Why is the motion jerky in a simple 2D game?

Question

I've been exploring the possibility of using Unity to build a 2D game. I'm looking at a number of 2D helper libraries, including ex2D and Orthello. The problem is: When I run the example projects that come with these libraries, the motion of the sprites is jerky. (The problem exists with both of the libraries, and actually just with Unity in general - more on that below.)

Let me explain what I mean: Every 1-2 seconds, there's very subtle but noticeable "blip" in the motion of the graphics. The hiccup is like a metronome; the motion is perfectly smooth for 1 second, and then for 1 instant (a single frame probably), the sprite noticeably jerks very subtly. Then it's smooth again for 1 second, then a frame blip, etc., ad infinitum. The jerky movement is probably only a matter of a pixel or two; you have to watch carefully to see it.

I dug a little deeper and created a simple Unity game from scratch with an orthographic camera and cube (and nothing else -- no 2D libraries, no physics, no nothing), and sure enough, the motion is still jerky. Every 1-2 seconds, like clockwork, a hiccup occurs in the motion.

I come from a Microsoft XNA background, and I experienced a problem just like this with XNA once upon a time. The solution was to disable "FixedTimeStep" for the game, and then the game ran perfectly smoothly. Is there a similar setting in Unity? I found the Time settings, but there's seemingly no way to disable fixed timestep.

For those curious, here are the objects/code for my simple project that exhibits the jerky animation behavior:

Main Camera: Orthographic, size 2, position(0,0,-1)
Cube: position(0,0,0)

And then this C# script on the Cube:

 using UnityEngine;
 using System.Collections;

 public class CubeScript : MonoBehaviour
 {
     protected float min;
     protected float max;
     protected float speed;
     
     void Start ()
     {
         speed = -0.01f;
         min = transform.position.x - 2f;
         max = transform.position.x + 2f;
     }
 
     void Update ()
     {
         transform.position = new Vector3(
             transform.position.x + (Time.deltaTime * speed),
             transform.position.y,
             transform.position.z
         );
         
         if (transform.position.x < min || transform.position.x > max)
             speed *= -1;
     }
 }

EDIT:

As the initial responses correctly pointed out, in my original example code I completely forgot to multiply by Time.deltaTime. But here's the weird thing: Multiplying by Time.deltaTime actually makes the problem more pronounced! I just updated my code to include Time.deltaTime, since that doesn't seem to be what's causing my specific problem. (Regardless, thanks for your responses Eric5h5 and malraux.)

More information: I just discovered that building the game (instead of playing it in the Unity editor) and running it at "Graphics quality: Fastest" seems to eliminate the motion jitter. The problem still exists if I run it at "Good" quality or higher. My video card is an Nvidia 8800GT with up-to-date drivers. I know it's not the greatest card in the world, but it runs plenty of other 3D games at decent quality with no frame rate hiccups. And my example project is about as simple as you can get: a single untextured cube moving back and forth.

Any other thoughts?

Answer 1

Answer by picobots · Apr 20, 2012 at 10:53 PM

So, this problem has been plaguing me for weeks, and I finally just solved it... sort of. My project has gone through a number of changes, so this solution doesn't necessarily apply to the question exactly the way I asked it. But I'm going to post this information here anyway, because I've never seen this solution mentioned in any of the jitter/judder/jerky movement questions about Unity. Hopefully this solution will help someone else out.

First of all, I'm now using Physics and Rigidbodies. And I still had the motion judder/jitter problem with a Rigidbody and FixedUpdate. But if you're using Physics, this solution may help you:

It all comes down to one little setting on the Rigidbody: Interpolate.

http://unity3d.com/support/documentation/ScriptReference/Rigidbody-interpolation.html

Setting the Interpolate property to Interpolate makes my object move completely smoothly now. Hope that helps someone else.

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plingativator · Jun 28, 2013 at 05:42 AM 0

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Thank you! Interpolate was exactly what I needed. This was plaguing me for weeks, my player would jitter once reaching a higher velocity and I thought it was a problem with my speed limiting. I set the Interpolate on and now it's incredibly smooth.

Andrii · Jan 03, 2017 at 02:49 AM 0

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Wow! $$anonymous$$ade my game look much more smooth!

gantar22 · Apr 03, 2018 at 04:22 PM 0

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Thanks so much! There are so many different discussions about jittering online and this was the one that fixed it for me.

salranax · Apr 04, 2018 at 09:43 AM 0

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thanks m8. Exactly what i was looking for! It also makes parallax effects smooth. I have having issues with high follow speed parallax effects and setting interpolate on makes it really smooth. TY

TaiNiePooNie · Nov 20, 2018 at 07:53 PM 0

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This answer is GOLD!

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Answer 2

I am currently having this problem. I hadn't noticed it until I got further along in my project, with more and more units on my screen.

It does seem heavily vsync/monitor refresh rate related. I get little frame "blips"/jittering about every 2-4 seconds, and it's really noticeable. At my flatscreen lcd monitors refresh rate of 60 hz, it's pretty bad jittering. When I switch to 100 hz refresh rate on my monitor, it gets a lot smoother, although it doesn't eliminate the "blipping" completely -- it's still there.

