Calculate 4 way direction of movement for 2d object

Question

Hello I have some code which uses velocity to tell which direction an object is moving. It does not always seem to work though and will sometimes say it is moving in the wrong direction. Any Advice? Thank You!

 private var prevLoc : Vector3 = Vector3.zero;
 var itsmoving = false;
 var direction : String;
 
 function Start () {
 
 }
 
 function Update () {
 
     //if moving stuff
     var curVel : Vector3 = (transform.position - prevLoc) / Time.deltaTime;
       if ((curVel.z > 0) && ((curVel.z - curVel.x > 0))) {direction = "up";}
       else if ((curVel.z < 0) && ((curVel.z - curVel.x < 0))){direction = "down";}
       else if ((curVel.x > 0) && ((curVel.x - curVel.z > 0))){direction = "right";}
       else if ((curVel.x < 0) && ((curVel.x - curVel.z < 0))) {direction = "left";}
       
       
     
     if((curVel.z > 0) || (curVel.x > 0) || (curVel.z < 0) || (curVel.x < 0))
     {
     // it's moving !!!!
     itsmoving = true;
  
         
     } else if ((curVel.z == 0) && (curVel.x == 0)) {
     // it's not moving
     itsmoving = false;
 
     
 
 }
  prevLoc = transform.position;  
 }

Answer 1

Answer by phren · Jun 13, 2012 at 12:50 PM

All those ifs are giving me a headache readability-wise so I ended up just thinking about it in a different way. :) You can do this quite easily by utilizing the dot products of vectors. In essence you can test with them if two vectors are "going in the same direction", in which case the dot product is positive (if they're going in separate directions the dot product is negative). Basically what you want to do is to just check:

 var dotUp : float = Vector3.Dot(curVel, Vector3.whateverIsUp);
 var dotRight : float = Vector3.Dot(curVel, Vector3.whateverIsRight);    
 if(dotUp > 0){
     direction = "up";
 } else if(dotUp < 0){
     direction = "down";
 } else if(dotRight > 0){
     direction = "right";
 } else if(dotRight < 0){
     direction = "left";
 } else {
     direction = "not moving";
 }

 if(curVel.magnitude > 0){
     itsmoving = true;
 } else {
     itsmoving = false;
 }

In this case I guess your "up" would be Vector3.forward and "right" would be Vector3.right. If you need help understanding how dot products work just ask. :)

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phren · Jun 13, 2012 at 12:51 PM 0

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Also I don't know how precise unity is with floating points, but I'd guess you're most likely not going to get itsmoving = false very often.

trothmaster · Jun 13, 2012 at 04:06 PM 0

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wow thank you so much for your help! So from reading the unity reference the Dot is used to compare two vectors facing directions against each other?

Also I tried to implement the script you gave and got this error: Assets/$$anonymous$$yscripts/Is$$anonymous$$oving.js(9,42): BCE0017: The best overload for the method 'UnityEngine.Vector3.Dot(UnityEngine.Vector3, UnityEngine.Vector3)' is not compatible with the argument list '(UnityEngine.Vector3)'.

here is how I tried to implement it...maybe i'm understanding this wrong?

private var prevLoc : Vector3 = Vector3.zero; var itsmoving = false; var direction : String;

function Update () { var curVel : Vector3 = (transform.position - prevLoc) / Time.deltaTime; var dotUp : float = curVel.Dot(Vector3.forward); var dotRight : float = curVel.Dot(Vector3.right);

 if(dotUp > 0){
     direction = "up";
 } else if(dotUp < 0){
     direction = "down";
 } else if(dotRight > 0){
     direction = "right";
 } else if(dotRight < 0){
     direction = "left";
 }
 
 if(curVel.magnitude > 0){
     itsmoving = true;
 } else {
     itsmoving = false;
 }

prevLoc = transform.position;

}

phren · Jun 14, 2012 at 06:19 AM 0

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Ah, I read the documentation wrong. I'll fix my answer to use the proper syntax. (also noticed a possible error)

Calculate 4 way direction of movement for 2d object

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