmoothStates();
singleStep(FIXED_TIMESTEP);
}
world.ClearForces();
smoothStates();
}
private function resetSmoothStates():void
{
for (var bb:b2Body = world.GetBodyList(); bb; bb = bb.GetNext())
{
if (bb.GetUserData() is MyUserData && bb.GetType() != b2Body.b2_staticBody )
{
//each of my bodies have a reference to their sprite (actor) in userData
var _userdata:MyUserData=bb.GetUserData();
_userdata.x = _userdata.bodyPreviousX = bb.GetPosition().x * RATIO;
_userdata.y= _userdata.bodyPreviousY = - bb.GetPosition().y* RATIO;
_userdata.rotation = _userdata.bodypreviousRotation= _userdata.bodypreviousRotation = - bb.GetAngle();
}
}
}
private function smoothStates():void
{
var oneMinusRatio:Number = 1.0 - fixedTimestepAccumulatorRatio;
for (var bb:b2Body = world.GetBodyList(); bb; bb = bb.GetNext())
{
if (bb.GetUserData() is MyUserData && bb.GetType() != b2Body.b2_staticBody )
{
var userdata=bb.GetUserData();
userdata.x = (fixedTimestepAccumulatorRatio * bb.GetPosition().x * RATIO + oneMinusRatio * userdata.bodyPreviousX) ;
userdata.y = (- fixedTimestepAccumulatorRatio * bb.GetPosition().y * RATIO + oneMinusRatio * userdata.bodyPreviousY) ;
userdata.rotation = (- fixedTimestepAccumulatorRatio * bb.GetAngle() + oneMinusRatio * userdata.bodypreviousRotation);
}
}
}
private function singleStep(dt:Number):void
{
Input();
world.Step(dt, velocityIterations, positionIterations);
}
What am I doing wrong?
Any help, suggestions would be highly appreciated.
Thanks

Answer 1

I did this once in a game I was working on so that I could lockstep the game to a certain update rate (this was in iOS). This was based on the code in Daley's book (Learning iOS Game Programming), which was based on some other article in the web (I believe, it may be Allen Bishop's). The code looked like this:
void GameManager::UpdateGame()
{
const uint32 MAXIMUM_FRAME_RATE = Constants::DEFAULT_OBJECT_CYCLES_PER_SECOND();
const uint32 MINIMUM_FRAME_RATE = 10;
const uint32 MAXIMUM_CYCLES_PER_FRAME = (MAXIMUM_FRAME_RATE/MINIMUM_FRAME_RATE);
const double UPDATE_INTERVAL = (1.0/MAXIMUM_FRAME_RATE);
static double lastFrameTime = 0.0;
static double cyclesLeftOver = 0.0;
double currentTime;
double updateIterations;
currentTime = CACurrentMediaTime();
updateIterations = ((currentTime - lastFrameTime) + cyclesLeftOver);
if(updateIterations > (MAXIMUM_CYCLES_PER_FRAME*UPDATE_INTERVAL))
{
updateIterations = MAXIMUM_CYCLES_PER_FRAME*UPDATE_INTERVAL;
}
while (updateIterations >= UPDATE_INTERVAL)
{
// DebugLogCPP("Frame Running");
updateIterations -= UPDATE_INTERVAL;
// Set the random seed for this cycle.
RanNumGen::SetSeed(_cycleManager->GetObjectCycle());
// Dispatch messages.
_messageManager->SendMessages();
// Update all entities.
_entityManager->Update();
// Update the physics
_gameWorldManager->Update(Constants::DEFAULT_OBJECT_CYCLE_SECONDS());
// Advance the cycle clock.
_cycleManager->Update();
}
cyclesLeftOver = updateIterations;
lastFrameTime = currentTime;
}
I can't put my finger on the specific item that is wrong in yours. However, this part is suspicious:
var nStepsClamped:int = Math.min(nSteps, MAX_STEPS);
Before this, you updated your accumulator with:
fixedTimestepAccumulator = fixedTimestepAccumulator - nSteps * FIXED_TIMESTEP;
But now the actual number of steps you are going to execute may be different because of the clamping (nStepsClamped). So your time accumulation is different than the number of steps you actually execute.
Was this helpful?

Answer 2

I decided to go with another approach. I'm using filtered delta times for the physics (I know this can cause some problems).
Here's what I'm doing now:
//Play around with this filter value if things don't look right
var filter:Number=0.4;
filtered_dt= time_between_frames * filter + filtered_dt * (1 - filter);
//Poll imputs and apply forces
// I use velocityIterations =6, positionIterations=3
world.Step(filtered_dt, velocityIterations, positionIterations);
//move sprites here
world.ClearForces();
Another thing you have to do is to scale the forces you apply to your bodies using filtered_dt so things don't "explode" when the frame rate changes a lot.
Hope this helps some one else ... it's not the perfect solution but it works for me.
If you have slow moving bodies the interpolation method above should work fine too.