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18
Question by picobots · Mar 16, 2012 at 06:12 AM · 2dorthographic

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

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?

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avatar image Eric5h5 · Mar 16, 2012 at 02:04 PM 0
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Your code is frame-rate dependent; you need to use Time.deltaTime. Physics can't be completely turned off, but you can set the fixed timestep to 10, which is the maximum allowed. Although if you have no physics, that won't really make much difference.

avatar image picobots · Mar 16, 2012 at 03:14 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!

avatar image Eric5h5 · Mar 16, 2012 at 04:39 PM 1
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$$anonymous$$aybe turn vsync on.

avatar image picobots · Mar 16, 2012 at 05:06 PM 0
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O$$anonymous$$, the VSync setting definitely has an effect. With VSync on ("Every VBlank"), the stutter problem is pretty bad. With VSync off, the stutter problem virtually disappears completely. (That's odd, I would've thought it would be the reverse.) I'm not going to answer my own question and accept the answer, so if someone would like to answer with something about VSync (preferably with some ideas about why it's causing this problem on my system), I'll gladly accept your answer. Thanks again for all of the responses.

avatar image iphonedeano · Oct 20, 2012 at 01:03 PM 0
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Hi All,

I was wondering if you ever got to the bottom of this problem? I am using the follow code to move the object and camera.

var Speed : float = 28.0;

function FixedUpdate ()

{

transform.position += transform.forward Speed Time.deltaTime; }

I can not make it any more simple, I have tried it on two pc's and on a mac book, and still I get jerky steps every 2 seconds. I have tried all the differnt update types. All that I can proove is that when I remove the delta time all works well. I am at my witts end with this problem. I am now going to look at using the constant from a RTC and forget this delta time has some kind of joke, I do know that if I load an old copy of Unity I do not have the problem. Can anybody throw any light on this problem. What I was thinking, is to read the delta time before each movment and remove any larger fluctuations that happen within a percentage of say 1 second intervals by avarage mearsurments, I will give this a shot and see if it can be smoothed somehow. Thanks All Dean

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Answer by felixwcf85 · Mar 26, 2017 at 02:16 PM

Solved my issue by switching LateUpdate function to FixedUpdate in my Camera script.

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Answer by alirezakhaan · Oct 09, 2017 at 10:04 AM

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;
 
     }
 


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