5 FPS Interpolated Time

Share with us how are you using the powerrrr of the force

Moderator: Alain

5 FPS Interpolated Time

Postby agi_shi » Sun Jun 29, 2008 9:36 am

Slow-motion can be simulated by either stepping the physics at a lower frame rate (like my little video), or just stepping them less often. Either way, you'll get choppy results. Implementing what "fix your timestep" says... Result: http://www.youtube.com/watch?v=htjS8NKRaHg

Both boxes are running at only 5 FPS (dt = 1.0 / 5.0), except one box is running in interpolated time (extremely smooth regardless of physics FPS), and the other in exact time (updates at same rate as physics).

The implementation is extremely easy, I'll walk you through it:
Code: Select all
// pseudo-code

double physicsDeltatime = 1.0 / 5.0; // physics frequency, for example, 5 FPS
class physicsBody {
    // previous and current position/orientation
    vector p0, p1;
    quaternion q0, q1;

    // accumulation time
    double time;

    PhysicsSetTransform(matrix)
    {
        // store the "old" position/orientation
        p0 = p1;
        q0 = q1;

        // store the "current" position/orientation
        p1 = matrix.getPosition()
        q1 = matrix.getOrientation()

        // start interpolating at 0 again
        time = 0;
    }

    // per-frame stepping
    step(deltaTime)
    {
        // figure out how much we need to interpolate
        double interp = deltaTime / physicsDeltaTime;
       
        // add this to our interpolation
        time += interp;

        // lerp the position and slerp the orientation
        vector position = p0 + (p1 - p0) * time;
        quaternion orientation = quaterion::slerp(time, q0, q1);

        // set this as the graphical body
        setGraphicalBody(position, orientation);
    }
}

// game loop
while (...)
{
    // implement accumulator stepping
    physicsTime += deltaTime;
    while (physicsTime >= physicsDeltaTime)
    {
        NewtonUpdate(world, physicsDeltaTime;
        physicsTime -= physicsDeltatime;
     }

     // for all physics objects
     object->step(deltaTime); // per-frame graphical interpolation
}


That's really all there is to it. Very pseudo-code, but give a good math library it shouldn't be hard to implement.

Note: this lags 1 frame behind. The problem is that we're interpolating from the previous position to the current position, when the physics tell use we're already at the current position. This really isn't a problem unless you're running some 100% exact scientific simulation, in which case I'm sure you won't be running the physics at 5 FPS, or even 60 FPS for that matter ;).
agi_shi
 
Posts: 263
Joined: Fri Aug 17, 2007 6:54 pm

Postby Julio Jerez » Sun Jun 29, 2008 10:57 am

Thank Shy, thei is very use full for many peopel.
I will put a copy of this post to the FAQ
Julio Jerez
Moderator
Moderator
 
Posts: 10954
Joined: Sun Sep 14, 2003 2:18 pm
Location: Los Angeles

Postby Leadwerks » Sun Jun 29, 2008 12:37 pm

You can just interpolate the matrices, which might be less expensive than quaternion maths.
User avatar
Leadwerks
 
Posts: 515
Joined: Fri Oct 27, 2006 2:54 pm

Postby JernejL » Sun Jun 29, 2008 1:20 pm

I admit that looks incredibly impressive, that way you can run physics simulation at constant 25 / 30 fps while graphics subsystem can run with interpolated data at 60 fps, but running anything on as low as 5 is only good for testing, so it is not exactly a real world usage scenario (but it sure does look incredible).
Help improving the Newton Game Dynamics WIKI
User avatar
JernejL
 
Posts: 1441
Joined: Mon Dec 06, 2004 2:00 pm
Location: Slovenia

Postby agi_shi » Sun Jun 29, 2008 2:10 pm

Leadwerks wrote:You can just interpolate the matrices, which might be less expensive than quaternion maths.

How do you interpolate between two 4x4 matrices? Either way, my final calculations require the transformation in the form of vector & quaternion, so using a spherical interpolation comes practically for free since I already have the target quaternion.

