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jPCT - a 3d engine for Java => Support => Topic started by: K24A3 on October 09, 2011, 10:58:07 am

Title: Calculate rotation angle from movement (translation)
Post by: K24A3 on October 09, 2011, 10:58:07 am

I'm trying to rotate an object so it's facing the way it's moving.

Is there a function in jPCT or Java that can calculate the rotation angle based on an object's translation movement?


If the object is travelling up Z using translation(0,0,2), the angle would be 270 degrees.
If the object is travelling down Z  using translation(0,0,-2), the angle would be 90 degrees (or PI/2).
If the object is travelling up X  using translation(2,0,0), the angle would be 0 degrees.
If the object is travelling down X  using translation(-2,0,0), the angle would be 180 degrees (or PI).
If the object is travelling down X and up Z using translation(-2,0,2), the angle would be 225 (270-180).

Title: Re: Calculate rotation angle from movement (translation)
Post by: EgonOlsen on October 09, 2011, 01:40:55 pm

You can calculate the direction vector from the old position to the new one, call one of the getRotationMatrix()-methods on the resulting SimpleVector and use that as the new rotation matrix of yout object. Working with derived angles from a matrix or something usually is a bad idea.

Title: Re: Calculate rotation angle from movement (translation)
Post by: K24A3 on October 09, 2011, 02:10:10 pm

The problem is that the destination vector is unknown since the object is moved on the fly or by random.

I created an algorithm myself which works fine, I was hoping there was a pre-existing function to do the math.


Here's the code if anyone needs it for something.

Code: [Select]

float fCombined = 0;
if(xMove  < 0) fCombined = -xMove;
else           fCombined =  xMove;
if(zMove  < 0) fCombined += -zMove;
else           fCombined +=  zMove;


float fAngle = fPI;
if(zMove > 0) fAngle = fPIhalf*3;
else          fAngle = fPIhalf*1;


if(zMove > 0) // 270
{
if(xMove > 0) fAngle += (xMove/fCombined) * fPI/2;
else          fAngle -= (-xMove/fCombined) * fPI/2;
}
else // 90
{
if(xMove > 0) fAngle -= (xMove/fCombined) * fPI/2;
else          fAngle += (-xMove/fCombined) * fPI/2;

}

                                myobject.clearRotation();
                                myobject.RotateY(fAngle)



Title: Re: Calculate rotation angle from movement (translation)
Post by: EgonOlsen on October 09, 2011, 02:12:05 pm

 ??? But once you know the translation, you know the destination....i don't get the point here. Anyway, if you have something that works for you, all is well...

Title: Re: Calculate rotation angle from movement (translation)
Post by: K24A3 on October 09, 2011, 02:42:24 pm

Hmm I guess I could place the Move variables into a simpleVector then scaleMul it to generate a destination. I'll use that technique if I need Y movement :)