[SOLVED] Rotate an object around another

rstralberg · 07-04-2015 12:39 PM

Hey. I'm a C++ programmer, but definitely not a mathematician.
So I need some help.

I want to rotate a object A around another object B at distance D.
Tested a bit with various Matrix commands and finally came to the solution.
"I need help" smile

Could anyone show the magic lines

Code:

void rot( Game.Obj& A, Game.Obj& B, Flt D )

{

     // rotate B around A at distance D

     // ????

}

3DRaddict · 07-04-2015 05:36 PM

Off the top of my head:
---------------------------------------------------------------------------
global variables:
Vec offset_new;
Vec offset;
----------------------------------------------------------------------------
Init:
offset = Vec( (position of A) - (position of B) );
----------------------------------------------------------------------------
Update:
Matrix3 m1;
m1.identity();
m1.setRotateY(Time.time());
offset_new=offset.mul(m1);
-----------------------------------------------------------------------------
Draw/Render:
Matrix m;
m.setPos(offset_new);

Draw A using m
------------------------------------------------------------------------------

Zervox · 07-04-2015 06:41 PM

Code:

Object needs some variables

Flt angle

/*Vec2*/ void functionRot(Game.Obj &cent, Game.Obj &point,Flt dist, Flt dang=30*Time.d())

{

   point.angle+=DegToRad(dang); // Convert it to radians

   Flt rotX = Cos(angle) * (dist - cent.x) - Sin(angle) * (dist-cent.z) + cent.x;

   Flt rotZ = Sin(angle) * (dist - cent.x) + Cos(angle) * (dist - cent.z) + cent.z;

   point.pos(rotX,cent.pos.y/*or point.pos.y*/,rotZ);

   //return Vec2(rotX, rotZ);

}

rstralberg · 07-04-2015 08:13 PM

Thanks both of you.
You made my day. Now I got my thing working.
In my case I needed rotation around y

Code:

// Rotate an object around a point in space (X-axis)

   static void rotate_around_x( Game.Obj* obj,  C Vec& point,  Flt angle_degree )

   {

      Matrix m;

      m  .identity()

      .setRotateX( DegToRad(angle_degree));

      m.setPos( Vec(obj->pos()-point).mul(m)+point);

      obj->matrix(m);

   }

3DRaddict · 07-05-2015 09:36 AM

Glad it all worked out for you

georgatos7 · 07-05-2015 06:25 PM

Hey i'm just adding a quaternion solution here also.

Code:

// Rotate an object around a point in space (X-axis)

void rotate_around_x( Game.Obj* obj,  C Vec& point,  Flt angle_degree )

{

   Quaternion myRotation;

   myRotation.setRotateX(DegToRad(angle_degree));   

   obj->pos( Vec(obj->pos()-point) * myRotation + point );

}

I think this should have been faster but it's not and i don't know why.

Tottel · 07-05-2015 07:55 PM

(07-05-2015 06:25 PM)georgatos7 Wrote: I think this should have been faster but it's not and i don't know why.

What do you mean with faster?

georgatos7 · 07-05-2015 08:05 PM

(07-05-2015 07:55 PM)Tottel Wrote:
(07-05-2015 06:25 PM)georgatos7 Wrote: I think this should have been faster but it's not and i don't know why.

What do you mean with faster?

Well i mean that Quaternion multiplication should be faster than 3x3 matrix multiplication so it shouldn't be as cpu intensive (though i haven't gone through the specifics with the number of calculations) but i tried to benchmark this with a couple of hundred thousand calcs and it seems to be the opposite.

rstralberg · 07-05-2015 08:21 PM

Reading here http://devcry.heiho.net/2011/05/quaterni...mance.html its said

Quaternions are apparently terrific for combined rotations. If you hardly do combined rotations, matrices will be faster. If you do combined rotations all the time (like for animating skeletons and such) then you probably already knew that quaternions are the way to go

Maybe thats the reason?

georgatos7 · 07-05-2015 11:15 PM

Thanks for the link, i'll make sure i give it a look tommorow.

[SOLVED] Rotate an object around another
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