The weird thing is that if I set my monitor refresh rate to 120 hz, it's almost as bad as at 60 hz. I am completely stumped. I've tried everything in this thread, and it helps smooth it out quite a bit, but again it's never completely eliminated. Even with setting my models rigidbodies to interpolate as suggested, and setting my monitors refresh rate to 100 hz, and setting vsync to off in my project settings.

Any leads as to how to fix this would be great. I know this is an old thread though. It is currently 8/15/2012 lol. Unity version 3.5.4f1. I'm beginning to think it's something the Unity programmers changed in the vsync option in one of the recent updates.

Update: I just tried building my game with settings on "fastest" in the project quality settings. Then selecting "fastest" in the selection tab upon starting my game up(with it built). This pretty much does work. I noticed almost no jittering/blipping -- possibly some very miniscule blipping, but almost unnoticeable. This is great! But the question is why? We shouldn't have to run our game on fastest in order to eliminate this terrible vsync blipping or whatever it is. Anyways, for now, at least there's one way to minimize it almost completely. Thanks for all the tips and advice so far.

Answer 3

I know this is an older thread, but I hope this helps somebody out:

I initially thought the jerkiness problem was related to Update() running in different threads by Unity, so I changed my objects to plain classes and update them manually from my "main", but this did not solve it.

The jerkiness problem is really related to variations in the frame rate, that's why -as far as I've seen- it happens regardless of the 'sync' scheme or Update vs FixedUpdate.

Let's say the last frame took for example 50ms, and you use that deltatime for your motion, but the actual current frame takes only 25ms to render, it will look to you as if the object jumped. This is particularly more evident if you have object 'seek' routines as I have, or if you're running lower framerates.

You can do this:

     Time.maximumDeltaTime=0.03; //<<just over your estimated average frame time.
     //or alternatively:
     t=Time.deltaTime;
     if(t>0.03){t=0.03;}//constrain it
     //Note that they also have Time.smoothDeltaTime, but it's not much help.

Now it's smooth as silk. What I'll probably do for a more robust solution is to keep a 1-second moving average of the frame rate, and then constrain 't' to that.

Edit: Sorry about the redundancy, I hadn't seen Deano's correct response.

Answer 4

i guess your game frame rate is default ( 30 FPS ). please study link below: Application.targetFrameRate , you can see your game frame rate by this component that i got from unity standard asset package :

 using System;
 using UnityEngine;
 using UnityEngine.UI;
 
 namespace UnityStandardAssets.Utility
 {
     [RequireComponent(typeof (Text))]
     public class FPSCounter : MonoBehaviour
     {
         const float fpsMeasurePeriod = 0.5f;
         private int m_FpsAccumulator = 0;
         private float m_FpsNextPeriod = 0;
         private int m_CurrentFps;
         const string display = "{0} FPS";
         private Text m_Text;
 
 
         private void Start()
         {
             m_FpsNextPeriod = Time.realtimeSinceStartup + fpsMeasurePeriod;
             m_Text = GetComponent<Text>();
         }
 
 
         private void Update()
         {
             // measure average frames per second
             m_FpsAccumulator++;
             if (Time.realtimeSinceStartup > m_FpsNextPeriod)
             {
                 m_CurrentFps = (int) (m_FpsAccumulator/fpsMeasurePeriod);
                 m_FpsAccumulator = 0;
                 m_FpsNextPeriod += fpsMeasurePeriod;
                 m_Text.text = string.Format(display, m_CurrentFps);
             }
         }
     }
 }

you can lock your frame rate to 60 FPS by this code :

     void Awake()
     {
         QualitySettings.vSyncCount = 0;  // VSync must be disabled
         Application.targetFrameRate = 60;
 
     }

Answer 5

Answer by malraux · Mar 16, 2012 at 02:20 PM

As Eric pointed out, use deltaTime:

 void Start ()
 {
     this.speed = -1f; // note I changed the name to reflect the usage
     min = transform.position.x - 2f;
     max = transform.position.x + 2f;
 }

 void Update ()
 {
     transform.position.x += this.speed * Time.deltaTime;

     if (transform.position.x < min || transform.position.x > max)
         this.speed *= -1;
 }

Note that I'm calling it speed now... deltaTime gives you the difference in time since the last update. Multiply that by a speed and you are interpolating the position value based on the speed, since speed (velocity) is distance over time.

Also, if you're using a RigidBody, make sure to use FixedUpdate instead of Update: MonoBehaviour.FixedUpdate

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picobots · Mar 16, 2012 at 03:15 PM 0

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Argh, yes, I had tested that previously but forgot it in this example code. Here's the weird thing: Correctly multiplying by deltaTime actually makes the problem more pronounced! I'm beginning to think that there must be some specific problem with my video card (Nvidia 8800GT) and fixed timesteps. It's truly bizarre. Regardless, thanks for your response!

malraux · Mar 16, 2012 at 04:31 PM 0

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@there$$anonymous$$ Did you try it in FixedUpdate?

picobots · Mar 16, 2012 at 05:10 PM 0

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Good question, I hadn't, but I just did. As it turns out, FixedUpdate produced the worst results of all: the motion jitter switches from once per second to pretty much constant. It was a worth a try though, thanks for the idea. As it turns out, it seems that having VSync enabled might be the culprit. (See my comment above.)

Why is the motion jerky in a simple 2D game?

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