Delfi wrote:admit that looks incredibly impressive, that way you can run physics simulation at constant 25 / 30 fps while graphics subsystem can run with interpolated data at 60 fps, but running anything on as low as 5 is only good for testing, so it is not exactly a real world usage scenario (but it sure does look incredible).

This usually comes in really handy with slow-motion. You can still run the physics at 60 FPS, but for slow motion you'll need to run the physics less frequently, causing the same choppiness (which is resolved using the interpolation). I just didn't really do the "real" slow motion in the video, instead I just took the easy route and ran the whole simulation at 5 FPS :lol: Technically, the results should be the same, other than obvious collision inaccuracies that are resultant from the 12x larger step in time.
agi_shi
 
Posts: 263
Joined: Fri Aug 17, 2007 6:54 pm

Postby Dave Gravel » Sun Jun 29, 2008 10:11 pm

Nice stuff Thanks.

I have a question.
I'm not sure if i'm wrong maybe you can make the test with your simulation to see if it happening for you too.
I think you can have a 13 milliseconds of delay when the vsync is enabled.
The video driver need around 13 milliseconds to apply the vsync.

Exemple you have a screen that running at 60hz with vsync enabled.
Imagine that you create a simulation and in this simulation you have a object that can accumulate 100 of speed.
Now imagine you fix the step to 60 internally.
When the vsync is not enabled you get high fps but the simulation running at 60.
Your object can get the 100 of speed normally,
imagine if you enable the vsync the simulation stay at 60 fps anyway but you lost around 13 milliseconds in the speed value.
Normally the deltatime is suposed to fix the delay but on my test it seen to lost the 13 milliseconds anyway.

Exemple without vsync the speed come to 100 normally but with the vsync the speed value seen to lose around 13 in value and come to around 87.

Maybe i'm totally wrong, thanks if you have time to test it in your simulation.

Edited:
Oh I think I have find from where the problem that I get coming.
It seen to come from the way that I use to update my 3d scene.
I have test a new method and now the problem seen to have go away.
Last edited by Dave Gravel on Sun Jun 29, 2008 11:40 pm, edited 1 time in total.
You search a nice physics solution, if you can read this message you're at the good place :wink:
OrionX3D Projects & Demos:
https://www.facebook.com/dave.gravel1
http://orionx3d.googlepages.com/
https://www.youtube.com/user/EvadLevarg/videos
User avatar
Dave Gravel
 
Posts: 690
Joined: Sat Apr 01, 2006 9:31 pm
Location: Quebec in Canada.

Postby Rob » Sun Jun 29, 2008 10:58 pm

Leadwerks wrote:You can just interpolate the matrices, which might be less expensive than quaternion maths.

This would cause shapes to scale at a greater degree as angle deltas increase, same reason it's generally not used in character animation interpolation. Could be LoD based, but in the grand scheme of things, doubt it'd make enough of a difference.
Rob
 
Posts: 7
Joined: Tue May 23, 2006 6:28 pm

Postby agi_shi » Mon Jun 30, 2008 8:04 am

k00m wrote:Maybe i'm totally wrong, thanks if you have time to test it in your simulation.

Edited:
Oh I think I have find from where the problem that I get coming.
It seen to come from the way that I use to update my 3d scene.
I have test a new method and now the problem seen to have go away.

I don't do the interpolation based on any velocity, so it's pretty much immune to the above issue. Instead, I simply interpolate between the current and previous physics frames. The problem here is that you get 1 frame lag (up to <physics frequency> milliseconds), but the visual smoothness is much worth it.
agi_shi
 
Posts: 263
Joined: Fri Aug 17, 2007 6:54 pm

Re: 5 FPS Interpolated Time

Postby Leadwerks » Fri Sep 05, 2008 12:12 am

you are right. i tried a linear interpolation of 4x4 mats. it was awful.
User avatar
Leadwerks
 
Posts: 515
Joined: Fri Oct 27, 2006 2:54 pm


Return to User Gallery

Who is online

Users browsing this forum: No registered users and 3 guests